Chemistry of the environment

Testing for water

Two chemical tests show that water is present:

• Anhydrous 无水 cobalt(II) chloride turns from blue to pink when water is added.
• Anhydrous copper(II) sulfate turns from white to blue when water is added.

These tests only show that water is there. To show that water is pure, you test its melting point 熔点 and boiling point 沸点: pure water melts at exactly $0\,{}^{\circ}\text{C}$ and boils at exactly $100\,{}^{\circ}\text{C}$. Any dissolved substance changes these values.

This is why distilled water 蒸馏水 is used in chemistry instead of tap water — it has far fewer chemical impurities 杂质.

What is in natural water

Water from rivers, lakes and the sea is not pure. It may contain dissolved oxygen 氧气, metal compounds, plastics, sewage 污水, harmful microbes 微生物, and nitrates 硝酸盐 and phosphates 磷酸盐 (which come from fertilisers and detergents).

Some of these are helpful:

• dissolved oxygen lets aquatic life 水生生物 (fish and plants) breathe;
• some metal compounds give essential minerals 矿物质.

Others are harmful:

• some metal compounds are toxic 有毒 (poisonous);
• some plastics harm aquatic life;
• sewage carries microbes that cause disease;
• nitrates and phosphates cause deoxygenation 缺氧 (loss of oxygen) in the water, which harms aquatic life.

Treating drinking water

To make water safe to drink, the water supply is treated in steps:

• sedimentation 沉降 and filtration 过滤 remove solid bits;
• passing it through carbon 碳 removes bad tastes and smells;
• chlorination 氯消毒 (adding chlorine) kills harmful microbes.

Fertilisers 肥料 are added to soil to help plants grow. Ammonium salts and nitrates are common fertilisers.

NPK fertilisers contain the three elements plants need most: nitrogen 氮气, phosphorus 磷 and potassium 钾.

Clean, dry air is approximately:

• 78% nitrogen ($\text{N}_2$)
• 21% oxygen ($\text{O}_2$)
• the rest is a mixture of noble gases 稀有气体 and carbon dioxide ($\text{CO}_2$).

Air pollutants and their sources

Effects of these pollutants

• Carbon dioxide 二氧化碳 and methane are greenhouse gases 温室气体: more of them causes global warming 全球变暖, which leads to climate change 气候变化.
• Carbon monoxide is a toxic gas.
• Particulates increase the risk of respiratory 呼吸 (breathing) problems and cancer 癌症.
• Oxides of nitrogen cause acid rain 酸雨, photochemical smog 光化学烟雾 and breathing problems.
• Sulfur dioxide causes acid rain.

Worked example. A power station burns coal containing sulfur, and a lake downwind slowly turns acidic. Name the pollutant, explain the link, and give one way to stop it. Burning a fossil fuel that contains sulfur releases sulfur dioxide, which causes acid rain, so the rain falling on the lake is acidic. To stop it at the source, use flue gas desulfurisation, where calcium oxide removes the sulfur dioxide from the waste gases before they leave the chimney, or switch to a low-sulfur fuel. Match the fix to the pollutant: a catalytic converter treats a car's oxides of nitrogen and would do nothing about a coal-fired power station's sulfur dioxide.

Reducing these problems

To slow climate change: plant trees, farm fewer animals, burn fewer fossil fuels, and use more hydrogen and renewable energy 可再生能源 such as wind and solar power.

To reduce acid rain: fit catalytic converters 催化转化器 in cars, use low-sulfur fuels, and use flue gas desulfurisation 烟气脱硫 with calcium oxide 氧化钙 to remove sulfur dioxide from waste gases.

How greenhouse gases warm the Earth

Greenhouse gases such as carbon dioxide and methane let sunlight through, but they absorb the thermal energy 热能 given off by the warm Earth, and send some of it back down. This reduces the thermal energy lost to space, so the Earth gets warmer.

In a car engine, the high temperature makes nitrogen and oxygen from the air react to form oxides of nitrogen. A catalytic converter removes them, for example:

Photosynthesis

Photosynthesis 光合作用 is the opposite of combustion — it removes carbon dioxide from the air. Plants use light energy and chlorophyll 叶绿素 to turn carbon dioxide and water into glucose 葡萄糖 and oxygen:

• Two tests show water is present (cobalt(II) chloride blue → pink; copper(II) sulfate white → blue), but only fixed melting and boiling points (0 °C and 100 °C) show it is pure.
• Clean, dry air is about 78% nitrogen, 21% oxygen and 1% other gases (mostly noble gases and carbon dioxide).
• Match each pollutant to its source and effect: carbon monoxide (incomplete combustion; toxic); sulfur dioxide and oxides of nitrogen (acid rain); carbon dioxide and methane (greenhouse gases → global warming).
• Greenhouse gases let sunlight through but absorb the heat the warm Earth gives off and send some back down, so the Earth warms up.