Bitesize has changed! We're updating subjects as fast as we can. Visit our new site to find Bitesize guides and clips - and tell us what you think!

Did you know?

We also have Bitesize study guides covering many subjects at National 4 and National 5 on our Knowledge & Learning BETA website.

Home > Chemistry > Elements and reactions > Chemical reactions


Chemical reactions


This topic builds on the work you have already done in S1 and S2. There are some terms and definitions that you will need to remember.

Physical changes

This type of change means that no new substances are made, but there is a change in the appearance of a chemical. Examples of physical change include state changes and dissolving.

State changes

All substances can be solid, liquid or gas.

an increase in heat turns water into ice and then stream. A decrease in heat turns water from steam into water and then into ice.

In this process, water (hydrogen oxide) has not been chemically changed, but by cooling or heating it, you can change its state.


Some chemicals can be dissolved in water, e.g. common salt (sodium chloride).

Sugar lumps dissolved in water make a sugar solution. Adding more sugar will make the solution more concentrated. If less sugar had been added, it would be a more dilute solution. Another way to dilute the solution would be to add more water (think of diluting orange!)

In a solution, the dissolving substance is called the solute [solute: A solute is the material that dissolves in a solvent to form a solution.. The liquid used to dissolve the solute is called the solvent [solvent: A solvent is the liquid in which the solute dissolves to form a solution.. For example, in the case of sugar and water, sugar is the solute, and water is the solvent.


Video: Changes of state

In order to see this content you need to have both Javascript enabled and Flash installed.



BBC © 2014 The BBC is not responsible for the content of external sites. Read more.

This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.