Joelle Dardene, St Anne's, Seaford

Some chemical science activities that you can include...

1. Ice in water and alcohol (designed for 5-6 but can easily do this one).
2. Kitchen slime Great fun!
3. Mixing oil and water
4. Physical Vs Chemical Reaction
5. Solids, liquids and gases
6. Viscosity

Vocabulary: matter, solid, liquid, gas, change, state, water, reversible, ice, steam, melt, boil, evaporate, condense, freeze.

What is Matter? Everything around us is made up of matter. The desk, your clothes, the air you breathe ... everything. Any matter can we weighed. It is said to have mass. Matter also takes up space. The amount of space it takes up is called its volume. Matter is classified into one of three types: solids, liquids or gases.

Solids, liquids and Gases: Solids can be tapped; liquids can be poured; and gases float. All matter is one of the three physical states: solid, liquid and gas. Its structure depends on temperature. All substances start as solids at the very lowest possible temperature, absolute zero (-273°C) and go through changes of state as they are heated. As a solid is heated, it will generally melt to become a liquid, and then boil to form a gas. Solids have a definite shape. Liquids take up the same volume within shape of its container even though the shape may change. Gases also do the same except they will always take up the same volume as that of the container. Gases can therefore be squeezed into a smaller container. Solids and liquids cannot.

What makes one state different from another? The state of a substance at any particular temperature and pressure is dependent on how big its particles are and how strongly they bond together. For example, substances which are solids under normal conditions (room temperature and standard air pressure) are usually made of large heavy atoms or molecules and are tightly bonded together. On the other hand, gases under these same conditions tend to be made of small, light atoms or very small molecules that do not stick together well. Liquids generally have particles which are somewhere in between these two extremes.

Changing state does not change the substance: When substances undergo a change of state they generally do not chemically change, or form new substances. They only undergo physical changes. Water is made of H₂O molecules, whether it is ice, liquid water or steam. Changes in state are usually easily reversed, by either heating or cooling, whereas chemical changes are more difficult to reverse. Changes in state are physical changes, not chemical. The substance itself remains chemically the same, i.e. water is always H₂O whether it is ice, water or steam.

Energy and changes in state: Changes in state occur because energy is either added or removed from a substance, affecting the way the particles interact with each other. If a substance is heated, energy is added and the particles will become more active; vibrating, rotating and even moving about faster. If the substance has enough energy, it can overcome the bonding forces holding the particles together and, in doing so, undergo a change in state. Cooling, on the other hand, removes energy thus making the particles less active and allowing the bonding forces to take hold within the substance.

Change of states: Melting (solid to liquid) is when the particles in a solid gain enough energy to overcome the bonding forces holding them together. The particles start to move about, staying close to nearby particles, then some of the bonds break. The temperature at which this change occurs is quite precise and is called the melting point of the substance. Freezing (liquid to solid) occurs when liquid is cooled and it turns back to a solid. Eventually the particles in a liquid stop moving about, bonds are reformed and settle into a stable arrangement, forming a solid. Boiling (liquid to gas) occurs when the particles in a liquid gain enough energy to overcome the forces holding them loosely in place and they become free, fast moving, individual particles in a gas. The temperature generally at which this change occurs is called the boiling point of the substance. Condensation (gas to liquid) occurs when a gas is cooled, its particles will eventually stop moving about so fast, some bonds re-form and a liquid results. Evaporation (liquid to gas) occurs when those remaining particles bonded together as a liquid break their bonds and fly off freely into the gas state, at a temperature called the boiling point of the liquid. When a puddle "disappears" after rain, the water has evaporated. Evaporation is dependent on individual particles gaining enough energy to escape the surface of the liquid and become gas particles. Sublimation is the change in state which occurs when a solid goes directly to a gas. Mothballs sublime and so does dry ice (solidified carbon dioxide gas). For a solid to sublime, individual particles on the surface of the solid obtain enough energy from their surroundings to jump off the surface of the solid and become individual gas particles directly.