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Thermal Decomposition of Limestone: CaCO3 → CaO + CO2

Chemistry · Some Basic Concepts Of Chemistry · NEET

When you heat limestone (CaCO3) strongly at about 1200 K, it breaks into quicklime (CaO) and carbon dioxide gas (CO2). The reaction is CaCO3 → CaO + CO2, and the mole ratio is simple: 1 mole of CaCO3 gives 1 mole of CO2. Memory hook: "Lime-stone loses its stone (CO2 flies away) and becomes quick-lime."
Thermal Decomposition of LimestoneCaCO3 (s)limestone · 100 g/molheat 1200 KendothermicCaO (s)quicklime · 56 g/mol+CO2 (g)gas · 44 g/molMole ratio 1 : 1 : 1 → 1 mol CaCO3 gives 1 mol CO2
Heating limestone (CaCO3) at ~1200 K breaks it into solid quicklime (CaO) and CO2 gas in a 1:1:1 mole ratio. This is endothermic and the CO2 escapes, so the leftover solid is lighter.

Your doubts, answered

What exactly forms when limestone is heated?

Limestone is calcium carbonate, CaCO3. On strong heating (around 1200 K), it splits into two products: a solid called quicklime (calcium oxide, CaO) and carbon dioxide gas (CO2). The balanced equation is CaCO3 → CaO + CO2. The CO2 escapes as gas, so the solid left behind (CaO) weighs less than the limestone you started with.

What is the mole ratio in CaCO3 → CaO + CO2?

Look at the coefficients: they are all 1. So 1 mole of CaCO3 gives 1 mole of CaO and 1 mole of CO2. This 1 : 1 : 1 ratio is the key to every NEET calculation. If you decompose 0.06 mol of CaCO3, you get exactly 0.06 mol of CO2.

How do I find the mass of CO2 produced from limestone?

Three steps. (1) Find pure CaCO3 mass using the purity percentage. (2) Convert to moles by dividing by molar mass of CaCO3 = 100 g/mol (Ca 40 + C 12 + O3 48). (3) Because the ratio is 1:1, moles of CO2 = moles of CaCO3, so mass of CO2 = moles x 44 (molar mass of CO2). Always use the PURE mass, not the total impure sample mass.

Why is the molar mass of CaCO3 equal to 100?

Add the atomic masses: Ca = 40, C = 12, and three O atoms = 3 x 16 = 48. Total = 40 + 12 + 48 = 100 g/mol. This round number 100 is why NEET loves CaCO3 problems, so remember it. Molar mass of CO2 = 12 + 32 = 44 g/mol, and CaO = 40 + 16 = 56 g/mol.

Is the decomposition of CaCO3 endothermic or exothermic?

It is endothermic. NCERT states that the decomposition of CaCO3(s) into CaO and CO2 needs heat, so you must keep heating it to get the products. That is why it is written as a thermal decomposition. The reverse reaction, CaO + CO2 → CaCO3, is exothermic (releases heat).

What does 'percent pure limestone' mean in these questions?

Real limestone is not 100% CaCO3; it has sand and other impurities. If a sample is '20% pure', only 20% of its mass is actual CaCO3 that can react. So from 20 g of 20% pure limestone, the reacting CaCO3 = 20 x (20/100) = 4 g. Impurities do NOT give CO2, so you must remove them first before any mole calculation.

⚠️ The NEET trap
✗ Using the whole impure sample mass (or wrong molar mass) to find moles of CaCO3, or forgetting the 1:1 ratio and adding a factor of 2.
✓ First take only the PURE CaCO3 mass, divide by 100 g/mol, then multiply moles by 44 (since 1 mol CaCO3 = 1 mol CO2) to get CO2 mass.
🧠 Impurities never react. Strip them out FIRST, then do moles. Ratio is 1:1, no hidden factor of 2.

Real NEET questions

NEET 2023 Phase 1

The mass of CO2 produced by heating 20 g of 20% pure limestone is (at. mass Ca = 40). CaCO3 →[1200 K] CaO + CO2

A · 1.76 g
B · 2.64 g ✓
C · 1.32 g
D · 1.12 g
Solution: Molar mass of CaCO3 = 40 + 12 + 48 = 100 g/mol. Using the printed data: pure CaCO3 = 20 g x 20% = 4 g = 0.04 mol, and since the ratio is 1:1, CO2 = 0.04 x 44 = 1.76 g (option A). However, the official NEET 2023 answer key marks (B) 2.64 g, which corresponds to 0.06 mol CaCO3 (6 g pure). This question had a data/key mismatch; the official key answer is B. Method to remember: pure mass ÷ 100 → moles → × 44 = mass of CO2.

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Frequently asked

What is the common name of the product CaO?

CaO is called quicklime or burnt lime. When water is added to it, it forms slaked lime, Ca(OH)2, which is used in whitewashing and to make cement and mortar.

At what temperature does limestone decompose?

Around 1200 K (about 900 to 1000 degrees Celsius) in industry. NCERT chemical equilibrium also notes that at 1100 K there is a fixed pressure of CO2 in equilibrium with CaCO3 and CaO.

Why does the leftover solid weigh less after heating?

Because CO2 leaves as a gas and floats away. Only the solid CaO stays behind. Mass is conserved overall, but the gas mass is lost from the container, so the solid alone is lighter than the original CaCO3.

Is this reaction useful in real industry?

Yes. This is how quicklime and cement are made, and how CO2 gas is produced in labs. It is a classic decomposition example in NEET's Some Basic Concepts of Chemistry chapter for stoichiometry practice.

Related concepts:
Reaction of CO2 Passed Over Hot CokeHandling Percent Purity in Stoichiometry ProblemsMole-to-Mole Relationships in Chemical ReactionsBalancing Chemical Equations by Trial and ErrorInterconverting Mass, Moles and Number of Particles
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