Day: April 29, 2010

1. Physical change is a reversible change.

Physical changes involve change in position. A physical separation involves moving one substance away from another. Iron can be moved away from sulfur and iron mixture by passing a magnet over a mixture containing the two substances. Water can be moved away from passing mixture of sand and water through a funnel and catching the sand in the funnel. We begin with two separate substances, which happen to be close to each other. We en up with the same two separate substances, but they are no longer close to each other. No new substances are formed and we can easily put the parts of the mixture together again.

Dissolving, melting, boiling, condensing, freezing and evaporating are all reversible changes.

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2. Chemical change is an irreversible change.

Chemical changes involve changes in the connections between substances. A chemical reaction can form a new substance. However, a chemical change involves the formation of a new substance, requires energy and cannot be reversed.

• Cooking can cause an irreversible change. For example: an egg can be heated and cannot be returned to its previous state.
• Burning materials, such as wood, paper, natural gas, causes an irreversible change.
• Irreversible changes can form new materials that can be useful e.g. plaster of Paris and water.
• Dead plants and animals decay and some metals rust. These chemical changes are permanent.

Our life is pull of forces. We cannot see them but we can see how they affect things. Forces can change the shape of something, its speed, or the direction that it is moving in.

This is an example of friction.

Forces are either pushes or pulls. These pictures show some changes caused by forces. Many forces need to touch an object before they can affect it. These are called contact forces.

Friction is the force between two touching objects. It can slow things down or make things stand still. The friction between our clothes and a chair stops us from sliding off it. Walking would be very difficult without the frictions between our feet and the floor – we would slip and slide everywhere.

Friction can also wear things away. The brake pads on a bicycle eventually wear away, and so do car tyres. Parts of your clothes get thinner as friction wears them away.

Friction produces heat and noise. If a car engine runs without any oil in it, the large friction between the moving parts inside the engine causes it to overheat and stop working. Rusty door hinges squeak and make a door difficult to open.

We can increase friction by using certain materials. Rubber produces a lot of riction which is useful to use. The tyres of a Formula One racing car stop the car from sliding off the road as it speeds round a sharp bend. The rubber mat in a bath stops us from slipping.

We can reduce friction by making surfaces smooth. Skiers wax the bottom of their skis to make them very smooth. This reduces friction and allows them to ski faster.