• There are four visual clues that indicate that a chemical reaction is likely occurring.
  • The most obvious sign is that the crust turns light brown.
  • Melting of a solid absorbs energy, while the condensation of a gas releases energy.
  • When zinc reacts with hydrochloric acid, the test tube becomes very warm as energy is released during the reaction.
  • The only way to be certain that a. chemical reaction has taken place is to test the composition of the substances after the change has taken.
  • place to see if they are different from the starting substances.
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