Bubbling the Same Gas Twice Undoes What It Just Did

Because a little carbon dioxide makes an insoluble solid, and more carbon dioxide dissolves that solid again.

Bubble the gas briefly through lime water and insoluble calcium carbonate forms as a white suspension — the milkiness. Keep bubbling and the excess gas converts that carbonate into calcium bicarbonate, which is soluble, so the milkiness disappears and the liquid clears.

So the milkiness went away does not mean the test failed. It means you passed too much gas, and that second stage is itself part of the identification. This page covers the third part of the ICSE Class 8 Chemistry chapter on carbon: preparing carbon dioxide, testing for it, its dangerous relative carbon monoxide, and its role in warming the planet.

By the action of dilute hydrochloric acid on calcium carbonate, using marble chips.

$${\text{CaCO}}_3+2\text{HCl}\to {\text{CaCl}}_2+{\text{H}}_2\text{O}+{\text{CO}}_2$$

The apparatus. Marble chips are placed in a flask or a Woulfe's bottle. Dilute hydrochloric acid is added through a thistle funnel whose stem dips below the acid, so gas cannot escape back up it. The gas leaves by a delivery tube.

How it is collected. By the upward displacement of air into an upright gas jar. Carbon dioxide is denser than air, so it sinks to the bottom of the jar and pushes the air up and out.

Why not over water. Carbon dioxide is fairly soluble in water, so water would absorb a good deal of it. This is the opposite of hydrogen, which is almost insoluble and therefore is collected over water.

Why dilute sulphuric acid is not used with marble. It forms calcium sulphate, which is insoluble. The calcium sulphate coats the surface of the marble chips as a hard layer, the acid can no longer reach the marble beneath, and the reaction stops after a brief start. Hydrochloric acid gives soluble calcium chloride, so the surface stays exposed and the reaction continues.

This is one of the most reliably asked questions on this preparation, and the reason — an insoluble product coating the reactant — is the whole answer.

Why not concentrated hydrochloric acid. It would give hydrogen chloride vapour as an impurity in the gas.

Drying the gas. If dry carbon dioxide is needed, it is passed through concentrated sulphuric acid, which is hygroscopic as the water chapter described.

An everyday version of the same reaction. Add a little dilute acid — or even lemon juice — to baking soda and carbon dioxide fizzes off at once. Marble is used in the laboratory rather than baking soda because the reaction is slower and easier to control.

Physical properties:

- Colourless and odourless, with a faintly acidic taste
- Denser than air. Its molecular mass is $44$, so its vapour density is $44÷2=22$, against air's $14.4$. It is about

$$\frac{44}{28.8}\approx 1.5$$

times as dense as air — which is why it sinks and why it can be poured from one jar into another.

- Fairly soluble in water, and more so under pressure
- Turns moist blue litmus faintly red, being weakly acidic
- Easily liquefied by pressure, and solidified to dry ice, which sublimes directly back to gas

Chemical properties:

It is neither combustible nor a supporter of combustion. A burning splint pushed into a jar of carbon dioxide goes out at once. This is the property behind every fire extinguisher, and combined with its density it is why the gas blankets a fire and smothers it.

With water it forms a weak acid:

$${\text{CO}}_2+{\text{H}}_2\text{O}\to {\text{H}}_2{\text{CO}}_3$$

Carbonic acid, which is what makes an aerated drink slightly sharp.

With an alkali it forms a carbonate:

$$2\text{NaOH}+{\text{CO}}_2\to {\text{Na}}_2{\text{CO}}_3+{\text{H}}_2\text{O}$$

With burning magnesium it is reduced. Push a burning magnesium ribbon into the gas and it keeps burning, leaving white magnesium oxide and black specks of carbon:

$$2\text{Mg}+{\text{CO}}_2\to 2\text{MgO}+\text{C}$$

So does not support combustion has a boundary: a metal reactive enough can take the oxygen straight out of the carbon dioxide. This is why a carbon dioxide extinguisher must not be used on a burning reactive metal.

The lime water test. Pass the gas through lime water, which is calcium hydroxide solution:

$$\text{Ca}(\text{OH}{)}_2+{\text{CO}}_2\to {\text{CaCO}}_3+{\text{H}}_2\text{O}$$

The insoluble white calcium carbonate makes the lime water milky. Continue passing the gas and:

$${\text{CaCO}}_3+{\text{H}}_2\text{O}+{\text{CO}}_2\to \text{Ca}({\text{HCO}}_3{)}_2$$

The soluble calcium bicarbonate forms and the milkiness disappears.

Both stages together make the test conclusive: turns lime water milky, and the milkiness disappears on passing excess gas. Boiling that clear solution reverses the second step and the milkiness returns — which is exactly the decomposition of temporary hardness from the water chapter.

Because it attaches to haemoglobin far more firmly than oxygen does, so the blood stops being able to carry oxygen at all.

Haemoglobin in red blood cells normally picks up oxygen in the lungs, carries it to the tissues and releases it. Carbon monoxide combines with haemoglobin to form carboxyhaemoglobin, which is much more stable than oxyhaemoglobin. Once formed it does not let go, so that haemoglobin is permanently out of service.

With enough haemoglobin blocked, the tissues are starved of oxygen even though the person is breathing normally. The symptoms — headache, dizziness, drowsiness — resemble ordinary tiredness, and unconsciousness can follow.

What makes it especially treacherous:

- Colourless, odourless and tasteless, so there is no warning whatever
- Almost the same density as air — its vapour density is $28÷2=14$ against air's $14.4$ — so it mixes evenly through a room instead of collecting where it might be noticed
- Neutral to litmus, and almost insoluble in water

Where it comes from. The incomplete combustion of any carbon fuel — a coal fire or angithi in a closed room, a faulty geyser, a vehicle engine in a shut garage. Complete burning gives carbon dioxide; burning with too little air gives carbon monoxide instead. This is why a coal fire must never be left burning in a closed bedroom.

Its chemical properties. Unlike carbon dioxide, carbon monoxide is combustible, burning with a blue flame:

$$2\text{CO}+{\text{O}}_2\to 2{\text{CO}}_2$$

And it is a good reducing agent, removing oxygen from heated metal oxides:

$$\text{CuO}+\text{CO}\to \text{Cu}+{\text{CO}}_2$$

$${\text{Fe}}_2{\text{O}}_3+3\text{CO}\to 2\text{Fe}+3{\text{CO}}_2$$

That second reaction is the heart of iron extraction in a blast furnace.

Its uses:

- As a fuel, in water gas and producer gas
- As a reducing agent in the extraction of metals
- In the manufacture of methanol

The contrast to hold on to. Carbon dioxide is not combustible and not poisonous in ordinary amounts; carbon monoxide is combustible and highly poisonous. Two oxides of the same element, differing by one oxygen atom, with opposite behaviour on both counts.

By letting sunlight in and holding the heat back from escaping.

The greenhouse effect. Sunlight passes easily through the atmosphere and warms the Earth's surface. The warmed surface then re-emits that energy as infrared radiation, which has a longer wavelength. Carbon dioxide and certain other gases absorb infrared radiation strongly, so instead of escaping into space, much of the heat is absorbed and re-radiated back downwards.

The result is that the lower atmosphere stays warmer than it otherwise would. Gases behaving this way are greenhouse gases — chiefly carbon dioxide, along with methane, water vapour, nitrous oxide and chlorofluorocarbons.

Why the effect itself is not the problem. Without any greenhouse effect the Earth would be far too cold for life. It is the increase that causes trouble.

What is increasing it. Chiefly the burning of fossil fuels — coal, petroleum and natural gas — and deforestation, which removes the trees that would have absorbed carbon dioxide by photosynthesis. Both raise the atmospheric concentration above what natural processes remove.

Global warming is the resulting rise in average temperature, and its consequences include melting glaciers and polar ice, rising sea levels that threaten coastal cities, shifts in rainfall and monsoon patterns, and harm to crops and to wildlife.

What can be done, each addressing a cause directly: burn less fossil fuel, use solar and wind energy, plant trees, use public transport, and improve the efficiency of engines and appliances.

Its uses, which are considerable:

- Fire extinguishers — it neither burns nor supports burning, and being denser than air it blankets the flame
- Aerated drinks — dissolved under pressure, as the solubility section explained
- Dry ice — solid carbon dioxide, used as a refrigerant for transporting food and medicines, and for stage effects. It sublimes rather than melting, so it leaves nothing wet behind
- Baking — baking soda or yeast releases carbon dioxide in the dough, and the bubbles make bread and cake rise
- Manufacture of urea fertiliser and of washing soda
- Photosynthesis — the raw material from which plants make all our food and release oxygen

The double character worth noticing. The same gas is a fire extinguisher, a food refrigerant, the reason bread rises, the raw material of all plant food, and the chief driver of global warming. None of those is a contradiction — they are all consequences of the properties listed in the earlier section.