Why the density of the matter changes during the physical change?

Answer 1

It depends on the physical change. The most obvious example would be evaporation, when a solid changes to a gas. In a gas, the particles have very high energy and are not bonded to each other. There is a large amount of space between them. On the other hand, in a liquid, the density is only slightly different to the density in a solid. All the particles are touching each other, although the bonds between particles are comparatively weak. Basically, as liquid is heated, the particles absorb more and more energy and this causes them to vibrate more and more vigorously. Essentially, once these vibrations overcome the weak bonds between the particles, they all spread apart, converting from a liquid to a gas. A liquid is more dense than a gas because they contain more particles per unit volume. That is, if you had a cubic centimetre of liquid and a cubic centimetre of gas, there would be far more particles in the liquid than in the gas, because there is less space between the particles. Other physical changes, such as when a liquid freezes into a solid, would not entail such a significant change in density. Also, there are sure to be a number of physical changes that don't entail any change in density at all. Again, it really depends on the physical change you're talking about.