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I am assuming you are weighing a hydrate in an evaporating dish, heating it to remove the water, and then weighing the evaporating dish to get the difference in mass. The reason you let the dish or crucible cool to room temperature is two-fold. The first reason is safety. You don't want to burn yourself on hot glass or metal. You also don't want to damage your balance with heat either. The other reason is warm objects surrounded by cooler air tend to create small air currents as a result of the hot air rising and colder air moving in to replace it. While these air currents are tiny and cannot be felt by a person, they can affect the mass reading on a balance. So you should wait until the dish is at room temperature so that the reading you get on the balance is steady and accurate.
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