Water topic

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Water
Water is covalently bonded, it has a boiling point of 100 degrees and freezing point of 0
degrees. Water is essential for all life on earth ­ it is used as a solvent, a coolant in power
stations and water is also decomposed in the Haber process to get hydrogen. Test for water,
it turns blue cobalt chloride pink and turns anhydrous copper sulphate blue.
The water cycle
The heat from the sun causes water from rivers, lakes and streams to evaporate. The water
vapour rises, cools due to the general cooling of the lower part of the atmosphere at high
altitudes, and condenses to form clouds. As the clouds cool further rain, hail, snow and other
forms of precipitate are produced. The rain (or precipitate) that falls starts off as fairly pure but
as it falls it will dissolve more and more carbon dioxide from the air. As the water falls it will
come into contact with rocks and dissolves some of the minerals /substances from the rocks
as it passes through them.( E.g. potassium, sodium) Water in different places will dissolve
different minerals into the water depending on the rocks present there. The water then runs
back into the sea completing the cycle. The cycle will then start over again.
Water is a solvent
Water dissolves most ionic compound. As water molecules are polar the negative oxygen
side attracts the positive ions in ionic compounds and the positive hydrogen side attracts the
negative ions in the ionic compound. Water molecules surround the ions of the compound and
disrupt the ionic bonding so the solid structure of the compound gradually falls apart.
This why drinking water is not pure as it contains dissolved substances and
chemicals. However most covalent compounds do not dissolve in water as their molecules
are too large and have no ions present and so the poles of the water can't attract to their ions.
Water dissolves lots of substances that it comes in contact with e.g. it can dissolve sulphur
dioxide from power stations to form acid rain.
Common question: How do minerals get into the water supply?
As rain and other forms of precipitate fall they come in contact with rocks and dissolve these
mineral ions from the rocks which then enter the water.
Purifying drinking water

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We get our water from reservoirs. Water flows into the reservoirs from rivers. For humans
water should contain low levels of dissolved salts/ions and microorganisms.
Water goes from reservoirs into water treatment works for treatment. Here water passes
through mesh screens to remove large solid material. The water is treated with ozone or
chlorine to kill harmful microorganisms. Chemicals are then added to make solids and
microorganisms stick together and sink to the bottom of the tank.…read more

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Hard water
Hard water is water containing dissolved calcium or magnesium ions ­ these ions are
dissolved in the water when it comes into contact with rocks. There are two types of hardness:
Temporary hardness is caused as when rain falls it dissolves carbon dioxide making
carbonic acid. This reacts with limestone (calcium carbonate) to form calcium
hydrogencarbonate which is soluble in water.…read more

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Scum
Soft water readily forms a lather but hard water it does not.
Hard water contains Ca2+ or Mg2+ ions in solution. These ions react with soap to form scum.
This is because soap is sodium stearate, which reacts with calcium ions to form insoluble
calcium or magnesium stearate which is scum Only after all of the magnesium or calcium
ions have reacted with the soap can the soap begin to form a lather.…read more

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