In basic inorganic chemistry, which of the following is the correct chemical formula for quicklime, the substance obtained by heating limestone strongly?

Introduction / Context:
Quicklime is an important industrial chemical used in construction, metallurgy, and environmental applications. It is produced by heating limestone, and it can further react with water to form slaked lime. Understanding the correct formula of quicklime and how it is produced is a standard topic in school level chemistry. This question asks you to choose the correct chemical formula for quicklime from several calcium compounds.

Given Data / Assumptions:

• Limestone is mainly calcium carbonate, CaCO3.
• When limestone is strongly heated (calcination), it decomposes into quicklime and carbon dioxide.
• Quicklime is also called burnt lime.
• Related calcium compounds include calcium hydroxide, calcium chloride, and others used in different processes.

Concept / Approach:
The thermal decomposition of calcium carbonate on heating is a classic example of a decomposition reaction. The balanced reaction is: CaCO3 (s) → CaO (s) + CO2 (g) The product CaO is called quicklime or burnt lime. When quicklime reacts with water, it forms calcium hydroxide, Ca(OH)2, commonly known as slaked lime. Calcium chloride, CaCl2, and other calcium compounds have different uses and are not formed directly in this decomposition. Therefore, the correct formula for quicklime is CaO.

Step-by-Step Solution:
Step 1: Identify limestone as calcium carbonate, formula CaCO3. Step 2: Recall that strong heating of CaCO3 causes it to lose carbon dioxide gas. Step 3: Write the reaction CaCO3 → CaO + CO2 and see that the solid residue CaO is quicklime. Step 4: Recognise that Ca(OH)2 is obtained only after adding water to CaO, giving slaked lime. Step 5: Choose CaO as the correct formula from the list of options.

Verification / Alternative check:
Industrial lime kilns operate by heating limestone to high temperatures, producing quicklime CaO, which is then often converted to calcium hydroxide for use in mortar and plaster. Textbooks consistently define quicklime as calcium oxide and slaked lime as calcium hydroxide. Material safety data sheets for quicklime list the formula as CaO. No standard reference refers to CaCO3, Ca(OH)2, or CaCl2 as quicklime, confirming that CaO is the correct choice.

Why Other Options Are Wrong:
CaCO3 is limestone itself, the starting material, not the product after strong heating. Ca(OH)2 is slaked lime, produced when CaO reacts with water. CaCl2 is calcium chloride, a salt often used as a drying agent and de icing compound, not related to lime production. CaO2 (if considered) would suggest calcium peroxide, which is not the common substance known as quicklime. Thus, only CaO correctly represents quicklime.

Common Pitfalls:
Some learners confuse quicklime with slaked lime and may mistakenly choose Ca(OH)2. Others may think that limestone and quicklime share the same formula because both are calcium compounds. To avoid these errors, remember the sequence: limestone (CaCO3) on heating gives quicklime (CaO), and quicklime plus water gives slaked lime (Ca(OH)2). Linking this three step chain in memory helps with many related questions.

Final Answer:
The correct chemical formula for quicklime is CaO (calcium oxide).