Definitions (Gases)

GASES

Carbon dioxide (CO₂): A gas made of carbon and oxygen that has no colour or smell and is roughly one and a half times as dense as air; burning fuels, breathing and rotting organic matter all release it.

Laboratory preparation of carbon dioxide: In the lab, marble chips or limestone (calcium carbonate) are placed in a Woulfe's bottle and dilute hydrochloric acid is poured over them; the fizzing releases CO₂ (CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂).

Collection of carbon dioxide: Because CO₂ is denser than air, the gas jar is kept upright so the gas sinks to the bottom and pushes the air out of the top (upward displacement of air).

Lime water test: Bubbling CO₂ through clear lime water makes it look milky, because insoluble calcium carbonate forms; if the gas keeps bubbling for a long time, the liquid turns clear again as soluble calcium bicarbonate forms.

Dry ice: Carbon dioxide in its frozen, solid state, made by compressing and cooling the gas to below about -78°C; it keeps things very cold and is used to store and transport food.

Carbogen: A medical gas mixture containing roughly 95 percent oxygen mixed with 5 percent carbon dioxide; doctors give it to patients, for example those with pneumonia, to make breathing easier.

Fire extinguisher: A device that produces carbon dioxide gas, usually from the reaction between sodium bicarbonate and sulphuric acid, which smothers a fire by cutting off its oxygen supply.

Ammonia (NH₃): A gas made of nitrogen and hydrogen with no colour but a sharp, choking smell; it is less dense than air, dissolves very easily in water and behaves as a base.

Laboratory preparation of ammonia: In the lab, ammonium chloride and calcium hydroxide (2 : 1) are mixed and heated in a hard glass test tube, which gives off ammonia gas (2NH₄Cl + Ca(OH)₂ → CaCl₂ + 2H₂O + 2NH₃).

Lime tower: A tower filled with calcium oxide (quicklime) through which ammonia gas is passed to dry it by absorbing the water vapour formed during its preparation.

Collection of ammonia: Ammonia cannot be gathered over water because water absorbs it very quickly. Because ammonia weighs less than air as well, the jar is held mouth-down: the gas rises inside and forces the air out from below (downward displacement of air).

White fumes test: Hold a rod wet with concentrated hydrochloric acid at the mouth of the jar: thick white smoke of ammonium chloride appears if ammonia is present.

Ammonium hydroxide: The compound formed when ammonia gas dissolves in water (NH₃ + H₂O → NH₄OH); it is responsible for ammonia's basic properties in solution.

Urea: A nitrogen-rich fertilizer (NH₂CONH₂) made in factories by combining ammonia with carbon dioxide under high pressure at about 150-200°C (2NH₃ + CO₂ → NH₂CONH₂ + H₂O). Note: some textbooks print 1500°C, which is a misprint.

Catalytic oxidation of ammonia: The industrial reaction in which ammonia burns in oxygen in the presence of a platinum catalyst at about 800°C to form nitric oxide, a key step in manufacturing nitric acid.

Sodamide: The compound NaNH₂, produced together with hydrogen gas when ammonia reacts with molten sodium (2Na + 2NH₃ → 2NaNH₂ + H₂).

Upward displacement of air: A method of collecting a gas that is heavier than air, in which the gas is allowed to settle at the bottom of a jar and push the lighter air upward and out.

Downward displacement of air: A method of collecting a gas that is lighter than air, in which the gas rises to the top of an inverted jar and pushes the heavier air downward and out.

Woulfe's bottle: A multi-necked laboratory bottle used for preparing and passing gases, commonly used in the laboratory preparation of carbon dioxide.

Combustible gas: A gas that burns when it is lit in air or oxygen, for example methane or hydrogen.

Supporter of combustion: A gas, such as oxygen, that helps another substance burn by supplying the oxygen needed for combustion; carbon dioxide and ammonia are both examples of gases that are not supporters of combustion.