Gases

Where carbon dioxide comes from

Carbon dioxide (CO₂) is made whenever something containing carbon burns: coal, wood, kerosene, fat, oil, candle wax, LPG. It is also made by the respiration of every plant and animal (including you), by volcanoes, and by the decay of dead plants and animals. Cutting down forests and burning fuels are the human-made sources that add the most.

Clean air is about 0.04% carbon dioxide by volume: about 425 parts per million, or 0.0425%, as of 2025. Sixty years ago it was about 0.03%; burning fuels has raised it.

Fact	Carbon dioxide
Formula	CO₂
Molecular mass	12 + 2 × 16 = 44
Mass of 1 liter (room temperature)	about 1.8 g (air: about 1.2 g)

Laboratory preparation of carbon dioxide

In the lab, carbon dioxide is made by the reaction of limestone or marble (both are calcium carbonate, CaCO₃) with dilute hydrochloric acid:

calcium carbonate + dilute hydrochloric acid → calcium chloride + water + carbon dioxide

$$\text{CaCO}_3\text{(s)} + 2\text{HCl(aq)} \rightarrow \text{CaCl}_2\text{(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}$$

Check the balance atom by atom: Ca 1 = 1; C 1 = 1; O 3 = 1 + 2; H 2 = 2; Cl 2 = 2. ✓

Two precautions, and why:

1. The bottom end of the thistle funnel must be under the acid. Otherwise the gas escapes up the funnel instead of going through the delivery tube. The acid acts as a door that lets acid in but no gas out.
2. The end of the delivery tube inside the bottle must be above the acid (not dipped into it). Otherwise the gas pressure would push acid up the delivery tube and into the jar.

Tests for carbon dioxide

1. A burning splint (or match) goes out. Carbon dioxide neither burns nor helps things burn. (But careful: nitrogen also puts out a flame, so this test alone doesn’t prove the gas is CO₂.)
2. Damp blue litmus turns red. Carbon dioxide dissolves in the water on the paper and makes a weak acid, carbonic acid. Phenolphthalein stays colorless, because it turns pink only in a base.
3. Lime water turns milky. This is the test that really proves it: of the common colorless, odorless gases, only carbon dioxide does it. (Sulphur dioxide turns lime water milky too, but you would know it at once by its sharp, choking smell.) Lime water is a clear solution of calcium hydroxide. Carbon dioxide reacts with it to make calcium carbonate, which doesn’t dissolve; its tiny white specks make the water look like milk.

$$\text{Ca(OH)}_2\text{(aq)} + \text{CO}_2\text{(g)} \rightarrow \underset{\text{milky}}{\text{CaCO}_3\text{(s)}} + \text{H}_2\text{O(l)}$$

Keep bubbling, and something surprising happens: the milkiness disappears. The extra carbon dioxide and water turn the chalky calcium carbonate into calcium hydrogencarbonate (calcium bicarbonate), which dissolves:

$$\text{CaCO}_3\text{(s)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)} \rightarrow \underset{\text{colorless solution}}{\text{Ca(HCO}_3)_2\text{(aq)}}$$

Properties of carbon dioxide

Physical properties

1. It is a colorless and odorless gas.
2. It dissolves only slightly in water (about one liter of the gas in one liter of water at room temperature). What dissolves makes carbonic acid, which tastes slightly sour: that is the bite of soda water.
3. It is about 1.5 times heavier than air (44 ÷ 29 ≈ 1.5).
4. It is an acidic oxide (the oxide of a non-metal): it turns damp blue litmus red.
5. It is not poisonous in small amounts: you breathe it out all day. But in large amounts it is dangerous. It pushes oxygen out of a closed space, and at about 10% in the air it makes people unconscious within minutes, even before the oxygen runs out. (See the well story below.)
6. Under high pressure and cooling, it turns into a liquid.
7. Cooled below −78 °C, it becomes a white solid called dry ice. At normal air pressure, dry ice never melts into a liquid: it turns straight into gas. This is called sublimation, and that is why it is “dry”.
8. It neither burns nor helps other things burn.

Chemical properties

1. It doesn’t burn, and puts out most flames, but not burning magnesium. Burning magnesium is so hot that it can pull the oxygen out of carbon dioxide. It keeps burning in a jar of CO₂ with a brilliant white light, leaving white magnesium oxide and black specks of carbon. This shows that carbon dioxide contains carbon.

$$2\text{Mg(s)} + \text{CO}_2\text{(g)} \rightarrow 2\text{MgO(s)} + \text{C(s)}$$

2. It dissolves in water to form carbonic acid, a weak acid. In soft drinks, CO₂ is dissolved under high pressure; when you open the bottle, the pressure drops and the gas escapes as bubbles.

$$\text{CO}_2\text{(g)} + \text{H}_2\text{O(l)} \rightleftharpoons \text{H}_2\text{CO}_3\text{(aq)}$$

3. It turns lime water milky, and with more CO₂, clear again (the two equations above).

4. It is absorbed by alkalis. Fill a test tube with carbon dioxide, put in a few drops of potassium hydroxide solution, and turn it upside down in water. The water climbs up the tube: the KOH has taken the CO₂ out of the gas, the pressure inside falls, and the air pressing on the water outside pushes water in.

$$2\text{KOH} + \text{CO}_2 \rightarrow \text{K}_2\text{CO}_3 + \text{H}_2\text{O}$$

5. Plants make food from it. In photosynthesis, green leaves use the energy of sunlight, caught by chlorophyll, to turn carbon dioxide from the air and water from the soil into glucose, and give out oxygen:

$$6\text{CO}_2 + 6\text{H}_2\text{O} \xrightarrow[\text{chlorophyll}]{\text{sunlight}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$$

6. With red-hot coke (carbon) at about 900–1000 °C it forms carbon monoxide, a poisonous gas. This happens in a coal stove or a brick kiln when air is short, and is why a makal (charcoal brazier) in a closed room can kill.

$$\text{CO}_2 + \text{C} \rightarrow 2\text{CO}$$

Uses of carbon dioxide

1. Soft drinks and soda water: CO₂ is dissolved under high pressure to give the fizz and sour taste.
2. Photosynthesis: green plants use it as the raw material for their food.
3. Fire extinguishers (below).
4. Dry ice keeps fruit, vegetables, meat, ice cream and medicines (some vaccines) cold, without leaving a wet mess.
5. Sugar mills use it to purify sugar cane juice (the carbonation process): lime is added to the juice, and carbon dioxide bubbled in makes calcium carbonate, which carries the impurities down as it settles.
6. Making chemicals: urea (NH₂CONH₂), washing soda (Na₂CO₃) and baking soda (NaHCO₃).
7. Carbogen, a mixture of 95% oxygen and 5% carbon dioxide, was given to patients to make them breathe more deeply (for example after carbon monoxide poisoning). Doctors use it rarely today.
8. Baking: bread and sel roti rise because carbon dioxide from yeast or baking powder makes bubbles in the dough.

The fire extinguisher

A fire needs three things: fuel, heat and oxygen. Take any one away and it goes out. Carbon dioxide, being heavier than air, settles over the fire like a blanket and keeps the oxygen away.

In the old soda-acid extinguisher, the cylinder holds a strong solution of sodium bicarbonate (sodium hydrogencarbonate). A small glass bottle of concentrated sulphuric acid hangs near the top, under a plunger. Strike the plunger, the bottle breaks, the acid mixes with the solution, and carbon dioxide is made so fast that it forces a jet of liquid and gas out of the nozzle onto the fire.

$$2\text{NaHCO}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{Na}_2\text{SO}_4 + 2\text{H}_2\text{O} + 2\text{CO}_2$$

Check: Na 2 = 2; H 2 + 2 = 4 = 4; C 2 = 2; O 6 + 4 = 10 = 4 + 2 + 4; S 1 = 1. ✓

Open the door of a cowshed in the morning, or walk past a public toilet, and you’ll smell it: a sharp smell that stings your nose and makes your eyes water. That is ammonia (NH₃).

Ammonia is found in nature both free (as the gas) and combined (in salts such as ammonium chloride and ammonium sulphate). It is made when things containing nitrogen, such as urine, dung and dead plants and animals, decay without enough oxygen. Ammonia gas was first collected by Joseph Priestley in 1774, by heating ammonium chloride with lime; he called it “alkaline air”.

Fact	Ammonia
Formula	NH₃
Molecular mass	14 + 3 × 1 = 17
Compared with air (about 29)	lighter: 17 ÷ 29 ≈ 0.6

The molecular mass of ammonia (17) is less than that of nitrogen (28), oxygen (32) and carbon dioxide (44), so ammonia is lighter than air.

Laboratory preparation of ammonia

In the lab, ammonia is made by heating a mixture of ammonium chloride (NH₄Cl) and calcium hydroxide (slaked lime, Ca(OH)₂) in a hard-glass test tube.

ammonium chloride + calcium hydroxide → calcium chloride + water + ammonia

$$2\text{NH}_4\text{Cl(s)} + \text{Ca(OH)}_2\text{(s)} \xrightarrow{\text{heat}} \text{CaCl}_2\text{(s)} + 2\text{H}_2\text{O(l)} + 2\text{NH}_3\text{(g)}$$

Check: N 2 = 2; H 8 + 2 = 10 = 4 + 6; Cl 2 = 2; Ca 1 = 1; O 2 = 2. ✓

The two are mixed in the ratio 2 : 1, as the equation says: two units of ammonium chloride for each one of calcium hydroxide.

Precautions, and why:

1. The test tube slopes slightly down towards its mouth. The reaction makes water. If the tube sloped up, drops of cool water would run back onto the hot glass at the bottom and crack it.
2. All the corks must be airtight, or the gas escapes into the room.
3. The gas is dried by passing it through a lime tower (quicklime, calcium oxide, CaO, takes up water). Ammonia can’t be collected over water, because it dissolves in water extremely well: the water would swallow it.

Tests for ammonia

1. Damp red litmus turns blue. Ammonia is the only common gas that is a base: it dissolves in the water on the paper and makes an alkali.
2. White fumes with hydrochloric acid. Bring a glass rod dipped in concentrated hydrochloric acid near the mouth of the jar: thick white fumes appear. These are tiny specks of solid ammonium chloride, made where ammonia meets hydrogen chloride gas in the air:

$$\text{NH}_3\text{(g)} + \text{HCl(g)} \rightarrow \text{NH}_4\text{Cl(s)}$$

Properties of ammonia

Physical properties

1. It is a colorless gas with a strong, pungent (sharp, stinging) smell. Ammonia smells like a dirty toilet or a cowshed. (The rotten-egg smell is hydrogen sulphide, H₂S, a different gas.)
2. It is lighter than air.
3. It is extremely soluble in water: at 20 °C, one liter of water dissolves about 700 liters of ammonia gas.
4. It is the only common basic gas: it turns damp red litmus blue.
5. It turns into a liquid at −33.4 °C, and freezes to a solid at about −78 °C. Under pressure it becomes liquid at room temperature, which is how it is stored and sold.

Chemical properties

1. It dissolves in water to make ammonium hydroxide (also called liquor ammonia or ammonia solution), a weak alkali:

$$\text{NH}_3\text{(g)} + \text{H}_2\text{O(l)} \rightarrow \text{NH}_4\text{OH(aq)}$$

2. Ammonia reacts with acids to make ammonium salts, with nothing else formed:

$$\text{NH}_3\text{(g)} + \text{HCl(aq)} \rightarrow \text{NH}_4\text{Cl(aq)} \quad \text{(ammonium chloride)}$$

$$2\text{NH}_3\text{(g)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow (\text{NH}_4)_2\text{SO}_4\text{(aq)} \quad \text{(ammonium sulphate)}$$

3. Ammonia solution reacts with acids to make a salt and water (neutralization):

$$\text{NH}_4\text{OH(aq)} + \text{HCl(aq)} \rightarrow \text{NH}_4\text{Cl(aq)} + \text{H}_2\text{O(l)}$$

$$2\text{NH}_4\text{OH(aq)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow (\text{NH}_4)_2\text{SO}_4\text{(aq)} + 2\text{H}_2\text{O(l)}$$

4. With carbon dioxide, at high temperature and pressure, it makes urea, the most used fertilizer in Nepal:

$$2\text{NH}_3\text{(g)} + \text{CO}_2\text{(g)} \xrightarrow[\text{high pressure}]{\text{about 190 °C}} \text{NH}_2\text{CONH}_2\text{(s)} + \text{H}_2\text{O(l)}$$

It needs two ammonia molecules, as the atom count shows: N 2 = 2, H 6 = 4 + 2, C 1 = 1, O 2 = 1 + 1. Factories run it at about 180–200 °C and 150 or more times the pressure of the air.

5. It burns in pure oxygen (not in air) with a yellowish flame, making nitrogen and water:

$$4\text{NH}_3\text{(g)} + 3\text{O}_2\text{(g)} \rightarrow 2\text{N}_2\text{(g)} + 6\text{H}_2\text{O(l)}$$

6. With a platinum catalyst at about 800 °C, ammonia and oxygen make nitric oxide (NO). This is the first step in making nitric acid:

$$4\text{NH}_3\text{(g)} + 5\text{O}_2\text{(g)} \xrightarrow[\text{Pt}]{\text{800 °C}} 4\text{NO(g)} + 6\text{H}_2\text{O(l)}$$

Count the nitrogen atoms to check the balance: 4 on the left, so 4NO on the right.

7. With molten sodium it makes sodamide and hydrogen:

$$2\text{NH}_3\text{(g)} + 2\text{Na(l)} \rightarrow 2\text{NaNH}_2\text{(s)} + \text{H}_2\text{(g)}$$

Uses of ammonia

1. Making fertilizers: urea, ammonium sulphate, ammonium nitrate, ammonium phosphate.
2. Making nitric acid, plastics, nylon and explosives.
3. Making washing soda (by the Solvay process, which uses ammonia and carbon dioxide).
4. Making medicines of ammonium salts (ammonium chloride in some cough syrups; smelling salts that wake up someone who has fainted).
5. Making blueprints of maps and building plans: in the diazo (“whiteprint”) copying process, ammonia vapor brings out the copied lines.
6. As a refrigerant: liquid ammonia takes in a lot of heat when it evaporates. Large cold stores for potatoes, apples and meat, and ice factories, use ammonia. (Home fridges use other gases, such as isobutane or R-134a, because ammonia is poisonous if it leaks.)
7. As a cleaning agent for grease and oil stains, as in some glass cleaners.

Activity 16.1: the ammonia fountain

Have you noticed that a cloudy night is warmer than a clear night? Or that a car parked in the sun becomes much hotter inside than the air outside? Or seen the plastic tunnels on farms around Kathmandu, where tomatoes and cucumbers grow even in winter?

These houses of glass or transparent plastic, used to grow plants, are called greenhouses. Sunlight gets in easily, but the heat it brings can’t get out easily, so the inside stays warmer than the outside. Trapping the Sun’s heat in this way is called the greenhouse effect. Greenhouses can be artificial (made of glass or plastic) or natural: the whole Earth is a natural greenhouse.

The Earth as a natural greenhouse

Here is what happens to sunlight, step by step:

1. The Sun is very hot, so most of its energy comes as short waves (visible light). Air lets these through easily. About 30% of the sunlight is reflected straight back to space by clouds, ice and the ground.
2. The rest is absorbed by the ground and the sea, which warm up.
3. The warm Earth gives out its energy again, but a much cooler body gives out long waves: invisible infrared rays (heat radiation, the warmth you feel from a hot iron without touching it).
4. Some gases in the air absorb infrared rays, and send much of that energy back down to the ground. These are the greenhouse gases: water vapor, carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), ozone (O₃) and chlorofluorocarbons (CFCs).
5. So the ground is warmed twice: by the Sun, and by the heat sent back from the air.

A cloudy night is warmer for the same reason: clouds (water) absorb the infrared from the ground and send it back down, while on a clear night it escapes to space.

Consequences of a stronger greenhouse effect

• a.The average temperature rises (global warming).
• b.The water cycle changes: heavier rain in some places, droughts in others, more floods and landslides.
• c.Human health suffers: heat waves, and diseases like dengue and malaria spreading to places that used to be too cold for mosquitoes (dengue has reached the hills of Nepal, even Kathmandu).
• d.Farm harvests fall in many places.
• e.Snow and glaciers melt: Himalayan glaciers are shrinking, and new glacial lakes form behind walls of loose rock that could burst. (In 2016 the Nepal Army lowered Imja Lake, below Everest, by 3.4 m to make it safer.)
• f.Sea level rises, flooding low coasts and islands.
• g.Biodiversity declines as plants and animals can’t move or adapt fast enough.
• h.Deserts spread (desertification).
• i.Ecosystems lose their balance.

Artificial greenhouses

An artificial greenhouse, or hot house, is a building of transparent glass or plastic where plants are kept. Short-wave sunlight enters and is absorbed by the soil, the plants and the floor, which warm up and give out long-wave infrared. Glass doesn’t let much of the infrared through, so it stays inside as heat, and the temperature rises. This is the artificial greenhouse effect.

Why artificial greenhouses are useful

It is warm inside a greenhouse, so plants can be grown in any season, and farmers can earn more by selling vegetables out of season. Plants that would die in a cold climate, and rare plants of cold places, can be kept alive in them.

• a.Plants of any season can be grown all year round.
• b.Food crops can be grown in very cold places (in Mustang and Jumla, high in the mountains, plastic tunnels let farmers grow vegetables).
• c.Flowers, leafy green vegetables, fruits and seedlings can be produced.
• d.The plants inside take up carbon dioxide and so help, a little, against pollution.
• e.Summer crops can be grown in winter.
• f.Plants of the summer season can be grown in cold places too.

How the Earth is a natural greenhouse

The Earth is wrapped in air containing carbon dioxide, water vapor, ozone and other greenhouse gases. Like the glass of a greenhouse, they let sunlight in but don’t let all the heat escape back to space. So the Earth stays warmer than it otherwise would be. That is why the Earth is called a natural greenhouse.

Ways to reduce the greenhouse effect

To slow down global warming, we must make and release less of the greenhouse gases:

1. Stop making and using chlorofluorocarbons. (This is largely done: under the Montreal Protocol of 1987, almost every country, Nepal included, has stopped making and importing CFCs, because they also destroy the ozone layer.)
2. Use less petrol, diesel and coal, and more renewable energy.
3. Promote hydroelectricity, solar and wind energy; Nepal makes almost all its electricity from water, so an electric vehicle or an induction stove in Nepal runs on clean energy instead of imported fuel.
4. Plant trees, and protect forests (thanks largely to community forests, Nepal’s forest cover has grown from about 29% of its land in the mid-1990s to about 40% in the 2015 national forest survey).
5. Reduce the release of carbon dioxide in every way: walk, cycle, use public transport, waste less.

What happens if acid falls on a leaf, or on a marble statue? Stone temples and statues in many cities of the world are slowly losing their faces and fine carving. Why is this happening, and what can we do?

What acid rain is

Burning coal, diesel and other fuels, and some factories, release gases such as sulphur dioxide (SO₂), oxides of nitrogen (nitric oxide, NO, and nitrogen dioxide, NO₂, from hot engines and kilns; some books say “nitrous oxide”, but nitrous oxide, N₂O, is a greenhouse gas, not a cause of acid rain), carbon dioxide and chlorine. In the air these gases react with oxygen and water vapor to make acids: sulphuric acid, nitric acid, carbonic acid and hydrochloric acid. These acids come down with the rain. This is acid rain.

$$2\text{SO}_2 + \text{O}_2 \rightarrow 2\text{SO}_3$$

$$\text{SO}_3 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_4 \quad \text{(sulphuric acid)}$$

$$4\text{NO}_2 + \text{O}_2 + 2\text{H}_2\text{O} \rightarrow 4\text{HNO}_3 \quad \text{(nitric acid)}$$

$$\text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{CO}_3 \quad \text{(carbonic acid)}$$

Even clean rain is slightly acidic, because it dissolves a little carbon dioxide from the air: its pH is about 5.6. Rain with a pH below 5.6 is called acid rain; it is usually between 4 and 5, and has been measured as low as 3 or less, as sour as vinegar.

Effects of acid rain

1. It eats away buildings, temples and statues made of marble or limestone (calcium carbonate), and corrodes metal, such as iron bridges and the copper and brass of temple roofs:

$$\text{CaCO}_3\text{(s)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{CaSO}_4\text{(s)} + \text{CO}_2\text{(g)} + \text{H}_2\text{O(l)}$$

The white marble of the Taj Mahal in India turned yellowish partly because of air pollution from factories and an oil refinery nearby; the government had to close or move many of them.

• 2.It makes the soil acidic, washing away the nutrients plants need, so crops and forests grow poorly.
• 3.It makes lakes and rivers acidic; fish eggs and many water animals die.
• 4.It can irritate the skin and eyes.
• 5.It harms human health in other ways: the same pollutant gases that make acid rain cause asthma and other lung diseases.

Ways to prevent acid rain

1. Release less sulphur dioxide and oxides of nitrogen: use low-sulphur fuels, fit factories and power stations with scrubbers, and keep vehicle engines tuned (catalytic converters in car exhausts remove oxides of nitrogen).
2. Use renewable energy (water, sun, wind) in place of fossil fuels.
3. Raise awareness about the causes and effects of acid rain.

1. Choose the correct option

2. Give reasons

3. Answer the following questions