Heating limestone powder in an evaporating dish.

 When limestone powder (which is primarily composed of calcium carbonate, CaCO3) is heated in an evaporating dish, several chemical reactions occur. Here's what happens:

1. **Decomposition of Calcium Carbonate**: The main reaction that takes place is the decomposition of calcium carbonate into calcium oxide (quicklime) and carbon dioxide gas. The chemical equation for this reaction is:

   CaCO3(s) → CaO(s) + CO2(g)

   In this reaction, heat energy is absorbed to break the bonds in calcium carbonate, resulting in the formation of calcium oxide and the release of carbon dioxide gas as a product.

2. **Formation of Calcium Oxide**: The resulting product, calcium oxide (CaO), is also known as quicklime. Quicklime is a white, caustic, and crystalline solid that has various industrial uses.

3. **Evaporation of Water**: If there is any moisture or water content in the limestone powder, it will evaporate during the heating process, leaving behind dry calcium carbonate before the decomposition reaction occurs.

4. **Release of Carbon Dioxide**: Carbon dioxide (CO2) gas is released during the decomposition reaction. This gas can be detected by effervescence, and it will escape into the air.

5. **Change in Mass**: Since carbon dioxide is released as a gas during the reaction, there will be a loss of mass in the evaporating dish as the carbon dioxide escapes. This loss of mass is due to the removal of carbon and oxygen atoms from the calcium carbonate molecule.

6. **Formation of Calcium Oxide Residue**: After the reaction is complete, you will be left with a residue of calcium oxide (quicklime) in the evaporating dish. Calcium oxide is an alkaline substance and can react with water to form calcium hydroxide, releasing a significant amount of heat in the process. This property makes quicklime useful in various applications, including in the production of cement and as a desiccant.

In summary, heating limestone powder in an evaporating dish results in the decomposition of calcium carbonate into calcium oxide and carbon dioxide gas. This reaction is commonly used in industries for various purposes, including the production of quicklime, which has numerous applications in construction, manufacturing, and agriculture.