Chemistry Unit 3 - Water

Breif set of over summary card from the cgp revision book and the aqa science text book on the topic water in the unit 3 chemistry aqa exam

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Water Cycle

The water cycle

  • Water in the rives, lakes and oceans on earth evapourates as the sum supplies it with energy 
  • The water vapour that it forms rises into the atmostphere where it cools and condenses (forms tiny water droplets rain)
  • The clouds rise further and they cool more and rain dropplets get bigger
  • Eventually the rain falls, replenishing hte water back into the lakes rivers and oceans.
  • This is called the water cycle
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What dissolves in water?

  • To some extent gases are soluble in water
  • Most ionic compounds
  • IMPOSSIBLE to dissolve many covalent substances in water

We call he amount of solute which we can dissolve in a certain amount of solvent the solubility of that substence

  • the solubility is measured in 100grams of the solvent at a particular temperature.
  • The solubility of most solid solutes increases as the temperature increases
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Saturated Solution

A saturated solution contains the maximum amount of solute that will dissolve at that temperature.

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What are the Solubilty Curves Key Points?

  • The solubility of most solid solutes increases as the temperature rises.
  • The solubility of gases decreases as the temperature rises
  • Solubility curves show how the solubility of a substence changes with temperature
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What factors effect the solubility of a gas in wat


As the temperature increases, the amount of gas which will dissolve in a certain volume of water decreases when we keep the pressure constant.

If we keep the temperature constant, the solubility of a gas increases as we increase the pressure

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Why must drinking water be checked for nitrate lev

  • nitrate gets into our water from washing off crops into lakes ect,
  • These must be checked at the nitrate ions in these can be very bad or babies and some adults. 
  • These therefore must be carefully monitored by companies who supply water to our homes. 
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Summary of Solubility Curves

The amount of a substance that will dissolve in water is shown by a solubility curve. The solubility of gases is affected by pressure and temperature. Gases are less soluble at high temperatures and more soluble a high pressures. 

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What ionic compounds dissolve in water?

  • salts of SODIUM (Na), POTASSIUM (K), AMMONIUM (NH4)
  • CHLORIDES (Cl) not silver or lead
  • SULFATES (SO4) exept for barium ad lead.
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How does water dissolve ionic compounds?

  • water molecules start to surround the ions
  • they disrupt the ionic bonding - so it gradually falls apart
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What are water molecules?

Water molecules are polar.

They have a positive hydrogen side, and a negative oxygen side.

The slightly negative side attracts the positives ions and the slightly positive side attracts the negative ions.

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Define solubility

  • The solubility of a substance is given solvent is the number of grams the solute (usually a solid) that dissolve in 100g of solvent (the liquid) at a particular temperature
  • The solubilty of (solid) solutes usually increase with temperature.
  • A saturated solution is one that cannot hold anymore solid at that temperature and you have to be able to see solid on the bottom to be certain it was saturated.
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How does a solubility curve show when a solution i

  • A solubility curve plots the mass of solute dissolved in a saturated solution at various temperatures.
  • The solubility of most solids increases as the temperature increases
  • This means cooling a saturated solution will use cause some solid to crytalise
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Example of a solubility curve question?

What mass of solid copper sulfate will crystallise when a saturated solution containing 100g of water is cooled from 100 c to 20 c? 

100 c reads 75g on graph

20 c read 20g

therefore its 75g - 20g= 55g

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Soluble Gases

''chlorine water'' is an example of a gas in water- used in pools to kill bacteria. 



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