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Water

In this GCSE Chemistry quiz we look at some of the factors which affect how well substances are dissolved in water. We also examine ions dissolved in impure water, hard water and limescale.

Water, Earth's most abundant natural resource, is vital for all life. The first life on Earth began in the water and remained there for millions of years until the Earth had an ozone layer and plants and animals had evolved sufficiently to colonise the land. Elsewhere in the Solar System, space probes have been looking for signs of water. On Mars, there are sedimentary rocks that seem to have been formed under water along with valleys that seem to have been carved by running water. Enceladus, one of the moons of the planet Saturn, has water geysers at the south pole and further research has confirmed that there is a liquid water ocean beneath an icy outer crust. Scientists also believe that an ocean, also beneath an icy crust, exists on one of the moons of Jupiter - Europa. It is possible that another of Jupiter's moons has an ocean underneath its icy outer crust.

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Water itself is a covalently bonded molecule made from one oxygen atom sharing electrons with two hydrogen atoms. Pure water therefore does not conduct electricity to any great degree. When ionically bonded chemicals are dissolved in water, there is a ready supply of ions that can transfer the electrical current throughout the solution. That is why water and electricity in the home are so dangerous - tap water is free from microbes but is far from pure. Even after using a water filter, there are still sufficient ions in the water to allow it to conduct electricity.

Our water supplies come from the ground, either from boreholes or reservoirs. When they are in contact with soil and rock, any soluble compound will dissolve into the water. Soluble calcium and magnesium compounds form hard water. These can be deposited on the inside of pipes, kettles, boilers, irons and on taps as limescale (sometimes referred to more simply as 'scale'). This clogs up pipes and makes heating appliances less efficient. Limescale can completely wreck dishwashers and washing machines as it blocks up their internal pipework. Soap does not work as well in hard water as it does in soft water because it forms scum. The ions that cause hard water can be removed in several ways. However, hard water does perhaps have a couple of advantages: firstly, some people say that it tastes better whilst medical research has found that the minerals in hard water could be beneficial to your heart.

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  1. What is a saturated solution?
    Solute = the substance that has been dissolved
  2. What effect does increasing the temperature have on the amount of solute that can be dissolved?
    As a saturated solution cools down, the solute crystallises and can be seen as a precipitate
  3. The amount of substance that will dissolve in a solvent is affected by the temperature of the solvent. The effect of the change in temperature can be seen on a graph called a...
    Solubility curves can be useful to predict how much solute will form when a hot solution is cooled down
  4. There is another factor that affects the solubility of gases apart from temperature. What is this other factor?
    Scuba divers sometimes suffer from the bends, medically called decompression sickness. This is a painful and potentially lethal condition. At depth, the increased water pressure causes more gases to be dissolved in the blood. If the diver comes back to the surface too fast, instead of these gases leaving the bloodstream in the lungs, they form bubbles in the blood itself
  5. As the temperature increases, the solubility of gases...
    This is the opposite way round to solubility of solids. Although tropical seas have a greater variety of life than polar seas, less oxygen and other gases are dissolved in it
  6. Pick the pair of compounds that are found in 'hard' water.
    If it is the hydrogen carbonate of these two ions that is dissolved in the water, heating it will cause the insoluble carbonate to form, removing the hardness. Permanent hardness occurs when the chlorides or sulfates are dissolved - they remain dissolved even when heated
  7. Pick the equation that shows the effect of heat on the dissolved calcium compounds in hard water.
    Most metal carbonates are insoluble in water
  8. What unwanted material can hard water produce in heating appliances, e.g. kettles and irons?
    This scale builds up on heating elements making them less efficient because the scale is a poor conductor of heat
  9. Soft water does not contain the dissolved substances that hard water does. It is possible to soften hard water by removing the offending ions. Pick the two methods for softening water.
    The washing soda precipitates out calcium and magnesium ions as insoluble carbonates. The ion-exchange column 'exchanges' calcium and magnesium ions for sodium ions which do not cause hardness
  10. There is a possible health advantage to drinking hard water. What is it?
    This has not been proven beyond doubt but there are a number or large scale studies which back up this claim

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