Is the density of a solid more than that of a liquid?
If you're talking about "any solid" vs "any liquid", then no, you cannot say that is true as a general rule. There will be some solids that are denser and some liquids that are denser. A simple test in each case is to place the solid on top of the liquid. If it sinks, the solid is denser. If it floats, the liquid is denser.
However, if you are comparing the solid and liquid forms of the same substance, it can be said, as a general rule, that the solid is generally more dense. The individual molecules have less energy and are less active as temperature decreases. Therefore, they take up less space (volume decreases). Density, by definition, is mass divided by volume. Since volume is smaller, density increases.
There are exceptions to this general rule, however, and the most well-known exception is water. Solid water (ice) is less dense than liquid water. This is because of the structure of the water molecule. It is a little more negative on the oxygen end, and a little more positive on the end with the two hydrogen atoms. These sets up a situation wherein the molecules can form loose electrostatic bonds between each other. In a liquid state, these so-called hydrogen bonds are not as controlling as they are when the molecules change state and become solid. As the molecules lose energy and begin to lock together forming the solid, those hydrogen bonds force the molecules apart slightly as they seek some kind of electrostatic equilibrium. The description that I just gave is very much an oversimplification. But it's nevertheless appropriate.