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A substance which is combined with another substance.
Physical Change
A change that can be reversed, since the internal structure of the substance isn’t changed.
Chemical Change
An irreversible change in which the reactants form a product as the internal structures are altered.
What observations are there in a chemical change?
Energy changes:
Heat energy may be liberated (exothermic reaction) or absorbed (endothermic reaction)
Light energy is given out
Sound energy is given out
A colour change.
A precipitate forms.
Effervescence is observed in liquids.
The law of conservation of mass
Mass can neither be created nor destroyed but is transformed from one form to another.
How are the particles in a solid arranged?
They are packed closely together and they are held in place by strong forces of attraction.
The particles can only vibrate about fixed points and solids have a fixed shape and volume.
How are the particles in a liquid arranged?
Particles are close to one another and there are some spaces between them, particles occur in clusters. There are quite strong forces of attraction between them.
They undergo random motion and roll over another, so they can flow. They take the shape of the container they are in.
How are the particles in a gas arranged?
Particles are far apart from each other and so can be compressed easily. There are negligible forces of attraction between the particles.
They move about in a random free way unless they are confined. They do not have a fixed volume/shape.
The Kinetic Theory
All matter is made up of tiny particles, the particles are in a state of constant motion and there are forces of attraction holding the particles together.
Melting
When a solid turns into a liquid.
Evaporation
When a liquid turns into a gas.
Condensation
When a gas turns into a liquid.
Freezing
When a liquid turns into a solid
Sublimation
When a solid turns into a gas.
Deposition
When a gas turns into a solid.
What happens to the temperature as a substance melts, boils, condenses or freezes?
It remains constant.
Note 
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