Water

Water in different parts of the UK varies in the amount of dissolved mineral ions it contains. This determines whether it is hard or soft water. There are advantages and disadvantages to each, but the damage that can be caused to water pipes and appliances by hard water means that water may need to be softened.

Rainwater is naturally weakly acidic because it contains carbonic acid, formed by the reaction between water and carbon dioxide in the atmosphere. When the rain falls, it flows over rocks or soaks into the ground and then passes through layers of rock. Compounds from the rocks dissolve into the water.Hard water contains dissolved compounds, usually calcium or magnesium compounds. For example, limestone contains calcium carbonate, CaCO₃. Carbonic acid in rainwater reacts with this to produce soluble calcium hydrogencarbonate:

carbonic acid + calcium carbonate → calcium hydrogencarbonate

H₂CO₃(aq) + CaCO₃(s) → Ca(HCO₃)₂(aq)

The presence of calcium ions and magnesium ions in the water makes it hard. Soft water readily forms lather with soap, but it is more difficult to form lather with hard water. The dissolved calcium ions and magnesium ions in hard water react with the soap to form scum, so more soap is needed. Soapless detergents do not form scum with hard water.

The types of rocks found in different regions determines how hard or soft the water will be.

The water in some parts of the country is soft because it has low levels of dissolved calcium and magnesium compounds, while the water in other parts of the country is hard because it has higher levels of dissolved calcium and magnesium compounds.

There are varying degrees of hardness in water - from slightly hard to very hard.

• One way to measure the hardness in water is to perform a titration with soap solution.A known volume of water is put into a conical flask. Soap solution is added to it from a burette or pipette. The mixture is swirled to help it form lather. The volume of soap solution that needs to be added to form permanent lather is recorded. The harder the water, the greater the volume of soap solution needed.

• Temporary hard water can be softened by boiling it. Permanent hard water stays hard, even when it is boiled. You should be able to tell temporary hard water from permanent hard water. The table shows the results of a titration experiment to distinguish between the two types.

• Temporary hard water contains dissolved hydrogen carbonate ions, HCO_3–. When heated, these ions decompose (break down) to form carbonate ions, CO₃^2–. The carbonate ions in the boiled water react with dissolved calcium and magnesium ions to form insoluble precipitates (calcium carbonate and magnesium carbonate). Permanent hard water contains dissolved sulfate ions, SO₄^2–. These do not decompose when heated. They remain dissolved and do not react with calcium and magnesium ions - so the water stays hard even when boiled.

Advantages;

• can improve the taste of the water
• are good for the development and maintenance of bones and teeth
• can help to reduce heart disease

Disadvantages;

• More soap is needed to produce lather, which increases costs. This happens with temporary or permanent hardness.
• The scum produced is unsightly - spoiling the appearance of baths and shower screens, for example.
• Temporary hardness can reduce the efficiency of kettles and heating systems. This is because limescale (a solid containing calcium carbonate) is produced when the water is heated. It coats the heating element in kettles, and the inside of boilers and hot water pipes. This means more energy is needed to heat the water, again increasing costs. Pipes may become blocked by limescale - causing the heating system to break down.

The damaging effect that hard water can have means that it may be beneficial to soften the water. Methods for softening hard water involve the removal of calcium ions and magnesium ions from the water.

There are two methods for softening hard water:

• adding sodium carbonate to the water

• using ion exchange column

Sodium carbonate, Na₂CO₃, is also known as washing soda. It can remove temporary and permanent hardness from water. Sodium carbonate is soluble but calcium carbonate and magnesium carbonate are insoluble.

The carbonate ions from sodium carbonate react with the calcium and magnesium ions in the water to produce insoluble precipitates. For example:

calcium ions + sodium carbonate → calcium carbonate + sodium ions

Ca²⁺(aq) + Na₂CO₃(aq) → CaCO₃(s) + 2Na+(aq)

The water is softened because it no longer contains dissolved calcium ions and magnesium ions. It will form lather more easily with soap.

However, the calcium carbonate and magnesium carbonate precipitates to form limescale. As well as being unsightly on your taps, it can also clog up pipes in heating systems (causing them to break down). This makes treatment with sodium carbonate suitable for softening water only in certain circumstances - such as softening water for hand washing clothes.

Commercial water softeners often use ion exchange resins. These substances are usually made into beads, which are packed into cylinders called ion exchange columns. These can be built into machines, such as dishwashers, or plumbed into water systems to continuously soften the water.

The resin beads have sodium ions attached to them. As the hard water passes through the column, the calcium and magnesium ions swap places with the sodium ions.

The calcium and magnesium ions are left attached to the beads, while the water leaving the column contains more sodium ions. The hard water is softened because it no longer contains calcium or magnesium ions. Some ion exchange resins use hydrogen ions instead of sodium ions.

Once the resin beads in dishwashers become saturated with calcium and magnesium ions, they must be regenerated by adding sodium chloride (common salt). The sodium ions it contains replace the calcium and magnesium ions on the beads. Sodium chloride is cheap and widely available, making this a convenient and cost-effective system.

When we turn our taps on, we naturally assume the water is safe to drink. This is because it is treated before it is supplied to our homes. In some parts of the country, fluoride is added to the water supply but this is controversial. Water can also be filtered at home – to help improve its taste and quality. In parts of the world where water is more scarce, sea water is distilled to provide drinking water.

Supplying safe water;

Water is essential for life - it acts as the solvent in our cells for chemical reactions to take place.

Water covers about two-thirds of our planet, but the vast majority of it cannot be drunk directly. This is because humans need drinking water with low levels of dissolved salts and microbes. To produce water of a sufficient quality, we must:

• choose an appropriate source of water

• filter the water

• chlorinate the water

sources of water;

Sources of water for drinking should be reliable, and they should also be fresh and free of toxic chemicals (such as heavy metals). In the UK, water resources include lakes, rivers, aquifers and reservoirs.An aquifer is an underground layer of permeable rock, gravel or sand that is soaked with water, while a reservoir is usually an artificial lake, made by building a dam to accumulate and save river water in the valley behind. In countries where water is scarce, boreholes are drilled to reach water underground.

filtering the water;

Solids in the water, such as leaves and soil, must be removed. The water is sprayed onto specially-prepared layers of sand and gravel called filter beds.Different-sized insoluble solids are removed as the water trickles through the filter beds. These are cleaned every so often by pumping clean water backwards through the filter.The water is then passed into a sedimentation tank. Aluminium sulfate is added to clump tiny particles together to make larger particles, which settle out more easily. The water is then passed through a fine filter, such as carbon granules, to remove very small particles.

Chlorinating the water

Chlorine is added to drinking water to sterilise it. The chlorine kills microbes - including microbes that cause potentially-fatal diseases such as typhoid, cholera and dysentery.

Results from scientific research indicate that fluoridated water can improve dental health by reducing tooth decay. Many areas of the country naturally have low fluoride levels present in the water supply. However, some local health authorities have made water companies add additional fluoride to the water.

Some people argue that extra fluoride should not be added to water, even if it does improve dental health. They claim that fluoridation:

• has been linked to tooth mottling (staining), bone disease and pain

• forces people to consume fluoride when they drink tap water - taking away their personal choice (making it unethical)

Water treatment in the UK means that the water from your tap is safe to drink. However, the water is not pure because it contains dissolved mineral ions and chlorine.Some people prefer to filter their water rather than use it straight from the tap. Filtering removes impurities and this can improve the taste and quality of the water. Filtering also helps to soften the water.Commercially-available systems use cartridges containing water filters. These may be fitted in jugs or kettles, or plumbed in to the home water supply pipework.

The filter cartridges can contain:

• silver to kill bacteria

• carbon (‘activated charcoal’) to absorb impurities, eg chlorine

• ion exchange resins to soften the water, and remove heavy metal ions (such as lead ions)

Silver nanoparticles have an antibacterial effect. Their presence in the filter prevents the growth of bacteria within the filter if water is left inside it for long periods. Silver nanoparticles also help break down harmful pesticides which might be in the water.

Seawater is a very abundant source of water, but its high salt content make seawater unsuitable as drinking water. However, pure water can be produced from seawater by distillation.During distillation, the seawater is boiled. The water vapour is then cooled and condensed to form pure water - leaving the salt behind.

The disadvantages of producing drinking water this way include:

• it is expensive because large amounts of energy are needed to heat the seawater

• it increases the use of fossil fuels - which are non-renewable resources

• carbon dioxide emissions from burning fossil fuels contribute to global warming

Distillation is common in some Middle Eastern countries that have little rainfall, but are wealthy due to their oil reserves.

Testing water purity

The purity of water can be tested by:

• measuring its boiling point

• evaporating it (to dryness) on an evaporating dish

Pure water boils at 100°C, but its boiling point increases as the concentration of dissolved salts increases.

Pure water will leave no solids behind when it is evaporated, whereas impure water will leave solids behind on the evaporating dish.