Why must the evaporating dish be allowed to cool before measuring its mass?

Because when the dish is hot, it weighs different than it normally would. 

Hello there!

When the dish rises in temperature, it's weight changes.

Hope this helps! :)

1) clean up you lab area2) dispose of the chemicals as instructed by your teacher3) put everything back where it belongs unless your teacher tells you otherwise4) dispose of gloves5) wash hands with soap and water