Everything from the 4 lessons of this topic on a few pages: the key idea of each lesson, the diagram to remember, the points to learn and the traps examiners set. Read it, cover it, test yourself.
Water is detected with anhydrous copper(II) sulfate, which turns from white to blue, or anhydrous cobalt(II) chloride, which turns from blue to pink. These tests show water is present, not that it is pure: pure water boils at exactly 100 °C and melts at exactly 0 °C. Distilled water is used in practical chemistry because it contains no dissolved substances that could interfere with results. Natural water contains some beneficial substances (dissolved oxygen for aquatic life, some metal compounds as minerals) and some harmful ones (toxic metal compounds, plastics, sewage, harmful microbes, and nitrates and phosphates from fertilisers that lead to deoxygenation of water). Domestic water is treated by sedimentation and filtration to remove solids, carbon to remove tastes and odours, and chlorination to kill microbes.
Filtration removes insoluble solids only: dissolved salts, nitrates and microbes small enough to pass through are not removed by it. Chlorine kills microbes; it does not remove solids, tastes or dissolved substances. And a positive copper(II) sulfate test shows that water is present, never that it is pure.
Ammonium salts (such as ammonium nitrate and ammonium sulfate) and nitrates (such as potassium nitrate) are used as fertilisers to increase crop yields. NPK fertilisers provide the three elements plants need most: nitrogen (N), phosphorus (P) and potassium (K). Ammonium fertiliser salts are made by neutralising ammonia with an acid, for example NH3 + HNO3 → NH4NO3. Extended: plants need nitrogen to make proteins for growth (healthy leaves), phosphorus for root growth, and potassium for flowers and fruit and resistance to disease.
Plants take up ions such as nitrate and ammonium dissolved in soil water, not nitrogen gas from the air, which is why the 78% nitrogen in air does not feed most crops. Fertilisers must be soluble. And too much fertiliser washes into rivers, where nitrates and phosphates cause deoxygenation of the water.
Clean, dry air is about 78% nitrogen, 21% oxygen and the remaining 1% is a mixture of noble gases (mainly argon) and carbon dioxide. Pollutants each have a source and an effect: carbon monoxide (incomplete combustion) is toxic; particulates (incomplete combustion) cause respiratory problems and cancer; oxides of nitrogen (car engines) cause acid rain, photochemical smog and respiratory problems; sulfur dioxide (burning fuels containing sulfur) causes acid rain; carbon dioxide and methane cause global warming. Acid rain is reduced by catalytic converters in vehicles, low-sulfur fuels and flue gas desulfurisation with calcium oxide. Extended: oxides of nitrogen form when nitrogen and oxygen from the air react at the high temperature inside an engine, and a catalytic converter removes them: 2NO + 2CO → N2 + 2CO2.
A catalytic converter does not remove carbon dioxide: it makes more of it, from carbon monoxide. Carbon dioxide causes global warming, not acid rain; acid rain comes from sulfur dioxide and oxides of nitrogen. And the nitrogen in NOx comes from the air, not from the fuel.
Carbon dioxide and methane are greenhouse gases: higher levels cause increased global warming, which leads to climate change. Extended: the Earth's surface absorbs energy from the Sun and emits thermal energy as infrared radiation; greenhouse gases absorb this infrared and re-emit it in all directions, including back towards the Earth, so less thermal energy is lost to space. Photosynthesis removes carbon dioxide: carbon dioxide + water → glucose + oxygen, using light energy in the presence of chlorophyll (Extended: 6CO2 + 6H2O → C6H12O6 + 6O2). Strategies to reduce climate change include planting trees, reducing livestock farming, decreasing the use of fossil fuels and increasing the use of hydrogen and renewable energy such as wind and solar.
The greenhouse effect is not the hole in the ozone layer, and acid rain does not cause global warming. Do not write that greenhouse gases "reflect" heat back or "trap the Sun's rays": they absorb and re-emit infrared given off by the warm Earth. And the greenhouse effect itself is natural and keeps the Earth habitable; the problem is the increase in greenhouse gases.