Question

Which of the following is a sign a chemical reaction has occurred? A liquid changes to a solid when cooled. A liquid forms bubbles when heated. Two clear liquids combine and mix together. Two clear liquids combine and turn cloudy.

Answer 1

The best answer is, “Two clear liquids combine and turn cloudy.”

When water freezes form a liquid to a solid. That’s a physical change.

When water boils, it forms bubbles. That’s a physical change.

When you mix ethanol and water, they form a clear solution. That’s a physical change.

Answer 2

The answer to your question regarding which statement describes a chemical reaction is: Two clear liquids combine and turn cloudy.

In general, there are two types of changes in substances: physical or chemical.

Physical changes occur without modification of the chemical composition, so the identity of the substance is not altered. Changes in the matter state are examples of physical changes. For example, when ice is melted, solid water becomes liquid water but is still water.

Chemical changes involve reactions so new substances are formed. They are often accompanied by changes in color or apparition of turbidity in a solution.

From the options, the following statements describe physical changes:

• A liquid changes to a solid when cooled.
• A liquid forms bubbles when heated.
• Two clear liquids combine and mix together.

The only option in which is described a sign of chemical reaction is Two clear liquids combine and turn cloudy (see attached image). Since the solution turned cloudy, a precipitate has been formed, so a new substance was formed by a chemical reaction.

You can learn more about physical and chemical changes here:

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