Here's why:

* Solubility and Temperature: Generally, the solubility of gases in liquids decreases as the temperature increases. This is because the higher kinetic energy of the molecules at higher temperatures makes it easier for the dissolved gas molecules to escape from the liquid.

* Chlorine's Nature: Chlorine is a gas at room temperature and is slightly soluble in water. The solubility is influenced by factors like pressure and temperature.

In practical terms:

* If you're using chlorine to disinfect water, it's more effective to add it to cold water.

* If you're using chlorine to sanitize surfaces, it's best to use cold or lukewarm water.

Let me know if you have any other questions!