Chemistry of the environment

Testing for water and testing its purity

Two substances give a colour change with water, and both must be anhydrous - that means they contain no water of their own, so any colour change must come from the liquid you add. Add the liquid to anhydrous copper(II) sulfate and a white powder turns blue. Add it to anhydrous cobalt(II) chloride and blue paper or powder turns pink.

Learn the direction of each change, because reversing it is the most common lost mark in this topic. Water turns white copper(II) sulfate blue and blue cobalt(II) chloride pink; heating the blue hydrated copper(II) sulfate drives the water off and turns it white again.

These tests show only that water is present. They say nothing about purity, because a dilute solution of salt or sugar will turn both reagents just as pure water does. To test the purity of a water sample you measure its melting point and its boiling point. Pure water melts at 0 °C and boils at 100 °C at standard pressure. Dissolved impurities lower the melting point and raise the boiling point, and they spread the change over a range of temperature instead of giving one sharp value.

This is why distilled water is used in practical chemistry rather than tap water. Tap water carries dissolved metal compounds and other chemical impurities that could react with your reagents or shift the end point of a titration, while distilled water contains far fewer chemical impurities.

• Anhydrous copper(II) sulfate: white -> blue when water is added.
• Anhydrous cobalt(II) chloride: blue -> pink when water is added.
• A positive result shows water is present; it does not show the water is pure.
• Pure water melts at 0 °C and boils at 100 °C at standard pressure.
• Impurities lower the melting point, raise the boiling point and widen the range.

What natural water contains: the beneficial and the harmful

Water from a natural source is never pure. It may contain dissolved oxygen, metal compounds, plastics, sewage, harmful microbes, nitrates that have washed out of fertilisers, and phosphates from fertilisers and detergents. Whether each of these matters, and whether it helps or harms, depends on what it is and how much is present.

Some of these substances are beneficial. Dissolved oxygen is essential for aquatic life: fish and other water organisms take it from the water to respire, exactly as you take oxygen from the air. Some metal compounds provide the essential minerals that living things need, for example calcium ions for bones and shells and magnesium ions for chlorophyll.

Others are potentially harmful. Some metal compounds, such as those of lead and mercury, are toxic. Plastics are not broken down quickly, so they harm aquatic life when animals swallow them or become tangled in them. Sewage carries harmful microbes that cause diseases such as cholera and typhoid.

Nitrates and phosphates are a particular problem because they are plant nutrients. A river or lake that receives them grows a dense bloom of algae. When the algae die, bacteria decompose them and use up the dissolved oxygen in the water. This sequence is called eutrophication, and the resulting deoxygenation kills fish and other aquatic life.

Treating water for a town supply

Water for a town supply must be made potable, which means safe to drink. The raw water is taken from a river, a lake or an underground source and then put through a sequence of physical and chemical stages, each with one job to do.

Sedimentation means letting the water stand in a large tank so that suspended solids sink to the bottom as sludge. Filtration then passes the water through beds of sand and gravel, which trap the smaller particles that did not settle. Together these two stages remove the solids.

Carbon, usually activated carbon, is used next to remove tastes and odours. The carbon has an enormous surface area, and the molecules responsible for an unpleasant taste or smell are held on that surface. Finally the water is chlorinated: chlorine is added because it kills the harmful microbes that could cause disease. Chlorine is added towards the end of the process so that the water stays safe as it travels through the pipes to homes.

Notice that water treatment does not remove every dissolved substance. The mineral ions that give tap water its taste survive the process, which is exactly why distilled water is preferred for practical chemistry.

• Sedimentation: the water is left to stand so that solids settle out.
• Filtration through sand and gravel: removes the remaining solid particles.
• Carbon: removes tastes and odours.
• Chlorination: kills the harmful microbes that cause disease.

Fertilisers: ammonium salts, nitrates and NPK

Plants take mineral ions from the soil, and every harvest removes some of them. Fertilisers replace them. Ammonium salts and nitrates are used as fertilisers because they supply nitrogen in a form plants can absorb; ammonium nitrate, NH4NO3, and ammonium sulfate, (NH4)2SO4, are two common examples.

An NPK fertiliser supplies three elements together: Nitrogen, Phosphorus and K (potassium). Each has its own job. Nitrogen is needed to make proteins and chlorophyll, so it drives strong leaf growth and a healthy green colour. Phosphorus is needed for DNA, for cell membranes and for root growth. Potassium helps control enzyme reactions and the movement of water in the plant, so it keeps the plant healthy and improves its resistance to disease.

The problem is that these compounds are soluble. Rain washes them out of the soil into streams and rivers, which both wastes the fertiliser and puts the nutrients into the water. There they trigger the sequence described above: rapid growth of algae, then decomposition, then deoxygenation and the death of aquatic life.

Farmers reduce the damage by applying only the amount the crop needs and by timing the application so that heavy rain is unlikely to wash it away.

Clean, dry air and the pollutants in it

Clean, dry air is a mixture, and its composition is worth knowing exactly: approximately 78% nitrogen, N2, 21% oxygen, O2, and the remainder - about 1% - a mixture of the noble gases and carbon dioxide, CO2. Nitrogen and oxygen are elements, and almost all of the noble gas fraction is argon.

Polluted air contains extra substances produced by human activity. For each pollutant you need its source and its adverse effect. Carbon dioxide comes from the complete combustion of carbon-containing fuels and contributes to global warming. Carbon monoxide and particulates both come from incomplete combustion; carbon monoxide is toxic, and particulates increase the risk of respiratory problems and cancer.

Methane comes from the decomposition of vegetation and from waste gases produced by digestion in animals, and it is also a greenhouse gas. Oxides of nitrogen form in car engines and cause acid rain, photochemical smog and respiratory problems. Sulfur dioxide comes from the combustion of fossil fuels that contain sulfur compounds and causes acid rain.

Acid rain forms when sulfur dioxide and oxides of nitrogen dissolve in rain water and make it acidic. It damages buildings and statues made of limestone or marble, acidifies lakes so that fish die, and leaches nutrients out of soils, so it harms plants as well as stonework.

The greenhouse effect: how carbon dioxide and methane warm the planet

Energy reaches the Earth from the Sun as radiation of short wavelength. The surface absorbs some of it, warms up, and then emits thermal energy back towards space. This outgoing radiation has a longer wavelength than the incoming sunlight.

Greenhouse gases in the atmosphere - mainly carbon dioxide and methane - absorb this outgoing thermal energy. They then re-emit it in all directions, including back down towards the surface. The result is that less thermal energy is lost to space, so the atmosphere and the surface stay warmer than they otherwise would. That is the greenhouse effect, and without it the Earth would be far too cold to support life.

The problem is not the greenhouse effect itself but the increase in greenhouse gases. Burning fossil fuels releases carbon dioxide, while livestock farming, decaying vegetation and waste tips release methane. More greenhouse gas means more absorption and re-emission, so less energy escapes and the average temperature of the Earth rises. This is global warming, and it drives climate change: melting ice, rising sea levels, more extreme weather and shifting growing seasons.

To reduce the effect you have to cut the emissions at source. Planting trees removes carbon dioxide from the air by photosynthesis and locks the carbon in wood. Reducing livestock farming lowers methane. Decreasing the use of fossil fuels and increasing the use of hydrogen and renewable energy such as wind and solar stop the carbon dioxide being produced in the first place, and burning hydrogen produces only water.

• Plant trees: they remove carbon dioxide from the air by photosynthesis.
• Reduce livestock farming: less methane from digestion in animals.
• Decrease the use of fossil fuels: less carbon dioxide released.
• Increase the use of hydrogen and renewable energy such as wind and solar: the fuel produces no carbon dioxide.

Photosynthesis, the carbon cycle and cleaning up exhaust gases

Photosynthesis is the reaction that takes carbon dioxide out of the air. In the presence of chlorophyll and using energy from light, a plant combines carbon dioxide with water to make glucose and release oxygen: carbon dioxide + water -> glucose + oxygen. The balanced symbol equation is 6CO2 + 6H2O -> C6H12O6 + 6O2. This is why planting trees is a strategy against climate change - the carbon that was in the air ends up stored in the wood.

Oxides of nitrogen form inside a car engine. At the high temperature of combustion, nitrogen and oxygen from the air react directly: N2 + O2 -> 2NO. The nitrogen monoxide can then be oxidised further to nitrogen dioxide, NO2. These gases cause acid rain, photochemical smog and respiratory problems, and no change to the fuel can prevent them, because the nitrogen comes from the air itself.

A catalytic converter in the exhaust system solves the problem. The exhaust gases pass over a catalyst of platinum and rhodium, and carbon monoxide and nitrogen monoxide react together in one step: 2CO + 2NO -> 2CO2 + N2. The carbon monoxide is oxidised to carbon dioxide and the nitrogen monoxide is reduced to nitrogen, so two harmful gases are converted into two that are far less harmful.

Sulfur dioxide has to be tackled differently, because the sulfur comes from the fuel rather than the air. One strategy is to use low-sulfur fuels, so that less sulfur dioxide is formed. The other is flue gas desulfurisation, in which the waste gases from a power station are passed over calcium oxide, which removes the sulfur dioxide: CaO + SO2 -> CaSO3. The calcium sulfite formed is a solid and is collected, so the sulfur dioxide does not reach the atmosphere.