Using Water

Using Water
HSW (How Scinece Works)
We all need water for drinking, cooking and washing ourselves and our our clothes. Some poeple also water their gardens and wash their cars. On average, each person in the UK uses about 150 litres of water a day.
The water that people in developed countries use comes from out of taps. The water is taken from lakes, rivers or wells. It is treated to make it clean and safe to use. When used water is let out of sinks or toilets are flushed, the water runs into a sewerage system, where it is treated again to make it safe.
We cannot use water from the sea for drinking, because sea water has salt dissolved in it. the salty water would make use ill.
In some part of the developing world, people have to get water from a well and carry it to their houses. In other places, people use water directly from rivers. Drinking water from rivers can spread diseases.
The Solution is Clear
What happens when things are mixed together
If lots of different things are jumbled up together, we have a mixture.
Sea water is a mixture. It is mostly water, but it has many different chemicals in it, including salt. It also has rubbish, seaweed and sand in it.
Seaweed and sand can be separated from sea water by filtering. The seaweed and sand are trapped in the filter paper, but the water runs through it.
Filtering will not separate the salt from the sea water, because the salt has dissolved in the water. When salt is stirred into water the salt grains seem to disappear. They break into very tiny pieces and mix with the water. A substance that dissolves is said to be solube. Something that will not dissolve is said to be insoluble.
A solid dissolved in a liquid make a solution. In a solution the liquid is called the solvent, and the solid is called the solute.
A solution is always transparent (see - through). If a liquid is cloudy or opaque, then it is not a solution.
Water is not the only solvent. Nail varnish does not dissolve in water, but it does dissolve in a liquid called propanone, which is in nail varnish remover. Some paints can be washed off with water, but gloss paint caonnot. A solvent called white spirit is used to remove gloss paint. Gloss pain is soluble in white spirit but not in water.
Solubility
What happens when something dissolves
When you make a solution, the solute does not go away. All the solute you use stays in the solution. The toal mass of a solution is always equal to the mass of the solvent added to the mass of the solute.
We can explain what happens using ideas about particles.
A. The particle in a solid are all held together in a fixed arrangement.
B. When the solid dissolves, the particles come away from each other. They mix with the particles in the solvent.
C. The paticles of the solid are too small to be trapped by the filter paper. This is why filtering cannot separate a dissolved solid from a liquid.
Top
Tip
Sea water contains about 35 g of
dissolved salt for every 100g of
water. Drinking water contains
less than 1 g of dissolved salt for
every 100g water
Even soluble solids like sugar cannot keep on
dissolving forever. For example, you wouldn't
expect a whole bag of sugar to dissolve in a
glass of water by adding it a little at a time.
Eventually you would find that no more sugar
will dissolve.
When no more sugar will dissolve, there will
be some crystals left at the bottom of the
glass. The solution is now saturated.
It contains as much dissolved solid as
it possibly can. If you add more suagr,
it will sink to the bottom and stay
undissolved.
The temperature also effects the amount of
a solid that dissolves. for most solids, more
solids dissolve when the water is hotter.

Main Menu

See Also

Main Menu