Water and solubility.
Water is cycled around the Earth and the atmoshere (the water cycle):
- Water evaporates from lakes, seas, rivers, etc.
- This produces water vapour which mixes with the air.
- The water vapour condenses into droplets which creates clouds and mist.
- The larger droplets fall as rain, this is called precipitation.
Ionic compounds, as well as many gases, are soluble in water.
Aquatic life needs dissolved O2 to survive.
Covalent substances cannot be dissolved in water, as there is little attraction between their molecules and water molecules.
Water that has been in contact with the ground many contain dissolved solids.
The amount of a solute that can be dissolved in a certain amount of solvent is called the solubility of a substance.
Solubility curves show how solubility changes with temperature.
They can also show how much solute will separate out of a saturated solution for a given change in temperature.
Solubility of a solid:
- Increases as temperature increases
Solubility of a gas:
- Two main factors are temperature and pressure
- Decreases as temperature increases, if a constant pressure is kept constant
- Increases as pressure increases, if temperature is kept constant
Water that lathers with soap easily is called soft water.
Hard water does not lather with soap easily and it produces an insoluble solid, scum, which floats on the water.
Scum is formed when soap reacts with dissolved salts, magnesium and calcium.
sodium stearate (soap) + Ca2+ and Mg2+ ions => calcium/magnesium stearate precipitate (scum) + Na+ ions (soluble in water)
When hard water is heated it produces scale which makes things like kettles heat up less efficiently.
Ca2+ (hardness) + 2HCO3- ==(heat)==> CaCO3 (limescale) + H2O + CO2
Hard water can be softened by removing the ions that react with soap.
There are two main ways of doing this:
1) by adding sodium carbonate (washing soda) which precipitates out the the insoluble ions.
Ca2+(aq)(hardness) + CO32+(from sodium carbonate) => CaCO3
2) by using an ion exchange column, they contain sodium ions which are exchanged with the magesium or calcium ions when the hard water pases through them. The columns are filled with a resin which needs to be kept topped up with salt (sodium chloride). The sodium ions do not react with soap.
Water that is safe to drink has low levels of harmful substances but it is not pure.
Filtration and sedimentation removes solids and then chlorine is added to kill bacteria. Boiling water can also kill microorganisms.
Water filters contain charcoal and an ion-exchange resin with silver or another substance to prevent bacteria growth.
Pure water (distilled water) is made by distillation or de-ionisation. De-ionisation uses an ion-exchange column to remove all ions execpt H+ and OH-.
Simple steps of water treatment:
- Passes through a screen to catch twigs and leaves and then along to a settlement tank where sand and soil settles out.
- Aluminium sulfate and lime are added to water so dirt clumps together and then water is passed through another filter of sand to properly clean the water.
- Finally chlorine is added to kill bacteria.
Water and solubility. continued.
Solubility is measured in g of solute per 100g of solvent at a given temperature.
(Solute + solvent = solution.)
A saturated solution contains the maximum amount of solute that will dissolve at the temperature of the solution.
If a hot saturated solution is cooled some of the solute will crystallise.
- Thermal pollution, when rivers are heated so the levels of dissolved O2 decrease causing fish to die.
- Nitrate ions from fertilisers dissolving in water which has touched the ground.
- Acid rain, pollution in the air from factories mixing with water vapour.
- Carbonated water, that is used for fizzy drinks, contains CO2 which has been dissolved at high pressures before bottling. The pressure is released when the bottle is opened and the gas comes out, providing the 'fizz!'.