Chemical Reactions: How Do You Know When They Happen?

Which of these indicates that a chemical reaction has occurred?

1) Steam forms above boiling water

2) A solid forms when a clear solution is frozen

3) A solid forms when two clear solutions are mixed

4) Sugar crystals form on the sides of a boiling pot of sugar

The student's question asks which of the provided options indicates that a chemical reaction has occurred.

Looking at each option:

Explanation:

Steam forms above boiling water - This is a physical change because water is turning from liquid to gas without changing its chemical structure.

A solid forms when a clear solution is frozen - This also describes a physical change, as the process of freezing involves water in the solution turning into ice, which is a physical state change of the same substance.

A solid forms when two clear solutions are mixed - This indicates a chemical reaction called a precipitation reaction, where a new solid, or precipitate, forms as a product of the chemical change.

Sugar crystals form on the sides of a boiling pot of sugar - This is a physical change where sucrose molecules arrange into a crystalline structure due to evaporation of water, but the sucrose remains chemically the same.

The conclusion is that option 3 is the correct answer because the formation of a solid precipitate when two solutions are mixed is a sign of a chemical reaction.