|Unlike chemical changes, physical changes can be ____.
|When water (a liquid) boils, it ____ into steam (a gas).
|Fossil fuels and nuclear power are ____ energy resources.
|A solution with a pH less than 7 is ____.
|Salt and sugar ____ quickly in hot water.
|When a spring is stretched, its potential energy ____.
|There are ____ meters (m) in one kilometer (km).
|Positive and negative charges ____ each other.