Which statement about a chemical reaction is true? it occurs only at low temperatures. it occurs only at high temperatures. it occurs whenever a physical change takes place. it occurs whenever a chemical change takes place.

 It occurs whenever a chemical change takes place.

Explanation:

A physical reaction is defined as the reaction in which a change in shape, size takes place. No new substance gets formed in these reactions.

Example: melting of ice

A chemical reaction is defined as the reaction in which a change in chemical composition takes place. A new substance is formed in these reactions.

Example: Oxidation of magnesium leads to formation of white magnesium oxide and is a chemical change.

Thus a chemical reaction occurs whenever a chemical change takes place.

 It occurs whenever a chemical change takes place.

Explanation:

A physical reaction is defined as the reaction in which a change in shape, size takes place. No new substance gets formed in these reactions.

Example: melting of ice

A chemical reaction is defined as the reaction in which a change in chemical composition takes place. A new substance is formed in these reactions.

Example: Oxidation of magnesium leads to formation of white magnesium oxide and is a chemical change.

Thus a chemical reaction occurs whenever a chemical change takes place.

indicate valleys, hills, steepness and gentleness of slopes.

explanation:

contour lines on a map indicate elevation, or depth. numeric notations on contour lines refer to the level of elevation, or height about sea level.