Exam practice

Carbon dioxide (molecular mass 44) is about 1.5 times heavier than air (about 29). It sinks to the bottom of an upright jar and pushes the lighter air out at the top. It doesn’t rise and escape through the open mouth, so the jar fills with it, and it can be collected by the upward displacement of air.

Ammonia is very soluble in water, but less soluble in warm water. In a warm bottle, ammonia gas comes out of the solution and builds up a high pressure under the lid; when the lid is opened, gas and solution can spurt out and hurt the eyes and nose. Cooling makes the gas dissolve back into the liquid, which lowers the pressure, so the bottle can be opened safely.

Principle: carbon dioxide is made by the reaction of marble or limestone (calcium carbonate) with dilute hydrochloric acid:
$$\text{CaCO}_3\text{(s)} + 2\text{HCl(aq)} \rightarrow \text{CaCl}_2\text{(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}$$

Apparatus: Woulfe’s bottle, thistle funnel, delivery tube, rubber corks, gas jar.

Method: Put marble pieces in the Woulfe’s bottle. Fit the thistle funnel and the delivery tube airtight with corks. Pour dilute HCl through the thistle funnel until the marble and the end of the funnel are covered. The carbon dioxide made passes through the delivery tube into an upright gas jar, where it is collected by the upward displacement of air, as it is heavier than air.

Diagram: as in Figure 16.2, labeling the thistle funnel, dilute HCl, Woulfe’s bottle, marble pieces, delivery tube, gas jar and CO₂.

Precautions: the lower end of the thistle funnel must dip into the acid; the end of the delivery tube must be above the acid; the apparatus must be airtight.

Test: the gas turns lime water milky.

• i.Carbon dioxide.
• ii.$\text{CaCO}_3 + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{H}_2\text{O} + \text{CO}_2$
• iii.Damp blue litmus paper: it turns red, as CO₂ is an acidic gas. (The surer test is lime water turning milky.)
• iv.Because carbon dioxide is heavier than air. In an upright jar it settles at the bottom and pushes the air out at the top (upward displacement of air).

1. It is a colorless, odorless gas, about 1.5 times heavier than air.
2. It neither burns nor supports burning, so it puts out fires.
3. It turns lime water milky, by forming insoluble calcium carbonate: $\text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O}$.

(Also: it is an acidic oxide that turns damp blue litmus red; it is slightly soluble in water, forming carbonic acid; it forms dry ice below −78 °C.)

1. To put out fires (fire extinguishers).
2. In soft drinks and soda water, for fizz.
3. As dry ice, to keep food and medicines cold.
4. By green plants, to make food in photosynthesis.

(Also: making urea, washing soda and baking soda; purifying sugar; in baking.)

Principle: ammonia is made by heating a mixture of ammonium chloride and calcium hydroxide in the ratio 2 : 1:
$$2\text{NH}_4\text{Cl} + \text{Ca(OH)}_2 \xrightarrow{\text{heat}} \text{CaCl}_2 + 2\text{H}_2\text{O} + 2\text{NH}_3$$

Apparatus: hard-glass test tube, stand, burner, delivery tubes, corks, lime tower, gas jar.

Method: Put the mixture in the hard-glass test tube and clamp it sloping slightly down towards its mouth. Fit a cork and delivery tube leading to a lime tower filled with quicklime (CaO), which dries the gas. From the top of the tower, lead the gas into a gas jar held upside down. Heat gently. Ammonia, being lighter than air, collects in the inverted jar by the downward displacement of air.

Diagram: as in Figure 16.7, labeling the mixture of NH₄Cl and Ca(OH)₂, the hard-glass test tube, burner, stand, delivery tube, lime tower (CaO), inverted gas jar and ammonia.

Precautions: the test tube slopes downwards so water can’t run back and crack it; the apparatus is airtight; the gas is not collected over water, as it is very soluble.

Test: damp red litmus turns blue; a rod dipped in concentrated HCl gives white fumes.

• i.Ammonia.
• ii.$2\text{NH}_4\text{Cl} + \text{Ca(OH)}_2 \rightarrow \text{CaCl}_2 + 2\text{H}_2\text{O} + 2\text{NH}_3$
• iii.Damp red litmus: it turns blue, as ammonia is a basic gas.
• iv.So that the water formed in the reaction runs out towards the mouth instead of flowing back onto the hot bottom of the tube, which would crack it.
• v.The lime tower is filled with quicklime (calcium oxide), which absorbs the water vapor, so we get dry ammonia. (Quicklime is used because it is basic and doesn’t react with ammonia.)

1. To make fertilizers such as urea, ammonium sulphate and ammonium nitrate.
2. As a refrigerant in cold stores and ice factories.
3. To make nitric acid, plastics and washing soda.
4. As a cleaning agent to remove grease and oil stains.

(Also: medicines of ammonium salts; blueprints.)

(i) The lime water turns milky, because insoluble calcium carbonate forms:

$$\text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O}$$
(ii) The milkiness disappears, because the calcium carbonate turns into soluble calcium hydrogencarbonate:

$$\text{CaCO}_3 + \text{H}_2\text{O} + \text{CO}_2 \rightarrow \text{Ca(HCO}_3)_2$$
(iii) The magnesium keeps burning brightly, leaving white magnesium oxide and black specks of carbon:

$$2\text{Mg} + \text{CO}_2 \rightarrow 2\text{MgO} + \text{C}$$
(iv) Ammonia gas is given off, with calcium chloride and water:

$$2\text{NH}_4\text{Cl} + \text{Ca(OH)}_2 \rightarrow \text{CaCl}_2 + 2\text{H}_2\text{O} + 2\text{NH}_3$$
(v) It dissolves readily, making ammonium hydroxide, a weak alkali:

$$\text{NH}_3 + \text{H}_2\text{O} \rightarrow \text{NH}_4\text{OH}$$
(vi) Ammonium chloride forms; as gases, they make dense white fumes:

$$\text{NH}_3 + \text{HCl} \rightarrow \text{NH}_4\text{Cl}$$