Which of the following is a sign a chemical reaction has occurred? a blue liquid is mixed into a yellow liquid, and the mixture turns green. a piece of metal is placed in water and produces sparks and gas. a liquid is heated on a hotplate, and bubbles form. a solid powder is mixed into a liquid and disappears.

A blue liquid is mixed into a yellow liquid, and the mixture turns green.

A piece of metal is placed in water and produces sparks and gas.

A liquid is heated on a hotplate, and bubbles form.

Explanation:

A chemical reaction occurs when reactants combine or decompose to form products which have a completely different composition and structure than the reactants. There are certain signs which indicate that a chemical reaction has taken place, the primary indicators include:

1) Formation of a precipitate

2) Formation of bubbles

3) color change

4) change in temperature

Based on the given examples:

1) A blue liquid is mixed into a yellow liquid, and the mixture turns green:

This involves a color change and hence a chemical process

2) A piece of metal is placed in water and produces sparks and gas.

Formation of sparks and gas signify a change in temperature and hence a chemical process

3) A liquid is heated on a hotplate, and bubbles form.

Formation of bubbles are indicate that a chemical reaction has occured