Breifly covers C3 3.1-6

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Water is constantly cycled around the earth and its atmosphere.

(copy this address to see the full water cycle) http://www.bbc.co.uk/scotland/education/int/geog/rivers/images/hyd_cycle/diag_cycle.gif

for this section all you need to know is that many ionic compounds and some molecular substances dissolve in water.

Also that a saturated solution has dissolved the maximum amount of solute at a given temperature.

Key Words:

  • Solute: the dissolved matter in a solution; the component of a solution that changes its state.
  • Solvent: A solvent is a liquid or gas that dissolves a solid, liquid, or gaseous solute, resulting in a solution.
  • Soluble: (of a substance) capable of being dissolved in some solvent.
  • Precipitation: The the formation of a solid in a solution during a chemical reaction
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Solubility Curves

The solubility of different substances at different temperatures can be plotted as a line graph.

Solubility graphs are used to plot solubility curves which can be used to find how much solute will dissolve at a particular temperature, they can also be used to find how much solute will separate out of a saturated solution for a given change in temperature.

The solubility of most solids increases as temperature increases.

The solubility of most gasses decreases as temperature decreases.

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Hard Water

Hard water creates 'scum' when it reacts with soap this is because of salts that are dissolved in the water (mostly magnesium and calcium.) Hard water also creates scale when it is heated.Water that lathers easily with soap is called soft water.
because hard water contains calcium it can be healthy for your bones and teeth and lower the chances of heart disease.

Removing Hardness: Hard water can be made soft by removing ions that react with soap (magnesium and calcium ions). This can be done by taking out the ions that react with soap to create scum or replacing them with ions that don't react with soap.

A way of doing this is to add washing soda, which is sodium carbonate - the carbonate ions react with with magnesium and calcium forming a precipitate, because the ions are no longer in solution, the ions cannot react with soap.

Another way of making hard water soft is to put hard water through ion-exchange resins which usually contain sodium ions. Hard water is passed through the ion-exchange resin and magnesium and calcium become attached to the resin and sodium ions take their place in the water and of course, sodium ions don't react with soap. If you want to get rid of the sodium ions then you'll have to wash the water with concentrated sodium chloride.

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Water Treatment

We all need water to drink, but before we do, it has to be safe to drink therefore we need to treat it and remove harmful substances such as bacteria and twigs or leaves.

We can remove solids from water by using filters, and remove microorgannisms by killing them with chlorine.

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