CHEMICAL CHANGES INVOLVING LIGHT

Photography

In photography, photographic films have silver bromide crystal coating. In the presence of light, silver bromide crystals form into silver atoms. Silver bromide crystal wich have not been exposed to light are removed by dissolving in a solution called "fixer".

The film is now called a "Negative", the opaque parts have been exposed to lights while the transparent parts are not exposed to light. When the film is exposed to light, the photograph will be formed.

This is an endothermic reaction where energy is absorbed(light energy).

CHEMICAL CHANGES INVOLVING ELECTRICITY

Electrolysis

It is the process whereby compounds which conduct electricity undergo chemical change during conduction. Compounds which conduct electricity and undergo electrolysis are called electrolytes.In electrolysis, electrical energy is converted into chemical energy.

An example is : Oxygen and hydrogen are produced when water goes through electrolysis.

This reaction is endothermic because bonds are broken.

CHEMICAL CHANGES INVOLVING MIXING

Reaction between a metal and an acid

This occurs when dilute acids react with metals to form a gas called hydrogen

An example is when magnesium and hydrochloric acid react together and magnesium chloride and hydrogen is formed.

Another example is sulfiric acid and calcium will combine to calcium sulfate and hydrogen gas.

This is an exothermic reaction as heat is given out.

Reaction between an acid and a metal carbonate

This occurs when a metal carbonate and an acid form a gas called carbon dioxide.

An example is when hydrochloric acid is mixed with calcium carbonate, it will form calcium chloride, carbon dioxide and water.

This is also an exothermic reaction as heat is given off.

CHEMICAL CHANGES INVOLVING MIXING

Neutralisation

Neutralisation is a process whereby a base reacts with an acid to form salt and water only.This means that the acid has lost its acidic properties.

Like an example, when sulfuric acid is mixed with magnesium oxide the end product are magnesium sulphate and water.

Another example is when hydrochloric acid is mixed with sodium hydroxide, the mixture will become sodium chloride and water.

This is an exothermic reaction as heat is produced.

CHEMICAL CHANGES INVOLVING HEAT

Combustion

Combustion is also a chemical change involving heat.

Combustion describes the chemical reaction where some substances react with oxygen in an exothermic reaction ( that is heat is produced). If flames are produced during combustion, it is called burning.

Example when magnesium is burnt in oxygen, a new product will be formed and heat is produced. The new product formed by the magnesium and oxygen is magnesium oxide.

Hydrocarbons are fuels which are burnt to produce energy.Hydrocarbons are compounds that consist of carbon and hydrogen. Example, when it is burned in oxygen, it produces carbon dioxide and water.

This is an exothermic reaction as heat is produced.

CHEMICAL CHANGES INVOLVING HEAT

Oxidation and and Reduction

Oxidation and Reduction are chemical reactions that involve heat.

Oxidation describes those chemical reactions in which:

(a)hydrogen is removed from the substance.

(b)oxygen is added to the substance.

(c)the substance loses electrons.

Reduction describes those chemical reactions in which:

(a)hydrogen is added to substance.

(b)oxygen is removed from the substance.

(c)the substance gains electrons.

1)An example is magnesium burn s in air, magnesium is oxidised by the addition of oxygen, and oxygen is reduced. The oxygen is the oxidising agent while magnesium is the reducing agent.

2)another example is : zinc dissolves in dilute hydrochloric acid. Zinc is oxidised by the addition of chlorine and hydrogen is reduced. The chlorine ions are neither oxidised or reduced. Zinc is the reducing agent while the hydrogen ions are the oxidising agent.