Preparation of Carbon Dioxide: 16 MCQs with answers

Dilute hydrochloric acid

The standard laboratory preparation uses dilute hydrochloric acid reacting with calcium carbonate (marble chips).

Calcium carbonate + Hydrochloric acid → Calcium chloride + Water + Carbon dioxide

The reaction between calcium carbonate and hydrochloric acid produces calcium chloride, water, and carbon dioxide.

The gas will escape because it is denser than air

Carbon dioxide is denser than air, so a mouth-down jar would let the heavy gas fall out instead of collecting.

Limewater turns milky when the gas is bubbled through it

The definitive test for carbon dioxide is that it turns limewater milky due to the formation of a white precipitate of calcium carbonate.

True

Carbon dioxide turns limewater milky due to the formation of a white precipitate of calcium carbonate.

It is slightly soluble in water and some gas would dissolve

Carbon dioxide is slightly soluble in water, so collecting it over water would result in loss of gas. Upward delivery is used instead.

Carbon dioxide will escape through the funnel instead of the delivery tube

The thistle funnel must dip below the acid surface to act as a seal. If it does not, gas escapes through the funnel rather than being delivered to the collection jar.

Heating calcium oxide strongly

Heating calcium oxide does not produce carbon dioxide. Heating calcium carbonate (limestone) does produce carbon dioxide, but calcium oxide itself does not.

Both A and R are true, and R correctly explains A

Upward delivery is used precisely because carbon dioxide is denser than air (so it sinks into the jar) and slightly soluble in water (so it cannot be collected over water). R directly explains A.

It provides $H^+$ ions that react with carbonate ions

The $H^+$ ions from the acid react with the $CO_3^{2-}$ ions in the carbonate, forming unstable carbonic acid that breaks down into water and carbon dioxide.

Carbon dioxide, because an acid reacts with a carbonate

Baking soda (sodium bicarbonate) is a carbonate, and vinegar is an acid. The reaction releases carbon dioxide — the same type of reaction as the laboratory preparation.

Denser than air and slightly soluble in water

Because carbon dioxide is denser than air, it cannot be collected mouth-down. Because it is slightly soluble in water, it cannot be collected over water. Upward delivery is the only viable option.

Excess carbon dioxide reacts with the calcium carbonate precipitate to form soluble calcium bicarbonate

While the source focuses on the initial milky precipitate, the disappearance on prolonged bubbling is due to formation of soluble calcium bicarbonate. The key exam point remains that the initial milkiness confirms carbon dioxide.

Carbon dioxide is collected by upward delivery only; oxygen can be collected over water or by upward delivery

Carbon dioxide is denser than air and slightly soluble, so upward delivery is used. Oxygen is only slightly denser than air and slightly soluble, so it can be collected over water or by upward delivery.

Carbon dioxide is denser than air, so it is collected by upward delivery

Carbon dioxide is about 1.5 times denser than air, not lighter. It is collected by upward delivery (mouth-up jar), not downward delivery.

Carbon dioxide smothers the fire by cutting off the oxygen supply

Carbon dioxide does not support combustion. When released onto a fire, it forms a blanket that cuts off the oxygen supply, smothering the flames.