7. What type of energy is contained in the matches within a matchbook?

A matchbook contains potential energy; just as a large stone at the top of a hill receives a little push, and the energy released as it travels downward, so a match when it is struck releases its stored, Potential energy as a flame. Potential Energy is the correct answer.

Explanation:

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the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.