Final answer:
The second half of the gas made by heating limestone in contact with water is primarily composed of carbon dioxide (CO₂).
Step-by-step explanation:
The second half of the gas made by heating limestone in contact with water is primarily composed of carbon dioxide (CO₂). When limestone (CaCO₃) is heated, it decomposes into calcium oxide (CaO) and carbon dioxide.
When this carbon dioxide reacts with water, it forms a milky solution due to the formation of calcium carbonate (CaCO₃), which is only slightly soluble in water.