13. a. Why are the units used to express the densi- ties of gases different from those used to
express the densities of solids or liquids?

Answer with Explanation:

The units used to express the densities of gases are different from those of solids and liquids because the particles in gas are widely separated from each other, unlike the particles in solid and liquid which are almost the same. The particles in solid are very close together. Considering it melts (if it's an ice), it will turn into a liquid and the change in volume is slightly greater only. However, if the liquid evaporates and transitions into a gas, the volume becomes largely different from its solid and liquid state. This is because the particles are much farther apart and free to move.

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chemical changes occur when a substance combines with another to form a new substance, called chemical synthesis or, alternatively, chemical decomposition into two or more different substances. these processes are called chemical reactions and, in general, are not reversible except by further chemical reactions. some reactions produce heat and are called exothermic reactions and others may require heat to enable the reaction to occur, which are called endothermic reactions. understanding chemical changes is a major part of the science of chemistry.