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Air and Atmosphere

ICSE Class 7 · Chemistry · Chapter 7

What this chapter covers

Key definitions

Air is a mixture of several gases, mainly nitrogen and oxygen, with small amounts of argon, carbon dioxide and water vapour, along with dust and smoke particles.

Oxygen is the gas in air that supports burning and that living things need for respiration.

Nitrogen is the most plentiful gas in air, and it does not react easily with other substances.

Carbon dioxide is the gas that green plants take in from air to make their food in sunlight.

Argon is an unreactive gas filled into some electric bulbs so that the hot filament does not burn away.

Combustion is a chemical reaction in which a substance combines with oxygen and gives out heat and light.

Acidic oxides are oxides, mostly of non-metals, that dissolve in water to form acids.

Basic oxides are oxides of metals that react with acids to form a salt and water, and some of them dissolve in water to form alkalis.

Respiration is the process in which living cells use oxygen to break down food such as glucose, releasing energy along with carbon dioxide and water.

Rusting is the slow reaction of iron with oxygen and water that forms a reddish-brown, flaky substance called rust.

Acid rain is rain that has become much more acidic than normal because acidic oxides of sulphur, nitrogen and carbon in polluted air have dissolved in it.

Air pollution is the presence in air of harmful gases, smoke or fine particles in amounts that can hurt living things or damage property.

Key points

Questions answered

Give two reasons why air is called a mixture and not a compound.
The composition of air varies from place to place, and each gas in air keeps its own properties. The components can also be separated by physical methods, and air has no chemical formula.
State one use each of oxygen, nitrogen and carbon dioxide.
Oxygen is given to patients in hospitals; nitrogen is used to make fertilisers such as urea; carbon dioxide is used in fire extinguishers.
A candle is covered with an upside-down glass in a plate of water. Why does the candle go out, and why does water rise into the glass?
The candle uses up part of the oxygen inside the glass, and no fresh air can enter. When the oxygen left becomes too little to keep the flame burning, the candle goes out, although some oxygen still remains. Water rises because the oxygen used up leaves less gas inside, and because the air inside cools and contracts after the flame dies.
When magnesium burns, the product weighs more than the metal. When a candle burns, it seems to lose mass. Explain both.
Magnesium combines with oxygen from the air to form solid magnesium oxide, which stays in the crucible, so the mass increases by the mass of oxygen taken in. The candle wax forms carbon dioxide and water vapour, which are gases and escape into the air, so the candle seems lighter.
6.0 g of magnesium burns completely to give 10.0 g of magnesium oxide. What mass of oxygen combined with the magnesium?
10.0 g − 6.0 g = 4.0 g of oxygen.
Write word equations for the burning of (a) sulphur, (b) phosphorus, (c) sodium and (d) potassium in oxygen.
(a) sulphur + oxygen → sulphur dioxide; (b) phosphorus + oxygen → phosphorus pentoxide; (c) sodium + oxygen → sodium oxide; (d) potassium + oxygen → potassium oxide.
What is a catalyst? Name the catalyst used when oxygen is prepared from hydrogen peroxide.
A catalyst is a substance that changes the speed of a chemical reaction without being used up itself. Manganese dioxide is the catalyst.
Describe the test for oxygen, and give two physical properties of the gas.
A glowing splint bursts into flame in oxygen. Oxygen is colourless and odourless; it is also only slightly soluble in water.