When a block of iron receives enough heat, it will melt into liquid iron. This happens at the temperature of 1 500 degrees Celsius. As a liquid, this iron can then be poured into molds. It will fill the molds evenly, and its surface will be completely horizontal. 

Mercury is a metal that melts at a temperature of -40 degrees Celsius. This means that it is already a liquid at room temperature. Mercury has traditionally been used in thermometers. 


Image on the left: When iron receives enough heat, it will melt into liquid. Image on the right: Mercury is a metal that exists in a liquid state in room temperature.

Similarly, when a block of ice receives enough heat, it will melt into liquid water. The melting point of ice is 0 degrees Celsius. The block of ice retains this temperature as long as it is melting. 

If you could see individual atoms, you would notice the difference between solid and liquid matter at the atomic level. In solid matter, the molecules that make up matter are packed tightly and do not almost move at all. When heat is increased, the molecules begin to move. Eventually, they will detach from one another and flow into a liquid. 


Image on the left: When ice receives enough heat, it melts into water. Image on the right: When a solid block of ice melts into a pool of liquid water, the movement of the molecules increases.