The Water cycle, Solubility of Compounds and Gases

Water is essential for the existance of life, a important raw material and an abundant substance. It has many uses:

• A solvent where substances are dissolved in.
• A coolant for the removal of heat from a system.
• In industrial processes e.g the manufacture of sulfuric acid
• Water molecules are polar because they have the postive hydrogen side and the negative oxygen side.

The following compounds dissolve in water:

• Salts of sodium(Na), potassium(K) and ammonium(NH4)
• Nitrates (NO3)
• Chlorides (Cl) except for silver and lead
• Sulphates (SO4) except for barium and lead

The sun provides the energy which drives the water cycle.

• Heat energy from the sun causes water in rivers,lakes and oceans to rise via evaporation
• As the water vapour rises higher in the atmosphere it cools and condensation occurs forming droplets of water which collect together and form clouds.
• As the clouds rise higher the temperature drops further and rain is produced when the droplets are large enough. (snow is produced when the clouds rise further and drop in temperature further)
• The rain then falls onto the land (precipitation) in which the following processes occur...

• Surface run off occurs when the rain that has fallen onto the land drains into rivers and flows into the sea.
• Interception is when trees absorb amounts of water to be used for the trees growth and during photosynthesis.
• Infiltration is the movement of water draining under the first layer of permeable rock.
• Through flow is the movement of water under the ground which eventually joins surface run off or runs into rivers or oceans.
• Percolation is the filtering of water through porous rocks (rocks with small holes which can allow water to filter through)
• Groundwater flow is the movement of water under porous rocks which is even deeper than throughflow.

Another process that occurs in the water cycle is transpiration and this is the loss of water vapour through the stomata of leaves on plants.

Cleaning water

• In order to process water of a good quality that is safe to drink, the water is passed through a filter bed in order to remove any solid particles.
• Chlorine Gas is added afterwards to kill any harmful micro-organisms.
• The levels of dissolved salts are kept sufficently low as by removing more dissolved substances from tap water the quality can be improved.
• This is done by passing the water through a filter containing carbon, silver, iron and exchange resins.
• Any water can be distilled to produce pure water which has NO dissolved substances in it.
• The process of distillation invoves boiling water to produce steam and condensing the steam by cooling it to produce pure liquid water.
• This process is expensive and uses large amounts of energy.

• Water molecules are POLAR as as they have a positive hydrogen side and a negative oxygen side.
• The amount of solute which can dissolve in a certain amount of SOLVENT is the solubility of the substance.
• Solubility is usually measured in grams per solute per 100 grams of solvent at a certain temperature.
• Most gases are soluble
• The solubility of most solid solutes increases as the temperature increases.
• Solubility curves show when a solution is saturated
• A saturated solution is a solution which is unable to dissolve any more solute so the solute will sit as crystals on the bottom of the container
• Solubility curves on a solubility graph show the effect on temperature on a solute or different solutes dissolving in a solvent e.g sodium nitrate into water.

• Hard water is created by the dissolving action of water.
• Water's chemistry is affected by the geology of the area through which the water passes.
• Temporary hard water is created when rainwater reacts with limestone.
• Rainwater is naturally acidic due to dissolved carbon dioxide.

• H₂O (l) [In rain] + CO₂ (g) [In air] ----> H₂CO₃ (aq) [Carbonic Acid]

This carbonic acid flows over chalk / limestone - reacts with calcium to form Calcium Hydrogen-carbonate. This is now soluble in water and can cause hard water

• CaCO₃(s) + H₂CO₃(aq) ---->Ca(HCO₃)₂(aq) [Calcium Hydrogen-carbonate]

Methods of removing temporary hard water are.....

• This can be removed by boiling because CO2 is driven off as the water heats up. Limescale is deposited (CaCO3)
• This reaction can occur at the edge of rainfalls and also in limestone cavens where stalacites and stalagmites.

Permanent Hard water

• This is created by dissolving other ions
• Hard water can be created by any of the following:

Calcium sulphate (CaSO4) Calcium chloride (CaCl2)

Magnesium sulphate (MgSO4) Magnesium chloride (MgCl2)

• Permanent hard water is removed by removing the calcium ions (Ca2+) and magnesium ions (Mg2+)

• Distillation removes all types of hardness as the Mg and Ca ions are removed.
• Adding washing soda- The sodium removes the Mg+ and replaces it with sodium in a displacement reaction as sodium is more reactive.

Ca2+(aq) + Na2CO3(aq) -> CaCO3(s) + 2Na+(aq)

Mg2+(aq) + Na2CO3(aq) -> MgCO3(s) + 2Na+(aq)

• Water can also be softened by using a ion exchange column- these columns contain sodium ion which are exchanges with the Mg and Ca ions in the hard water as they pass through the column. This is how a domestic water softening system works e.g dishwasher.

Advantages:

• The dissolved substances give it a pleasant taste.
• The calcium compounds in it are good for bones and teeth
• There are fewer heart attacks in areas with hard water.
• A coating of CaCo3 inside pipes and boilers helps prevent corrosion.

Disadvantages:

• When the coating develops a scale it makes pipes and boilers less efficient and has to be removed to prevent blockages.
• Hard water uses more soap than soft water does.
• It leaves a scum which is tougher on laundry

Scum is a precipitate that is formed when soap reacts with calcium or magnesium ions.

Advantages:

• It makes lather easier and gets clothes cleaner.
• Laundry uses less soap aand can be done at lower temperatures
• Less scum in your bathtub
• It leaves no scale on pipes or boilers

Disadvantages:

• Soft water contains more sodium ions than hard water. Sodium is linked to heart disease.
• Soft water dissolves in metals such as cadium and lead; lead is poisionous and cadium can lead to hypertension but it also dissolves in copper which the body needs

Scale is formed when heat decomposes dissolved calcium or magnesium compounds. (CaCo3 or MgCo3)