When Ag+ is combined with Na2CO3, the substances formed are Ag2CO3 and Na+. In this case, Ag performs single substitution over the element Na to form another set of substances. There are other types of reactions like double displacement, decomposition, etc.
When something is a solid the molecules are impact together and have a small sense of vibration. But as the solid melts away for example ice, the molecules become more loose forming into a liquid
