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Recognizing Chemical Reactions

Describes the four main signs that a chemical reaction has occurred.

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Recognizing Chemical Reactions

Look at the girl’s hair in the photo above. It has obviously changed color. The process in which this occurred involved chemical reactions. How do you know that chemical reactions have occurred? The change in color is the most obvious clue.

What’s Your Sign?

A change in color is just one of several potential signs that a chemical reaction has occurred. Other potential signs include:

  • Change in temperature-Heat is released or absorbed during the reaction.
  • Production of a gas-Gas bubbles are released during the reaction.
  • Production of a solid-A solid settles out of a liquid solution. The solid is called a precipitate.

Examples of Chemical Reactions

Look carefully at the Figures below, below, and below. All of the photos demonstrate chemical reactions. For each photo, identify a sign that one or more chemical reactions have taken place. 

A burning campfire is a chemical reaction

A burning campfire can warm you up on a cold day.

Dissolving an antacid tablet is a chemical reaction

Dissolving an antacid tablet in water produces a fizzy drink.

Adding acid to milk to form cottage cheese is a chemical reaction

Adding acid to milk produces solid curds of cottage cheese.

Q: Did you ever make a “volcano” by pouring vinegar over a “mountain” of baking soda? If you did, you probably saw the mixture bubble up and foam over. Did a chemical reaction occur? How do you know?

A: Yes, a chemical reaction occurred. You know because the bubbles are evidence that a gas has been produced and production of a gas is a sign of a chemical reaction.


  • Potential signs that chemical reactions have occurred include a change in color, change in temperature, production of a gas, and production of a solid precipitate.


  1. How can you tell whether a change in matter is caused by a chemical reaction?
  2. When water freezes, it changes to ice. Is this a chemical reaction? Why or why not?


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