The properties of air; water and its properties

Air is a mixture of gases. Nitrogen does not burn and cannot be breathed, but it is part of proteins; some bacteria take it from the air and give it to the soil. That is where its Uzbek name comes from: “azot” means “lifeless”. Oxygen is needed for breathing and burning: without oxygen a fire goes out and a person cannot live more than a few minutes. Carbon dioxide is needed by plants: leaves make sugar from it. Properties of air: it fills every space (pour water on dry soil and bubbles appear — that is the air in the soil); it is colourless, transparent and odourless (a smell means vapour of another substance mixed in); it has mass; warm air expands, becomes lighter and rises while cold air sinks; it is elastic — compressed air returns to its form when released, which is why balls and tyres are pumped up. In fog, water vapour in the air turns into tiny droplets and we can see them.