Chemical and Physical changes.

Any substance which is submitted to heat, added to water or mixed with another substance etc. can be changed. If a change takes place, it can be either chemical or physical.

Chemical Change/Reaction

A chemical reaction occurs when two or more chemical substances are mixed together and change into new substances. For this to happen, the bonds between atoms and molecules must break up and then re-form in different ways. Because the bonds can be strong, energy, (usually in the form of heat), is often needed to start a reaction. The new substances (products) have different properties from those of the original substances (reactants).

Chemical reactions do not occur only in laboratories; they happen all around us – for example, when metal rusts, when food is cooked and in firework displays.

Remember; when a new chemical substance is produced, the change is known as a chemical change.

* Chemical Substance Produced = Chemical Change*

Firework displays

The fireworks used in displays today are of many different types but all still rely on chemical change. Many fireworks are produced by mixing gunpowder with compounds with coloured flames( e.g. strontium compounds burn with a red flame ) and/or distinctive noises(e.g. magnesium compounds produce a hiss.)

Signs of a Chemical Changes

You can tell whether a chemical reaction has taken place by these three signs;

1. One or more new chemical substances are formed

The products have changed form and appearance. For example;

iron + sulphur   = iron sulphide

(Black fillings) (Yellow powder) = (Black solid)

2. Energy is taken in or given out, during the reaction.

A reaction, which gives out heat energy, is known as exothermic (link).

A reaction, which takes in heat energy, is endothermic (link).

For example:

Fireworks are exothermic, as is the reaction above.

Frying eggs is an endothermic reaction.

Reactions not only produce heat but light and sound.

3. The change is usually difficult to reverse.

Several reactions would have to be carried out to get iron and sulphur from iron sulphide.

Chemical equation

A chemical equation is a way of describing a chemical reaction. It uses symbols and numbers to show the names and proportions of the various substances involved in a reaction. The reactants are shown on the left side of the equation, and the products are shown on the right. Matter is never created or destroyed during a chemical reaction. This means the equation must be balanced-the number of atoms on each side must be equal.

For example;

CuO      +         H₂SO₄                 =           CuSO + H₂0

Copper oxide + sulphuric acid = Copper sulphate + water

Mixtures and compounds

In a mixture particles are mixed closely together, but are not bonded so the particles of the substance still behave, as they should. During a chemical reaction, however, the atoms of each substance form ions which bond to each other to become one and act as one

Remember; A compound contains atoms of different isotopes chemically joined together.

Mixture diagram (link or picture.)

Compound diagram (link or picture.)

Physical Changes

A chemical change is usually contrasted with a physical change, in which a substance changes form but keeps its chemical properties remains the same.

No new chemical substances are formed in these changes.

For example;

Although ice, water and steam look different, they are all made up of water molecules, and have the formula H₂0.

Remember; A physical change is when no new chemical substance is formed.

*No chemical substance produced = No chemical change*

Physical changes are usually easy to reverse e.g. water back to ice.

The distinction between physical and chemical change is not absolute, and some important types of change, such as the dissolving of salt in water, could be regarded as either being chemical or physical, or both.

Thanks to Laura Gallagher/ 10B /Loreto College, Coleraine