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pond water, an example of natural water, contains impurities.
impurities in natural water include soluble impurities, insoluble impurities and micro-organisms.

some micro-organisms in natural water, such as E. coli and Amoeba, may cause illnesses.

sedimentation is the process in which large and heavy solids are allowed to settle at the bottom of the water, the layer of impurities settled at the bottom is called sediment.

in the process of sedimentation, adding alum helps remove the fine solid impurities suspending in water.

by passing the pond water through a filter column, most of the insoluble impurities in the water can be removed.
by passing the pond water through a piece of filter paper, most of the insoluble impurities in the water can be removed.

filtration can remove solid impurities in water.

the liquid which passes through the filter is called  filtrate  , the solid which cannot pass through the filter is called residue.

filtered pond water contains micro-organisms and soluble impurities.

filtration cannot remove micro-organisms and soluble impurities in water.

distillation can remove all the impurities in water.

adding chlorine to water can kill most micro-organisms in it.

chlorine, ozone, ultra-violet light and boiling are commonly used to kill micro-organisms in water.

in Hong Kong, drinking water is treated at water treatment works before it is distributed to us.

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the water treatment process in Hong Kong involves sedimentation, filtration, chlorination and fluoridation.
in the water cycle, water keeps changing between liquid water and water vapour.

fresh water is a precious resource. we must conserve it.

water pollution is mainly caused by sewage from homes. restaurants, farms and factories, solid waste and oil from accidental spills.

every one has the responsibility to reduce water pollution.

the Hong Kong government controls water pollution by running sewage treatment works. treating waste water that contains harmful chemicals and legislation.

water is a good solvent, but it cannot dissolve everything.

when a substance dissolves in a liquid, a solution is formed. the substance that dissolves is called a solute. the liquid used to dissolve it is called a solvent.

a solute will not dissolve completely when too much of it is added to a fixed amount of solvent.

more/less salt can dissolve than sodium sulphate in a fixed amount of water of the same temperature.

when no more solute can dissolve in a solvent, a saturated solution is formed.

a substance with a higher solubility can dissolve a large amount in a fixed amount of water of the same temperature.

in general , a solute dissolves faster when:
the temperature of the solvent is higher.
the solute pieces are smaller in size.
the solution is stirred/not stirred.

small crystal can be formed by cooling down a hot saturated solution quickly.

crystal have      shapes and    surface.

crystals can also be formed by cooling down a hot saturated solution slowly, the crystals formed in this way are larger/smaller in size.

crystals of the same kind may have the same sizes but their shapes are different/same.

some substance such as oil, dissolve in solvent other than water.

there are potential dangers in using solvents other than water . we must handle these solvents with care.

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chlorinationdistillate
distillation
filtrate
filtration
fluoridation
insoluble
residue
sediment
sedimentation
soluble
water treatment works
convection current
evaporation
water cycle
pollutant
saturated solution
solute
solution
solvent
crystal