Properties of carbon dioxide

Physical properties first — the ones you notice without a reaction.

• Carbon dioxide is colourless and, in the dilute concentrations met in a classroom, essentially odourless (a faint sharp smell is sometimes noted only in concentrated form).
• It is denser than air, roughly one and a half times as heavy for the same volume. This single fact explains two lab observations at once: CO₂ is collected by upward displacement of air (it sinks to the bottom of the jar instead of escaping), and it can be tipped like a liquid from one gas jar into another, dousing a candle flame in the second jar even though nothing touched it.
• It is moderately soluble in water, and because it reacts with water to a small extent, the resulting solution is weakly acidic — this is the same solubility that makes fizzy drinks fizz.
• It does not support combustion: a burning splint lowered into a jar of CO₂ goes out. Combined with its high density, this is exactly why carbon dioxide is used in fire extinguishers — it blankets a fire, cutting off the oxygen supply, without itself burning.

Chemical properties — where the acidic character shows up. Carbon dioxide is the oxide of a non-metal, and non-metallic oxides are acidic oxides. An acidic oxide reacts with a basic oxide to form a salt, exactly the way an acid reacts with a base:

where: $\text{Na}_2\text{O}$ and $\text{MgO}$ are basic (metal) oxides, and the product in each case is the metal carbonate — the salt of the "carbonic acid" character that CO₂ carries. The single most important chemical property, though, is how carbon dioxide behaves with lime water, a solution of calcium hydroxide, $\text{Ca(OH)}_2(aq)$. Passing CO₂ through it gives a two-stage result that examiners love to test in full:

where: the calcium hydroxide (a base) reacts with carbon dioxide (an acidic oxide) to give calcium carbonate, an insoluble white solid that clouds the solution, plus water. This is stage one: milky. If CO₂ is passed on in excess, that same white solid dissolves again:

where: calcium hydrogen carbonate, $\text{Ca(HCO}_3)_2$, unlike calcium carbonate, is soluble in water, so the suspended white solid disappears and the liquid turns clear again. This is stage two, and it is the part students most often forget to mention.

There is no single formula to derive here — the properties of carbon dioxide are read off from its chemical identity, so the useful thing is to understand why it behaves acidic rather than simply memorise that it does, and why the lime-water reaction has two opposite-looking stages instead of one.

Step 1. Start with what "acidic oxide" actually means. Carbon dioxide is $\text{CO}_2$, carbon combined with oxygen, and carbon is a non-metal. Oxides of non-metals dissolve in water to give weakly acidic solutions and react with bases the way an acid does, forming a salt and water. That single classification rule predicts, before you even run the experiment, that CO₂ should react with a base like calcium hydroxide.

Step 2. Apply that rule to lime water. Calcium hydroxide, $\text{Ca(OH)}_2$, is a base, so an acid-base reaction gives a salt plus water — here the salt is calcium carbonate, $\text{CaCO}_3$. Calcium carbonate is insoluble in water (a fact to know, not derive), so the moment it forms it appears as a fine white suspension, and the once-clear solution looks milky. That is the entire explanation for stage one.

Step 3. Now explain the reversal. Passing more CO₂ does not undo the first reaction — it starts a second one. Calcium carbonate reacts further with carbon dioxide and water to form calcium hydrogen carbonate, $\text{Ca(HCO}_3)_2$, and this second salt is soluble. The suspended solid is chemically converted into a soluble compound, and the liquid clears as that compound dissolves. This only happens with CO₂ kept flowing in excess, because both reactions compete for the same solid.

Step 4. The general pattern: many insoluble carbonate precipitates can be redissolved by excess of the acidic gas that produced them, because the corresponding hydrogen carbonate is more soluble than the carbonate. Recognising this pattern, not just this one equation, is what lets you predict similar two-stage behaviour elsewhere.

Question: A student bubbles carbon dioxide gas through a test tube of freshly prepared lime water for one minute, observes a change, and then continues bubbling gas through for several more minutes. (a) Describe what is seen at each stage. (b) Write balanced equations for both reactions. (c) State the property of carbon dioxide that this whole experiment demonstrates.

Step 1. Identify what lime water is: an aqueous solution of calcium hydroxide, $\text{Ca(OH)}_2(aq)$, which starts perfectly clear.

Step 2. Describe stage one (after roughly a minute of CO₂). The solution turns milky/cloudy due to a fine white precipitate. Write the equation:

Step 3. Check the balance of Step 2's equation before moving on: calcium atoms, 1 on each side; oxygen atoms, $(2+2)=4$ on the left and $(3+1)=4$ on the right; hydrogen atoms, 2 on each side; carbon atoms, 1 on each side. The equation is balanced.

Step 4. Describe stage two (after continued, excess CO₂). The milky suspension gradually clears and the tube returns to a colourless, transparent liquid. Write the equation:

Step 5. State the property demonstrated. This experiment demonstrates the acidic chemical nature of carbon dioxide — it reacts with the base calcium hydroxide in an acid-base fashion to give a salt and water (stage one), and, because it is present in excess, it goes on to convert the insoluble salt into a more soluble one (stage two). Together these two equations form the standard laboratory test used to confirm the presence of CO₂ gas.

Final answer: the lime water turns milky within about a minute, then clears again on continued excess CO₂; the two equations are as written above; the test confirms both the presence of CO₂ and its acidic nature.

Competitive stretch

If a question asks you to identify an unknown colourless gas using only lime water, why is "it turned milky" alone sometimes an insufficient answer? Reporting only "it turned milky" is risky because that shows just one of the two diagnostic reactions; a sharper answer states that the gas turns freshly prepared lime water milky within a short time, and, when passed in excess, clears that milkiness again — a two-step signature that carbon dioxide alone reliably gives among the gases met at this level. Quoting both stages, with both equations, separates a complete answer from a partial one.

• Describing only the first half of the test — "lime water turns milky" — and stopping there, without mentioning that excess CO₂ clears the milkiness again by forming soluble calcium hydrogen carbonate.
• Calling carbon dioxide a "basic" gas. It is the oxide of a non-metal, so it is an acidic oxide, and it reacts with bases and basic oxides, not with acids.
• Treating the fire-extinguishing behaviour as a chemical reaction. It is mainly a physical effect: CO₂ neither burns nor supports burning, and being denser than air, it smothers a flame by cutting off its oxygen supply — no new chemical bond is being tested there.
• Writing an unbalanced equation, or leaving out state symbols and the downward arrow (↓) that marks the precipitate, for the lime-water reactions.

Ask the tutor: if the two-stage lime-water reaction keeps tripping you up, ask your AI tutor to walk through both equations side by side and explain why the second reaction only kicks in once the first has already happened.