Basic chemistry help for students and teachers


Interactive Chemistry Worksheets for Students


Periodic table

Naming compounds

Basic formula

Simple compounds

Ionic compounds 1

Ionic compounds 2

Chemical suffixes

Old chemical names

Hydrocarbons - Alkanes

Alkanes, alkenes & alkynes

Alkanols to alkanoic acids


Common compounds

Common formula quiz

Acids, bases and salts

Covalent compounds

Printable worksheets


Decomposition reactions

In a decomposition reaction a chemical substance is broken down into simpler substances.

The general equation for the reaction is AB A + B

Substance AB decomposes or breaks apart producing substances A and B.

Decomposition can be achieved by 1. heat, 2. electricity and 3. a catalyst.

Note: Some extremely unstable chemicals can be set of by sound waves!

Examples of decomposition reactions

1. Thermal decomposition of copper carbonate.

If a test tube containing green copper carbonate is heated in a Bunsen burner the powder turns black and occasionally ‘jumps’ as carbon dioxide gas is given off.

Copper carbonate =heat=> Copper oxide + carbon dioxide

CuCO3 =heat=> CuO + CO2

2. The electrolytic decomposition of water

Electrolysis is the process using electrical energy to speed up or cause a chemical reaction.

Pure water is a poor electrical conductor. Sodium sulfate salt or sulfuric acid needs to be added to the water first. Note: DO NOT use sodium chloride salt or hydrochloric acid as toxic chlorine gas is produced!

Two inert electrodes (carbon or platinum) are placed into the water and are connected to an external D.C power pack or series of batteries. Hydrogen gas is produced at the negative electrode and oxygen gas at the positive electrode in a ratio of 2:1.

Water =electrolysis=> Hydrogen and Oxygen

2H2O =electrolysis=> 2H2 + O2

3. The catalytic decomposition of hydrogen peroxide

A catalyst is a substance that speeds up a chemical reaction without being used up in the reaction.

Hydrogen peroxide is broken done quite rapidly into oxygen gas and water when a catalyst like potassium iodide or manganese dioxide is added. Potassium iodide salt is preferred if available as it makes less mess. The black manganese dioxide powder is difficult to clean up.

One to two spatulas of potassium iodide salt is added to a conical flask containing 50mL of 5% hydrogen peroxide solution. The solution is gently mixed and bubbles of gas are observed to form. If a glowing splint is placed into the conical flask it will re-ignite due to the production of oxygen gas.

Hydrogen peroxide =catalyst=> Oxygen and water

2H2O2 =catalyst=> O2 + 2H2O