After lime water turns milky: hydrogen carbonates (bicarbonates), formation and decomposition HCO3 - @sergei-chemistry

Sergeis Chemistry If carbon dioxide is bubbled through, lime water turns milky on formation of insoluble calcium carbonate. But if the gas is allowed to bubble on, solution soon becomes clear again as soluble calcium hydrogen carbonate is formed. It is unstable and on heating it decomposes giving precipitate of CaCO3.

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Key words: lime water, carbon dioxide test, calcium carbonate, calcium hydrogen carbonate, thermal decomposition, CO2, Ca(OH)2, CaCO3, Ca(HCO3)2.

The test for carbon dioxide is well-known: lime water turns milky when the CO2 gas is passing through it. Insoluble white precipitate of CaCO3 is formed:
CO2 (g) + Ca(OH)2 (aq) → CaCO3 (s) + H2O (l)
In other words, acidic gas CO2 reacts with alkali Ca(OH)2 making insoluble salt CaCO3 and water.

But if we allow the gas to bubble on the reaction proceeds further.
CaCO3 is basic and still can react with acidic CO2 in water.
Precipitate of CaCO3 disappears as it is reformed into soluble calcium hydrogen carbonate:
CaCO3 (s) + CO2 (g) + H2O (l) → Ca(HCO3)2 (aq)
This reaction is responsible for formation of caves in limestone.

Hydrogen carbonates are mainly soluble, in contrast to carbonates, which are mainly insoluble.

But hydrogen carbonates are unstable. Except for hydrogen carbonates of Na+, K+ and NH4+ they can exist only in solutions, not in the solid state. On heating or drying they break down, for example:
Ca(HCO3)2 (aq) → CaCO3 (s) + CO2 (g) + H2O (l)

As Ca(HCO3)2 breaks down on heating we again observe appearance of milky precipitate of CaCO3.
This reaction is responsible for formation of stalactites and stalagmites in caves.

Chapters:
00:00 Test for carbon dioxide with lime water: milky CaCO3 precipitate is formed
00:44 CaCO3 precipitate disappears as hydrogen carbonate is formed
01:13 Hydrogen carbonate decomposes on heating; CaCO3 reappears as milky precipitate

updated 4 years ago

After lime water turns milky: hydrogen carbonates (bicarbonates), formation and decomposition HCO3 -

Sergeis Chemistry 2022-02-14 | If carbon dioxide is bubbled through, lime water turns milky on formation of insoluble calcium carbonate. But if the gas is allowed to bubble on, solution soon becomes clear again as soluble calcium hydrogen carbonate is formed. It is unstable and on heating it decomposes giving precipitate of CaCO3.

TUTORING Introductory and Advanced high school Chemistry CBSE, Edexcel, etc
inquire at sergei@auroville.org.in

Key words: lime water, carbon dioxide test, calcium carbonate, calcium hydrogen carbonate, thermal decomposition, CO2, Ca(OH)2, CaCO3, Ca(HCO3)2.

The test for carbon dioxide is well-known: lime water turns milky when the CO2 gas is passing through it. Insoluble white precipitate of CaCO3 is formed:
CO2 (g) + Ca(OH)2 (aq) → CaCO3 (s) + H2O (l)
In other words, acidic gas CO2 reacts with alkali Ca(OH)2 making insoluble salt CaCO3 and water.

But if we allow the gas to bubble on the reaction proceeds further.
CaCO3 is basic and still can react with acidic CO2 in water.
Precipitate of CaCO3 disappears as it is reformed into soluble calcium hydrogen carbonate:
CaCO3 (s) + CO2 (g) + H2O (l) → Ca(HCO3)2 (aq)
This reaction is responsible for formation of caves in limestone.

Hydrogen carbonates are mainly soluble, in contrast to carbonates, which are mainly insoluble.

But hydrogen carbonates are unstable. Except for hydrogen carbonates of Na+, K+ and NH4+ they can exist only in solutions, not in the solid state. On heating or drying they break down, for example:
Ca(HCO3)2 (aq) → CaCO3 (s) + CO2 (g) + H2O (l)

As Ca(HCO3)2 breaks down on heating we again observe appearance of milky precipitate of CaCO3.
This reaction is responsible for formation of stalactites and stalagmites in caves.

Chapters:
00:00 Test for carbon dioxide with lime water: milky CaCO3 precipitate is formed
00:44 CaCO3 precipitate disappears as hydrogen carbonate is formed
01:13 Hydrogen carbonate decomposes on heating; CaCO3 reappears as milky precipitate

Crystals in a drop: Pb(NO3)2 + KNH4Br lead nitrate and ammonium bromide make lead bromide crystals

Sergeis Chemistry 2025-04-01 | Two substances meet in a drop of water: lead (II) nitrate and ammonium bromide.
Needle-like lead (II) bromide crystals are forming.
Ionic equation: Pb2+ (aq) + Br- (aq) → PbBr2 (s)
White precipitate is expected; at high magnification one can see crystals formation.

Movement of colored ions under the action of electricity: KMnO4 on a moist filter paper, DC current

Sergeis Chemistry 2025-03-29 | Ionic substances, such as salts, are made of ions, charged particles.
Some of them are colored, like permanganate ions here.
If ions are charged, they should move under the action of electric field, created by DC power source.
The video (accelerated about 7000 times) demonstrate the slow motion of negative permanganate ion from the negatively charged clamp to the positive one.

Matchstick test for sulfates; burned wood reduces SO42- into sulphides; nitroprusside turns purple.

Sergeis Chemistry 2025-03-29 | Colorful test for sulphates.
Burning wood reduces sulfates into sulphides.
Sulphides turn potassium nitroprusside solution purple.

Water vapour fountain - vapor condenses, pressure drops, atmospheric pressure pushes water in

Sergeis Chemistry 2025-03-29 | ...

Crystals in a drop: Pb(NO3)2 + KI lead nitrate and potassium iodide crystals in a drop of water

Sergeis Chemistry 2025-03-28 | 2KI (aq) + Pb2+ (aq) → PbI2 (s) (yellow precipitate) + 2K+ (aq)
Yellow precipitate of PbI2 is made of golden shiny crystals

Cu2+ drop and Fe filings; copper metal is formed from solution of its ions; iron is reducing agent

Sergeis Chemistry 2025-03-28 | Drop of CuSO4 (copper (II) sulphate) and iron (Fe) filings.
Copper ions are reduced by iron atoms, copper ions are leaving solution becoming copper metal.
Full equation: CuSO4 (aq) + Fe (s) → Cu (s) + FeSO4 (aq)
Ionic equation: Cu2+(aq) + Fe (s) → Cu (s) + Fe2+ (aq)

Drops: KI and CuSO4 drops react forming white precipitate of CuI (copper (I) iodide) and iodine

Sergeis Chemistry 2025-03-28 | Copper (II) ions oxidize iodide ions to iodine; at the same time copper (II) is reduced to copper (I) which form insoluble copper (I) iodide.
2Cu2+ + 4I- → 2CuI (s) (white precipitate) + I2 (aq) (brown solution)

Dying planet test: potassium ferrocyanide turns a drop containing Cu2+ ions chocolate brown

Sergeis Chemistry 2025-03-28 | Copper (II) sulfate added a drop of potassium ferrocyanide making a brown complex:
Cu2+(aq) + K4[Fe(CN)6] (aq) → Cu2[Fe(CN)6] (brown
ppt.)+ 4K+

Drops: CuSO4 + NH3 ammonia added to a drop of copper (II) sulphate solution makes deep blue complex

Sergeis Chemistry 2025-03-28 | Drops of CuSO4 + NH3
Addition of excess of ammonia to copper (II) sulphate solution make deep blue complex:
[Cu(H2O)6]2+ (pale blue complex) + 4NH3 (aq) →
[Cu(NH3)4(H2O)2]2+ (aq) (deep blue complex) + 4H2O (l)

Matchstick test for sulphate ions (confirmatory test for sulfates)

Sergeis Chemistry 2024-10-27 | Confirmatory test for sulphates; more precisely it detects the presence of sulfur atoms in the tested substance.

Mixture of a salt to be tested and NaHCO3 are put on the wooden end of a matchstick which is heated strongly in a flame.
On heating sulphate is reduced to sulphide by carbon of the charcoal into which the wood is turned. This is confirmed by the colour change of sodium nitroprussade solution from colourless (or pale brown-green) to purple.

Test for phosphate ions with ammonium molibdate (testing H2PO4- with (NH4)6Mo7O24 solution)

Sergeis Chemistry 2024-10-15 | Confirmatory test for phosphate ions:
1. Acidify the tested solution with concentrated nitric acid.
2. Add ammonium molibdate solution (NH4)6Mo7O24 (aq)
3. Heat up the test tube with solution in the flame of the burner (do not boil) or in a hot water bath.
4. Formation of bright yellow precipitate confirms the presence of phosphate ions.

Yellow precipitate is ammonium phosphomolibdate (NH4)3PMo12O40
The equation for the reaction is:
12 (NH4)6Mo7O24 + 7 NaH2PO4 + 58 HNO3 → 7 (NH4)3PMo12O40 + 51 NH4NO3 + 7 NaNO3 + 36 H2O

Nickel Ni2+ confirmatory tests: dimethyl glyoxime and test with NaOH and bromine water Br2 (aq)

Sergeis Chemistry 2024-10-13 | Ni2+ tests; confirmatory tests for nickel (II) ions:

1. Dimethyl glyoxime test
Add dimethyl glyoxime CH3C(NOH)C(NOH)CH3 solution in propan-2-ol to the tested solution (solution should be neutral or alkaline).
Red precipitate formation confirms Ni2+ presence.

2. Sodium hydroxide + bromine water Br2 (aq) test
To the tested solution add NaOH followed by bromine water Br2
formation of black precipitate of Ni(OH)3 confirms Ni2+ presence in original solution
Ni(OH)2 (pale green precipitate) + ½ Br2 (aq) + OH- (aq)→ Br- (aq) + Ni(OH)3 (black precipitate)

00:00 Introduction; nickel (II) ammonia complex, nickel (II) hydroxide
00:46 Dimethyl glyoxime test for Ni2+ ions
01:47 Dimethyl-nickel (II) complex structure
02:51Soduim hydroxide + bromine water test
05:18 Two confirmatory tests for Ni2+ ions - final run

TUTORING High School Chemistry - inquire at sergei@auroville.org.in

More videos for the positive ions / basic radicals confirmatory tests:
Fe3+ : youtu.be/d0MGataJtM0
Cu2+ : youtu.be/TTi-APBOwvQ
Mg2+ : youtu.be/RDuagPJ6Qtg
Pb2+ : youtu.be/eoFnWm5eLso

Confirmatory tests for Cu2+ cupric ions - hydroxide, ammonia, pot.ferrocyanide, KI and iron filings

Sergeis Chemistry 2024-10-06 | Confirmatory qualitative tests for copper (II) Cu2+ ions presence in solution.

High School Chemistry TUTORING, Edexcel, CBSE, Cambridge. Inquire at sergei@auroville.org.in

1. Addition of NaOH (aq) - formation of gelatinous blue precipitate of Cu(OH)2
Cu2+(aq) + OH-(aq) → Cu(OH)2 (s)
CuSO4 (aq) + NaOH (aq) → Cu(OH)2 (s) + Na2SO4 (aq)

2. Addition of NH3 (aq) - formation of deep blue solution.
[Cu(H2O)6]2+ (pale blue complex) + 4NH3 (aq) → [Cu(NH3)4(H2O)2]2+ (aq) (deep blue complex) + 4H2O (l)

3. Addition of KI (aq) - formation of brown solution with white precipitate.
Cu2+ (aq) + 2KI → CuI (s) (white ppt.) + ½ I2 (aq) (brown) + 2K+(aq)

4. Addition of potassium ferrocyanide K4[Fe(CN)6] (aq) - chocolate brown precipitate.
Cu2+(aq) + K4[Fe(CN)6] (aq) → Cu2[Fe(CN)6] (chocolate brown ppt.) + 4K+

5. Addition of Fe (s) metal to a solution of Cu2+ salt - formation of brown-pink Cu metal.
Cu2+ (aq) + Fe (s) → Fe2+ (aq) + Cu (s)

More videos for the positive ions / basic radicals confirmatory tests:
Fe3+ : youtu.be/d0MGataJtM0
Ni2+ : youtu.be/ozE2ZiWhmSI
Mg2+ : youtu.be/RDuagPJ6Qtg
Pb2+ : youtu.be/eoFnWm5eLso

Test for magnesium ions Mg2+ using Magneson Reagent (Azo Violet)

Sergeis Chemistry 2024-09-28 | Magneson reagent (Azo Violet) gives blue precipitate (blue lake) with solutions of magnesium salts. Here I tested magnesium salt plus several other salts for control.
Azo Violet can be used as a pH indicator. It is blue at pH above 13 and yellow at pH below 11.

TUTORING Introductory and Advanced high school Chemistry CBSE, Edexcel, etc
inquire at sergei@auroville.org.in

Thanks to NESS high school, Auroville, Tamil Nadu, India for chemicals used in the video.

More videos for the positive ions / basic radicals confirmatory tests:
Fe3+ : youtu.be/d0MGataJtM0
Ni2+ : youtu.be/ozE2ZiWhmSI
Cu2+ : youtu.be/TTi-APBOwvQ
Pb2+ : youtu.be/eoFnWm5eLso

Confirmatory tests for Fe3+ ferric ions: hydroxide, potassium ferrocianide, potassium thiocyanate

Sergeis Chemistry 2024-09-22 | Tests for iron (III) ions Fe3+
1. NaOH sodium hydroxide test:
add to analyte solution 3 drops of NaOH (aq); brown precipitate suggests presence of Fe3+ ion. Add 6 drops of HCl (aq); precipitate redissolves into yellow solution.
2. K4[Fe(CN)6] potassium ferrocyanide (potassium hexacyanoferrate (II)) test:
to analyte solution add several drops of potassium ferrocyanide; formation of blue precipitate (prussian blue) confirms Fe3+
3. KSCN potassium thiocyanate test:
to analyte solution add 3 drops of KCNS (aq); formation of blood red colour confirms Fe3+ ion.

TUTORING Introductory and Advanced high school Chemistry CBSE, Edexcel, etc
inquire at sergei@auroville.org.in

More videos for the positive ions / basic radicals confirmatory tests:
Ni2+ : youtu.be/ozE2ZiWhmSI
Cu2+ : youtu.be/TTi-APBOwvQ
Mg2+ : youtu.be/RDuagPJ6Qtg
Pb2+ : youtu.be/eoFnWm5eLso

Thanks to NESS high school, Auroville, Tamil Nadu, India for chemicals used in the video.

Paramagnetism of aluminium, eddy currents, dumping oscillations with a magnet.

Sergeis Chemistry 2023-11-13 | A powerful magnet is tried on a number of nonmagnetic metals.
Aluminum is not exactly nonmagnetic, it is paramagnetic, very weakly attracting a magnet. This attraction is difficult to feel directly but it can be observed with a help of a powerful magnet and sensitive scales.

There is another puzzling effect.
Paramagnetic (Al) or diamagnetic (Cu) metals are both attracted to and repelled from a magnet depending on their relative motion. The faster we bring the metal and the magnet close to each other, the bigger their mutual repulsion, the faster we take then away, the bigger is attraction.

Explanation of this observation is eddy currents.

What are these currents and how they are generated?
If I forcefully bring a magnet close to nonmagnetic metal, changing magnetic field of approaching magnet will pass into the metal and generate circular electric fields there. Circular or eddy currents are created as electrons are moving along the field lines. The direction of the eddy current is such that its magnetic field will oppose the change which brought it about.

Magnetic fields generated by eddy currents would repel the magnet which is approaching the metal and would attract the magnet which is being removed.

Eddy currents explain damping effect of magnetic field on movement of nonmagnetic metals through the area of the field.

TUTORING Introductory and Advanced Physics Edexcel, etc
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Tests for Pb2+ ions with K2CrO4 and KI. Redissolving lead iodide in hot water. Recrystallization.

Sergeis Chemistry 2023-10-29 | Confirmatory test for lead (II) ions in solution using nickel sulfate as a control.
1. Lead (II) precipitates on addition of K2CrO4 forming yellow solid PbCrO4.
2. Lead (II) precipitates with KI forming yellow PbI2.
3. PbI2 redissolves on heating.
4. Cooling the solution makes it recrystallize back forming glistening crystals.

TUTORING Introductory and Advanced high school Chemistry CBSE, Edexcel, etc
inquire at sergei@auroville.org.in

More videos for the positive ions / basic radicals confirmatory tests:
Fe3+ : youtu.be/d0MGataJtM0
Ni2+ : youtu.be/ozE2ZiWhmSI
Cu2+ : youtu.be/TTi-APBOwvQ
Mg2+ : youtu.be/RDuagPJ6Qtg

Reaction of iron (Fe) and sulphur (S) forming iron (II) sulphide; mixture and compound

Sergeis Chemistry 2023-10-08 | Two elements form a compound. Elementary example of a chemical change: iron and sulfur react on heating forming iron (II) sulphide.

TUTORING Introductory and Advanced high school Chemistry CBSE, Edexcel, etc
inquire at sergei@auroville.org.in

Bromine water test for unsaturated compounds (C=C double bond)

Sergeis Chemistry 2023-10-06 | TUTORING: Introductory and Advanced Chemistry. Inquire at sergei@auroville.org.in
Video:
Unsaturated organic compounds decolorise bromine water (Br2 (aq) color changes from brown to colorless).
The reason: unsaturated compounds have double or triple bonds between carbon atoms. Bromine adds to the molecule across the double bond.
Saturated substances give negative result - the brown colour remains.

TUTORING Introductory and Advanced high school Chemistry CBSE, Edexcel, etc
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Brown ring test for nitrate ions NO3- . Formation of [Fe(H2O)5(NO)]SO4

Sergeis Chemistry 2023-10-02 | Test for nitrates (NO3- ions) is based on the ability of nitrogen monoxide gas (NO) to form brown coloured complex with Fe2+ ions. For the test we add FeSO4 (iron (II) sulfate) to the solution which we are going to test and then carefully pour down along the wall of the test tube concentrated sulphuric acid H2SO4 which goes to the bottom.

On the contacting surface with sulphuric acid nitrates are protonated and form nitric acid HNO3 which is then reduced by Fe2+ (ions of iron (II)) forming NO (nitrogen monoxide). NO immediately form complexes with Fe2+ ions which are still left intact, forming brown colouration.

This takes place at the borderline between sulphuric acid at the bottom and the tested solution with FeSO4 at the top -- the brown colour is formed there. This is seen at a certain angle as a brown ring -- positive result for the presence of NO3- ions.

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Fehlings solution test for aldehydes; using glucose for positive result and starch for control

Sergeis Chemistry 2023-09-27 | Fehling's solution test for aldehydes: if aldehyde is added to the solution, on heating it is turning from blue (due to copper (II) ions) to brown or red - precipitate of Cu2O (copper (I) oxide) is formed.

Essentially we are testing for extremely good reducing properties of aldehydes, which can reduce even such weak oxidising agent as Cu2+ in alkaline medium.

As the result of the reaction aldehyde is oxidised forming carboxilate ion.

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Tests for sulfide (sulphide) S2- with cadmium carbonate, lead acetate and sodium nitroprusside

Sergeis Chemistry 2023-09-24 | There are at least three ways to test for sulphide (sulfide) ions S2- presence in solution.
Here I use NaS solution for the positive result and ZnSO4 solution as a control.

1. Cadmium carbonate test.
Add a pinch of CdCO3 (white powder) to the tested solution. Change of colour from white to yellow confirmes the presence of sulphide.

2. Lead acetate test.
Add a few drops of lead acetate or lead nitrate solution to the tested solution. Formation of black precipitate (insoluble PbS) confirms the presence of sulphide ions.

3. Sodium nitroprusside test.
Add a few drops of sodium nitroprusside solution to the analyte. Formation of purple-violete complex (sodium thionitroprusside) confirms the presence of sulphides.

TUTORING Introductory and Advanced high school Chemistry CBSE, Edexcel, etc
inquire at sergei@auroville.org.in

00:00 Introduction. Cadmium carbonate test.
01:19 Lead acetate test.
02:18 Sodium nitroprusside test.
03:18 All three tests for sulphides (sulfides) at once.

Test for water with anhydrous CuSO4, preparing anhydrous copper (II) sulfate from hydrated crystals

Sergeis Chemistry 2023-09-03 | On addition of water white anhydrous copper (II) sulphate turns blue.

We can prepare white anhydrous CuSO4 powder by heating up blue hydrated crystals of CuSO4.5H2O driving the water of crystallization away.

TUTORING Introductory and Advanced high school Chemistry CBSE, Edexcel, etc
inquire at sergei@auroville.org.in

Supersaturated sodium acetate (ethanoate) solution preparation, crystallization, building castles

Sergeis Chemistry 2023-08-24 | Supersaturated solution crystallizes on disturbance or on addition of a seed crystal. It is a semi-stable liquid, containing more dissolved solid than it have at equilibrium.

On heating solubility in water of most of the salts increases. Some of them can even dissolve in their own water of crystallization. Sodium acetate is an example. When the liquid cools down in absence of the solid phase of the salt it remains liquid even at room temperature.

It immediately solidifies on a contact with a seed crystal.

TUTORING Introductory and Advanced high school Chemistry CBSE, Edexcel, etc
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Silver mirror test for aldehydes: Tollens reagent test, Tollens reagent preparation, washing up.

Sergeis Chemistry 2023-08-22 | Appearance of silver mirror on the walls of the test tube containing Tollen's reagent on heating after the addition of a tested substance is a positive test for aldehydes.

Tollen's reagent is an alkaline solution of silver ions which addition of ammonia is keeping in dissolved state. Silver ions are weak oxidizing agents, but they are strong enough to oxidize very reducing aldehydes.

Soluble silver ions on receiving electrons from aldehydes become insoluble silver atoms. Some of them deposit on the walls of the test tube creating a "silver mirror."

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Schiffs reagent test for aldehydes (formaldehyde, benzaldehyde and effect of reagent on skin)

Sergeis Chemistry 2023-08-20 | Schiff's reagent is a de-colorized pigment which regains its color on contact with an aldehyde. If aldehyde is present it colorless Schiff' reagent turns magenta or red or pink.

Many aldehydes are insoluble in water, so the test is usually done in ethanol.

Beware: glucose (an aldehyde) does not give positive result with Schiff's reagent.

TUTORING Introductory and Advanced high school Chemistry CBSE, Edexcel, etc
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Deriving 2as = v2 - u2 kinematic equation for accelerated motion; its meaning, one sample problem

Sergeis Chemistry 2023-04-21 | Deriving 2as = v2 - u2, kinetic energy theorem.

TUTORING High School Physics -- Edexcel, etc
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Sulfur burns in oxygen: properties of sulfur dioxide; test with acidified potassium dichromate paper

Sergeis Chemistry 2023-04-20 | Sulfur burning in oxygen, test for sulfur dioxide, acidity and reducing properties of SO2.

Sulfur burns in oxygen with bright blue flame. Gaseous sulphur dioxide is produced:
S + O2 → SO2

Sulfur dioxide is an acidic gas; dissolving in water it produces sulfurous acid, a weak acid.
SO2 + H2O → H2SO3
Acidity of the gas can be measured with wet pH paper inserted into it or - which is essentially the same - by mixing water, containing universal indicator, with the gas. Indicator in paper or universal indicator in water would turn red, proving acidity of SO2.

SO2 gas also has reducing properties. Sulfur in sulfur dioxide or in sulphite ion (which is formed when the gas dissolves in water) has oxidation state +4, thought the stable oxidation number for sulfur is +6, as in sulfate or in sulfuric acid. It means that two more electrons can be lost by sulfur atoms and obtained by some oxidizing agent. Oxidizing agent will be reduced, and this can be easily observed if oxidized and reduces forms of oxidizing agent have different colors. Acidified potassium dichromate fulfills this requirement, turning from orange to green on reduction in acidic conditions.

To test for sulphur dioxide we use potassium dichromate paper: filter paper dipped into mixture of K2Cr2O7 and H2SO4. On contact with SO2 it changes color from orange to green, turning orange K2Cr2O7 (oxidation number for Cr in it is +6) to green Cr3+ ion (oxidation state Cr is +3).

Half equations for oxidation of sulphite into sulfate and for reduction of dichromate ions into chromium (III) are given in the second part of the video.

TUTORING Introductory and Advanced high school Chemistry CBSE, Edexcel, etc
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Diffraction of light on a slit using Vernier Caliper and table lamp

Sergeis Chemistry 2022-05-30 | To see a diffraction of light on a slit in a simplest way use a Vernier Caliper - the slit created by the jaws is one of the finest easily available.
Have a narrow source of light - the narrower is the light the finer will be the diffraction picture.
Bring the jaws of the caliper very close to each other and then bring the caliper very close to your eye.
Look through the slit at the light in a relaxed way, as if at infinity: you would see a central white maximum surrounded by rainbow-looking secondary maxima.
Changing the width of the slit will change the resolution of the diffraction picture.

TUTORING Introductory and Advanced high school Chemistry CBSE, Edexcel, etc
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Ammonium carbonate (NH4)2CO3 thermal decomposition; tests for NH3, H2O and CO2 which are formed

Sergeis Chemistry 2022-05-09 | Decomposition of ammonium carbonate and the tests on the products: ammonia, water and carbon dioxide.

Ammonium carbonate decomposes on heating forming three gases:
(NH4)2CO3 (s) → 2NH3 (g) + H2O (g) + CO2(g)

These gases can synthesize the solid back in the colder areas:
2NH3 (g) + H2O (g) + CO2(g) → (NH4)2CO3 (s)

The tests for all three products of decomposition are demonstrated:

The test for ammonia is turning damp litmus or damp pH paper blue.
The test for water is turning blue cobalt (II) chloride paper pink.
The test for carbon dioxide is turning lime water milky. This is however complicated by the presence of ammonia which would prevent formation of the precipitate. Ammonia is removed from the gaseous mixture by bubbling it first through sulfuric acid.

TUTORING Introductory and Advanced high school Chemistry CBSE, Edexcel, etc
inquire at sergei@auroville.org.in

Chapters:
00:00 Introduction
00:10 Thermal decomposition of ammonium carbonate
01:06 Test for ammonia
01:38 Test for water
02:00 Test for carbon dioxide. Removal of ammonia.

Reduction of copper (II) oxide by carbon; 2CuO + C = 2Cu + CO2

Sergeis Chemistry 2022-04-26 | Carbon is above copper in reactivity series so element carbon can reduce ions of copper, such as Cu2+ ions in CuO:

2CuO (s) + C (s) → 2Cu (s) + CO2 (g)

Reaction takes place after a few minutes of heating of the mixture. Examination of the products confirms the presence of a microscopic nuggets of copper.

TUTORING Introductory and Advanced high school Chemistry CBSE, Edexcel, etc
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Magnesium metal reaction with cold water and hot water vapor; on heating: Mg + H2O (g) → MgO + H2

Sergeis Chemistry 2022-04-24 | Rate of magnesium reaction with cold water is very slowly indeed:
Mg (s) + 2H2O (l) → Mg(OH)2 (aq) + H2 (g)
Solution is turned alkaline and some bubbles of hydrogen gas are formed slowly. Magnesium hydroxide formed is practically not soluble in water and is blocking the access of water to the surface of the metal and the reaction eventually stops.

Reaction of Mg with water vapour is violent if vapor is heated up to several hundred degrees. Reaction is essentially the same, but instead of magnesium hydroxide magnesium oxide is formed:
Mg (s) + H2O (g) → MgO (s) + H2 (g)

In both reactions Mg acts as a reducing agent: hydrogen in water is reduced to its elementary form.

A side reaction is taking place In the areas where Mg touches glass:
SiO2 (s) + 2Mg (s) → Si (s) + 2MgO (s)
Glass turns black because silicon element is produced.

TUTORING Introductory and Advanced high school Chemistry CBSE, Edexcel, etc
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Magnesium metal burning in air, basic properties of magnesium oxide; 2Mg + O2 → 2MgO

Sergeis Chemistry 2022-04-21 | Magnesium metal: how it looks, how it burns in air and the properties of its oxide.

Mg metal is grey and shiny, that is when it is cleaned, it is quite soft.
It burns in air (reacting mostly with oxygen) with blinding white flame forming white smoke and white solid of magnesium oxide:
2Mg (s) + O2 (g) → 2MgO (s)
Filming at low light sensitivity allows us to see the inner structure of the flame.

Magnesium oxide reacts with water forming magnesium hydroxide:
MgO (s) + H2O (l) → Mg(OH)2 (s)
Magnesium hydroxide is not very soluble in water but it dissolves well enough to show its alkalinity. Phenolphthalein indicator changes color from colorless to pink.

TUTORING Introductory and Advanced high school Chemistry CBSE, Edexcel, etc
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Test for halide ions using silver nitrate; ammonia to distinguish between chloride, bromide, iodide

Sergeis Chemistry 2022-04-08 | Test for presence of Cl-, Br- or I- ions:

1. Acidify the tested solution with HNO3
2. Add AgNO3 solution
3. Silver halides are insoluble. If precipitate is formed it shows, depending on the color:

white precipitate: presence of Cl- ions
cream precipitate: presence of Br- ions
yellow precipitate: presence of I- ions

Addition of NH3 (ammonia) helps to distinguish precipitates which can look very similar to each other:
AgCl is soluble in dilute ammonia
AgBr is insoluble in dilute but is soluble in concentrated ammonia
AgI is not soluble in ammonia, dilute or concentrated.

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Concentrated sulfuric acid reaction with sugar and filter paper - dehydrating carbohydrates

Sergeis Chemistry 2022-03-23 | Concentrated sulfuric acid has two remarkable properties: it is non-volatile - means it does not easily form vapors in contrast to hydrochloric and nitric acids - and also it is dehydrating: it readily absorbs water and can be used as a drying agent to acidic gases.

It can do more than drying: it can and will absorb water from the very structure of organic molecules. Carbohydrates, such as cellulose or sucrose, have structure Cm (H2O)n. In theory any carbohydrate can be broken down to just water and carbon. Concentrated sulphuric acid does this in practice.

On addition to filter paper it eats up holes in it leaving a charred residue behind.

With sugar (sucrose) the reaction is even more spectacular. A spongy column of carbon is rising up from a beaker, hissing and fuming is taking place.

Absorption of water by concentrated sulphuric acid is very exothermic. This is the reason why the reaction takes a lot of time to start and speeds up enormously by the end (the higher the temperature the higher the rate).

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Electrolysis with active electrodes: copper Cu and iron Fe anodes; oxidation of metallic electrodes

Sergeis Chemistry 2022-03-11 | Electrolysis with active electrodes: iron or copper anodes

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Normally ANODES are made of graphite or platinum. They are called inert electrodes, as they do not participate in reactions of electrolysis. Cathode just supplies electrons to the species in the solution, anode takes them. In this cases there are no reactions which change the substance of the anode.

In case of the solution of sodium chloride NaCl (aq) at anode either Cl- ions or H2O molecules are oxidized, depending on concentration of the solution.

Anode reactions are:
2Cl- → Cl2 + 2e- (if the concentration of halide ions is high enough)
2H2O → O2 + 4e- + 4H+ (if the concentration of halide ions is low)

If the anode is made of a metal other than platinum, for example from Fe or even Cu, there is another species from which electrons can be taken: the atoms of the anode themselves.

In these cases we have the following anode reactions:
Fe (s) → Fe2+ (aq) + 2e-
Cu (s) → Cu2+ (aq) + 2e-

Colored transition metal ions will go into the solution.
If environment is alkaline they will form colored precipitates of metal hydroxides. If the medium is acidic, we can test for the presence of the ions adding NaOH (aq) and observing the color of the precipitate:
Fe2+ (aq) + OH- (aq) → Fe(OH)2 (s) (green precipitate)
Cu2+ (aq) + OH- (aq) → Cu(OH)2 (s) (blue precipitate)

CATHODE electrodes are always inert, they are never oxidized because the cathode is the place of reduction. Electrons are given to water, to hydrogen ions or to ions of metals below hydrogen in reactivity series (if they are present in the solution).

Cathode reactions in case of electrolysing H2SO4 with copper anode:
2H+ (aq) +2e- → H2 (g)
Cu2+ (aq) + 2e- → Cu (s) (this will happen after a while, when copper atoms accumulate in the solution)

Chapters
00:00 Iron anode
01:15 Copper anode

KNO3 - thermal decomposition, test for oxygen evolved and oxidizing properties of potassium nitrate

Sergeis Chemistry 2022-03-10 | On heating the following reaction takes place:
2KNO3 (l) → 2KNO2 (l) + O2 (g)

Nitrates of the first group metals decompose just to nitrite and oxygen; in contrast to other nitrates there is no formation of brown NO2 gas during decomposition.

We can test the gas evolved for oxygen; the gas relights a glowing splint.

Potassium nitrate is an oxidizing agent. It is a main component of a black gunpowder. At a high temperature it will oxidize carbon, sulfur or any organic material. Even a glowing splint – which is demonstrated here.

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Chapters:
00:00 Introduction; melting and thermal decomposition of KNO3
00:33 Test for oxygen evolved
00:55 The salt "eats up" glowing splint

Electrolysis of KBr and KI solutions: micro-scale; pH paper and starch is used to test the products

Sergeis Chemistry 2022-03-08 | Electrolysis of KI and KBr solutions in water.

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Few drops of 0.5 M of KBr or KI solution in a watch glass, graphite electrodes (pencil leads), potential difference between electrodes is about 8V.

Cathode is supplying electrons to particles which can take it, reduction is taking place at the cathode.
Anode is taking electrons from any particles, be it atoms, ions or molecules. This is oxidation.

Halide ions in water solution will be oxidized at anode if they are present at high concentration.

Particles from solution which can be oxidized at the anode: H2O (OH-), I-
Iodide ions are oxidized and the black solid – iodine – is made which is soluble in KI (aq).

Anode half equation:
2I- → I2 + 2e-
Test: starch solution turns blue if iodine is present.

Particles which can be reduced at the cathode: H2O (H+), K+
Potassium ions cannot be reduced in water solution, this is not possible for any metal above Zn in reactivity series. Hydrogen in water is reduced instead.

Cathode half equation:
2H2O + 2e- → H2 + 2OH-
To make test for hydrogen gas it has to be collected, which is difficult at micro-scale, but we can test for hydroxide ions formation with pH paper.
Test: hydroxide ions make red litmus paper blue (make pH paper blue).

Full ionic equation:
2I- + 2H2O→ I2 + H2 + 2OH-

Full chemical equation for electrolysis of water solution of potassium iodide:
2KI + 2H2O→ I2 + H2 + 2KOH

Equations for KBr are analogous.
When Br2 is formed it dissolves in water, making it brown. One can distinguish it from iodine with the help of starch solution – on its addition there is no color change.

Anode half equation:
2Br- → Br2 + 2e-
No color change on addition of starch solution.

Cathode half equation:
2H2O + 2e- → H2 + 2OH-
Test: hydroxide ions make red litmus paper blue (make pH paper blue).

Full ionic equation:
2Br- + 2H2O→ Br2 + H2 + 2OH-

Full chemical equation for electrolysis of water solution of potassium bromide:
2KBr + 2H2O→ Br2 + H2 + 2KOH

Chapters:
00:00 Titles
00:10 Electrolysis of KBr (aq)
01:13 Electrolysis of KI (aq)

Electrolysis of NaCl (sodium chloride) solution; simple test of the products with pH paper

Sergeis Chemistry 2022-03-07 | Electrolysis of brine (NaCl water solution)

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A few drops of NaCl solution in a watch glass, graphite electrodes (pencil leads), potential difference between electrodes is about 8V.

Cathode is supplying electrons to particles which can take it. This is reduction.
Anode is taking electrons from any particles, be it atoms, ions or molecules. This is oxidation.

Particles from solution which can be oxidized at anode: H2O (OH-), Cl-
Hydroxide ions at standard conditions will be oxidized in preference to chloride ions, but their concentration in water is very low. Chloride ions are oxidized instead, forming chlorine molecules. Some chlorine dissolves in water making it acidic, some bubbles off as a gas. Solution of chlorine in water bleaches indicator paper.

Anode half equation:
2Cl- → Cl2 + 2e-
Test: chlorine is bleaching wet indicator paper.

Particles which can be reduced at cathode: H2O (H+), Na+
Sodium ions cannot be reduced in water solution, this is not possible for any metal above Zn in reactivity series. Hydrogen in water is reduced instead.

Cathode half equation:
2H2O + 2e- → H2 + 2OH-
To make test for hydrogen gas it has to be collected, which is difficult at micro-scale, but we can test for hydroxide ions formation with pH paper.
Test: hydroxide ions make red litmus paper blue (make pH paper blue).

Full ionic equation:
2Cl- + 2H2O→ Cl2 + H2 + 2OH-

Full chemical equation for electrolysis of water solution of sodium chloride:
2NaCl + 2H2O→ Cl2 + H2 + 2NaOH

Chlor-alkali industry is using this process for production of Cl2 and NaOH.

Diffusion of gases: white ring experiment. NH3 and HCl diffuse towards each other forming NH4Cl

Sergeis Chemistry 2022-03-02 | Diffusion is a net movement of particles from the aria of high concentration to the aria of low concentration dew to their random thermal motion.

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Here we compare the relative rate of diffusion of two gases: ammonia (NH3) and hydrogen chloride.
We can observe the spreading of the gases tracking their passage with the help of pH paper. Ammonia is alkaline making the paper blue and green and hydrogen chloride is acidic, making it red. The place where they meet is marked by formation of a solid crystals of NH4Cl salt. The "white ring" of the solid is formed on the walls of the tube.

NH3 (g) + HCl (g)→ NH4Cl (s)

Particles at a given temperature have the same kinetic energy. Therefore the lighter particles should have bigger velocity, since kinetic energy is half of the product of masses of particles by the squares of their velocities.

Molar mass of NH3 is about two times less than that of HCl. We expect it particles to move faster and therefore to diffuse faster.

We sent these two gases diffusing towards each other from the opposite ends of a tube. It is observed that the solid NH4Cl, marking the point at which the gases meet, is formed farther away from the source of NH4 and closer to the source of HCl.

This supports the idea that the lighter particles are moving faster.

Tests for bicarbonates (hydrogen carbonates): heating with magnesium sulfate or phenolphthalein

Sergeis Chemistry 2022-02-16 | Test 1:
Add MgSO4 (aq) (or any other calcium or magnesium salt solution) to the tested liquid; there should be no precipitate.
White precipitate is formed on heating.

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Test 2:
Add phenolphthalein to the tested solution; phenolphthalein should turn pink.
Solution becomes violet-red on heating

Three main ideas are behind the tests:
1. Hydrogen carbonates decompose into carbonates on heating
2. Soluble carbonates are more alkaline than hydrogen carbonates
3. Hydrogen carbonates of group II metals are soluble but their carbonates are insoluble

pH
Hydrogen carbonates are weakly alkaline:
HCO3- + H2O → H2CO3 + OH-
Reaction is reversible and equilibrium is on the left

Carbonates are more alkaline than hydrogen carbonates: they are better equipped to receive protons.

Soluble carbonates are strong alkalis
CO3- + H2O → HCO3- + OH-
Equilibrium is on the right here.

On heating hydrogen carbonate, a weak alkali, turns into carbonate which is a strong alkali.
2NaHCO3 (aq) → Na2CO3 (aq) + CO2 (g) + H2O (l)
Ionic equation:
2HCO3- (aq) → CO32- (aq) + CO2 (g) + H2O (l)

During this process pH is significantly increased.
This is shown by changing colors of indicators.

Solubility
HCO3- are soluble in water.

Most of hydrogen carbonates (except for the 1st group ones and ammonium) can exist only in solution and decomposed on drying.

This is especially important for group II metals.
Calcium and magnesium hydrogen carbonates are main components of hard water.

On heating hydrogen carbonates of magnesium or calcium will give precipitate of carbonates, which are insoluble, as most carbonates are.

This is a reason for the test “heating with Mg or Ca salt solution and observing formation of white precipitate”.

Decomposition of hydrogen carbonate:
2NaHCO3 (aq) → Na2CO3 (aq) + CO2 (g) + H2O (l)
Precipitation of insoluble carbonate of the 2nd group metal:
MgSO4 (aq) + Na2CO3 (aq) → MgCO3 (s) + Na2SO4 (aq)

As ionic equations:
2HCO3 (aq) → CO3 2-(s) + CO2 (g) + H2O (l)
Mg2+ (aq) + CO3 2-(aq) → MgCO3 (s)

Chapters:
00:00 Main ideas behind the tests
00:17 Hydrogen carbonates and carbonates: comparison of properties
01:30 Test 1: using magnesium sulfate (MgSO4 (aq))
02:47 Test 2: using phenolphthalein

Existence of ions demonstration. Negative permanganate ions move to anode.

Sergeis Chemistry 2022-02-16 | Demonstration of existence of ions.

In electrolytes under the action of electric field positive and negative move in opposite direction. Anions, negative ions, move towards anode, positive electrode.

Movement of negative colored manganate (VII) ions is observed on this video accelerated x 40

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CaCO3 thermal decomposition; reaction of CaO with H2O; testing Ca(OH)2 formed with phenolphthalein

Sergeis Chemistry 2022-02-13 | Thermal decomposition of marble pieces, mainly calcium carbonate, CaCO3, at temperatures above 650 C:
CaCO3 (s) → CaO (s) + CO2 (g)

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CaO formed is tested with water; reaction is very exothermic:
CaO (s) + H2O (l) → Ca(OH)2 (s)

On addition of more water Ca(OH) dissolves and dissociates into ions:
Ca(OH)2 (s) + aq → Ca(OH)2 (aq)
Ca(OH)2 → Ca2+ + 2OH-

Testing with phenolphthalein gives violet color, which means pH is more than 10, confirming the alkaline nature of the product.

Original marble pieces do not change color of phenolphthalein.

Chapters:
00:00 Heating up marble chips
00:31 Cooling down produced calcium oxide CaO
00:55 Reaction of CaO with water; formation of Ca(OH)2
01:27 Testing original marble pieces with phenolphthalein
01:42 Testing hydrated decomposed chips with phenolphthalein
01:58 Conclusion

Reduction of potassium permanganate (manganate (VII)) with sulphite at different pH with equations

Sergeis Chemistry 2022-02-03 | KMnO4 - potassium permanganate or manganate (VII) is a well-known oxidizing agent.

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In acidic conditions purple potassium permanganate is reduced to colorless manganese (II) ions.

If pH is high reduction goes not to to completion as there is not enough H+ ions to remove oxides from the manganate (VII) complex. Reduction goes to form MnO2 precipitate in neutral conditions and green unstable MnO2- (manganate (VI)) solution in alkaline conditions.

The list of ions and complexes containing Mn at different oxidation states:

MnO4- oxidation state +7 purple
MnO42- oxidation state +6 green, disproportionates reduction at pH 14
MnO2 oxidation state +4 brown precipitate reduction at pH 7
Mn2+ oxidation state +2 colorless reduction at pH 1

Calcium carbonate thermal decomposition; calcium oxide reaction with water and phenolphthalein

Sergeis Chemistry 2022-01-28 | A marble chip - CaCO3 - is decomposed to CaO on intense heating (about 850C). CaCO3 breaks down into CaO and CO2 gas.

Calcium oxide is cooled down and is reacted with H2O. It reacts exothermically; water boils and hisses and crumbly Ca(OH)2 is formed.

Ca(OH)2 is dissolved in water and solution is tested with phenolphthalein. Phenolphthalein turns pink, showing that Ca(OH)2 is an alkali.

In old English wording: thermal decomposition cases formation of quick lime (CaO) from a limestone (CaCO3). Then slaked lime (Ca(OH)2) is formed if we slake quicklime with water. On addition of more water some Ca(OH)2 dissolves, forming lime water.

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Reaction rates 2 Definition, introduction to graphs, collision theory, simple diagrams illustration

Sergeis Chemistry 2021-12-17 | Reaction rate is defined as an amount of change (of concentration, mass or volume) per unit of time.
We can plot graphs depicting the process of reaction showing products: the graphs will rise, as they are produced during the reaction, or we can plot reactants: the graphs will fall, as the reactants are consumed.

Collision theory:

1. For particles to react, they have to collide first.
Factors increasing the rate of collisions (the number of collisions per unit of time) will speed up the reaction: increasing the concentration of solutions, increasing the surface area of solids and increasing the pressure of gases.

2. For collision to end up in a chemical change (i.e. to be successful) the particles have to collide with energy not less that some minimum amount.
This minimum amount of the energy of collision, needed for it to be successful, is called activation energy.
The factors which increase the rate of successful collisions (the number of successful collision per second) are: higher temperature (by increasing the number of molecules with energy higher than activation), and presence of a catalyst - by creating an alternative pathway for the reaction, which requires less activation energy.

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Reaction rates 1 Elementary introduction: informal intro to activation energy, temperature, catalyst

Sergeis Chemistry 2021-12-17 | Informal - there is no formulas! - introduction to the idea of activation energy and catalyst. Elementary chemistry.

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I introduce the idea of activation energy and use it to explain the effect of increasing temperature and the presence of catalyst on the rate of a reaction.

Molecules react when they collide. Increased rate of collisions between particles (number of collisions per unit of time) can explain increasing rate when concentration of reactants is increased, or surface area of a solid reactant is increased, or a pressure of reacting gases is increased.

Presence of a catalyst or increasing temperature have nil or minimal effect on increasing the rate of collisions, but they increase the rate by increasing the number of SUCCESSFUL collisions.
For molecules to react collision is necessary, but it is not sufficient. In addition the molecules should collide with the energy which is higher than some minimum amount, which is called activation energy.

Activation energy is the minimum energy the colliding molecules should have for the reaction to take place.

Increasing temperature we minimally increase the number of collisions, but we significantly increase the proportion of collisions which are successful, which have energy higher than activation.

Catalyst is a substance which speeds up the rate of a chemical reaction without being consumed in the process.

Catalyst increases the rate because it provides an alternative pathway for the reaction to proceed, which has lover activation energy.

Adding vectors: introductory physics. Rule of triangle and rule of parallelogram.

Sergeis Chemistry 2021-12-14 | Elementary introduction to adding vectors.

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Vectors are mathematical objects which have direction and magnitude. They can be modeled as arrows. They are used in introductory physics first of all to represent forces and calculate their sum. How to add forces acting on an object? For that we have to know how to add vectors.

To find a sum of a group of vectors we draw them in a chain, connecting the base of the one with a tip of another. The directed line connecting the base of the first with the tip of the last represents their sum.

The order in which you add vectors does not matter, the result will be the same.

To add just two vectors use a rule of triangle: draw the base of one starting at the tip of another, than complete the triangle: the line completing the triangle is their sum.

Rule of parallelogram allows us to add vectors drawn with their bases positioned at the same point (common case with forces on a free body force diagram). Draw two vectors with their bases at the same spot. Complete the parallelogram with two additional sides parallel to the vectors. The diagonal of the parallelogram coming from the common base of the vectors is their sum.

These rules are essentially equivalent.

Colors of iodine solutions in water, ethanol, KI (aq) and hexane. Sublimation and starch reaction.

Sergeis Chemistry 2021-12-14 | How solid iodine looks and how it sublimes, solutions of iodine in water, ethanol, potassium iodide and hexane.
Explanations are accompanied by high-quality photos.

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The colors of iodine:
solid - shiny grey-black
vapor - purple
solution in water - virtually not soluble, yellow or pale brown
in ethanol - brown
in KI(aq) - brown
in hexane - purple
with soluble starch - blue-black

Iodine is not soluble in water as its molecules are nonpolar. It is soluble in ethanol (due to interaction with its nonpolar ethyl group). It is soluble in hexane (which is nonpolar).
It is soluble in aqueous KI due to formation of complexes between iodine molecules and iodide ions.

Iodine reversibly reacts with soluble starch forming deep blue color. In iodine redox titrations soluble starch is added when iodine solution pales to straw color to emphasize the end-point, at which the solution titrated changes from blue to colorless.

Chapters:
00:00 Solid iodine appearance
00:22 Gaseous iodine in equilibrium with solid
00:42 Sublimation of iodine
01:07 Solubility of iodine in water
01:43 Solubility of iodine in ethanol
02:13 Solubility of iodine in iodide solutions (KI (aq))
02:57 Solution of iodine in hexane (in alkanes)
03:21 Distribution of iodine in layers of hexane and KI (aq); solvent extraction
04:10 Overview of solutions of iodine in different solvents
04:30 Iodine and starch reaction
05:09 Main points of the video revisited