The Water Cycle

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Water has the opportunity to dissolve substances, both when it is falling as rain, and when it is flowing across soil and rocks.

H₂O + CO₂ à H₂CO₃ (Carbonic Acid)
CaSO₄ + Aq à Ca²⁺ + SO₄^2-

Calcium Sulphate is particularly soluble.

Hardness of Water

Soft water easily forms a lather with soap. Hard water is difficult to obtain a lather with soap and forms a scum precipitate.

Calcium Carbonate and Magnesium Carbonate are present in rocks. They are insoluble, but they will dissolve in water which is weakly acidic because of dissolved Carbon Dioxide.

H₂O + CO₂ + CaCO₃ à Ca (HCO₃)₂

The Carbonate ion has acted as a base here as it has accepted a proton.

CO₃^2- + H⁺ à HCO₃^-

It is Calcium ions and Magnesium ions in solution which interfere with the soap.

Soap is an ionic compound which contains a Sodium Cation and a very large anion called a Stearate. Sodium Stearate is soluble in water. Calcium/Magnesium Stearate are in-soluble in water. This is the cause of the scum precipitate when soap is used in hard water areas.

Ca²⁺ + Stearate^- à Ca(Stearate)₂
This means that the Stearate ions cause the soap to lather. It can't operate until all the Ca²⁺ and Mg²⁺ have been removed from the solution as the scum.
Most metal ions will interfere with soap as seen above, but Ca²⁺ and Mg²⁺ are the ones commonly found in water. Soapless detergents "ignore" hardness: they can operate in water.

Benefits of Hard Water

Hard water contains dissolved compounds which are good for health. It often provides calcium to strengthen bones, and there is evidence of people who live in hard water areas as they have reduced risk of heart disease. Using hard water can increase cost as more soap is needed.
Some hard water decomposes when it is boiled.

Ca(HCO₃)₂ à CaCO₃ + H₂O + CO₂

This is the reverse of the reaction which forms the hard water in the first place.
The Calcium Carbonate solid which forms is the "Scale" in hot water pipes, and the "fur" in kettles. This reduces the efficiency of kettles so more current id required. The "scale" can constrict water flow and even block central heating systems meaning explosions are possible, needing services. Water which is hard because of dissolved CaSO₄ or MgSO₄ doesn't decompose on heating. This type of water is referred to as having "Permanent Hardness" opposed to "Temporary Hardness."

Removal of Hardness

If water is to be softened, Ca₂ and Mg₂ must be removed.
This temporary hardness can be removed by boiling

Ca(HCO₃)₂ à CaCO₃ + H₂O + CO₂

It may also be removed by adding Sodium Carbonate crystals (Washing Soda) or Sodium Carbonate solution which will remove Ca₂₊ and Mg₂₊ from solutions as their Carbonate precipitates.

Ca²⁺ + CO₃^2- à CaCO₃
Mg²⁺ + CO₃^2- à MgCO₃

Ions are no longer in the solution, the water is softened.

Ion Exchange resins

These contain long chained covalently bonded molecules which have various "Sites" which hold ions loosely. Ion exchanges which contain Sodium ions will replace Calcium and Magnesium ions, which will pass through the ion exchange. Although the water is now soft, it is not suitable for drinking as it has high concentrations of Na⁺. In some ion exchanges, all the positive ions for H⁺ and all the negative ions for OH^-. These ions combine in a neutralisation reaction so the water coming out the bottom of the resin is completely pure; the water has been de-ionised.
Distillation of hard water removes all hard compounds. These are left behind in the distilling glass.

We remember that distillation followed by condensation, dissolve in the apparatus.

Drinking water

Water used for drinking must be of high quality. It is treated by:

Filtration - To remove small solid particles (Done through beds of sand).
Chlorination - Chlorine is added in large quantities to kill bacteria, but not enough to damage humans.

Chlorine can bleach materials aswell as kill bacteria. When excess Chlorine is used in swimming pools, problems may arouse.

Dissolved Gases

Gases become less soluble with increasing temperature, and more soluble with increasing pressure.
"Fizzy Drinks" are Carbonated (CO₂) under high pressure.

When the can or bottle is opened, the pressure of gas is lowered, which means the drink holds less gas so it comes out of the solution in the form of gas. If the can or bottle is in a warm place, the gas solubility falls. More gas enters the space above the drink and the pressure increases. This means that when the can is opened, the drop in pressure is larger, and the dissolved gases come out much quicker causing a spray.

We remember that dissolved oxygen is important to aquatic life. Hot water emissions from industrial places raise the temperature so that the solubility of oxygen falls. his means there is less oxygen available, thus causing death of aquatic life.

Problems caused by fertilisers

The use of artificial fertilisers has caused waters to be contaminated by dissolved Nitrate ions (Na^3-) and from Ammonia ions (NH₄⁺). Nitrates can cause kidney problems in young children. So the levels of dissolved Nitrates are carefully monitored, as they are not removed by normal purification processes.

Stream waters containing too much nitrate will encourage the rapid growth of small plant organisms "Algae". When these die, they are decomposed by bacteria which use oxygen. When all the oxygen is used up, other water creatures will die. This process is known as eutrophication, and can be very serious.

Tests for Water

Tests for water is that it melts at 0⁰C and boils at 100⁰C. The presence of dissolved impurities, lowers the freezing point and raises the boiling point.

Some ionic solids have Hydrated forms; the contain water of crystallisation. The solid is not wet - the water molecules are trapped in the lattice. Ionic solids with no crystallisation of water is said to be anhydrous. Sometimes the hydrated form has a different colour to anhydrous form, and this can be used to test for the presence of water.

COCl₂ 6H₂O à CoCl₂ + 6H₂O
(Pink) HEAT (Blue)

COCl₂ + 6H₂O à CoCl₂ + 6H₂O
(Blue) EXCESS (Pink)

When the pink hydrated Cobalt Chloride crystals are warmed gently, the water trapped inside them comes out and the solid dissolves. If heating continues, the water evaporates, leaving behind dull powdery blue anhydrous Cobalt Chloride.

Solubility of Solid Solutes

Most ionic compounds are soluble in water.
Most covalent compounds are insoluble in water.

A Saturated solution is one which no more solute will dissolve at a particular temperature. The solution is usually in contact with the solute.
The solubility of a solute with water or any solvent is usually given in grams of solute per 100g of water (or Solvent) g/100g H₂O for a saturated solution at a particular temperature.
The solubility varies with temperature: most solids become more soluble as the temperature increase. When a hot saturated solution cools, some of the solute will separate from the solution.