The composition of a mixture is variable.

 

Def: A compound is formed when two or more elements are chemically combined.

 

Compounds have a fixed composition: the proportions of the elements from which they are made are fixed.

 

Def: A Mixture is formed when two or more substances are put together but are not combined chemically. E.g. Air

 

Tea (Solvent) + sugar (soluble substance) = Cup of tea (Solution)

 

Def: A solution is obtained when one substance is dissolved in another.

 

	Dilute and concentrated solutions: Add a little blue copper sulphate (CuSO4) to water in a beaker and stir until it dissolves. This is a dilute solution. Add more Copper Sulphate and stir until it dissolves, this is a more concentrated solution. Another way to make this solution more concentrated is to evaporate off some of the water.
	Soluble/Insoluble:  Some substances don’t dissolve in others they are soluble. E.g. Salt is soluble in water but isn’t in petrol. E.g. Candle wax is insoluble in water but soluble in petrol.

 

Def: A saturated solution is a solution that has dissolved as much solute (solid) as it can hold at that temperature.

 

Def: The process of forming crystals of a compound by cooling a saturated solution is called crystallisation.

 

Def: Suspension is where fine particles are spread throughout the liquid and do not dissolve. i.e. they are insoluble. E.g. Muddy water.