Human life always depended on water. Therefore for a long time people watched it carefully, having found out that on cold water freezes, that is turns into solid substance — ice which when heating becomes water again.
Instruction
1. Many substances, both natural, and artificial, can be in several phase states, that is the forms of existence replacing each other depending on external conditions. Now more than ten such phase states most of which is achievable only in laboratories are known. In the nature most often meet firm, liquid and gaseous.
2. The most part of the water which is on our planet — liquid. It means that its molecules quickly move and are poorly connected among themselves. Therefore liquid takes any form, but is not capable to keep it in itself.
3. When heating a molecule of liquid begin to move even quicker, and substance gradually turns into gaseous state. Are in gas of a molecule further away from each other therefore gas can be rarefied or compressed strongly, and it not only does not keep a form, but also occupies any available volume.
4. But if to cool liquid, then it can pass into a firm state. Its molecules slow down so that between them stable relations are formed. There is a solid body having own internal structure. If this structure is ordered, then it is called crystal. For example, ice — crystal substance. Its crystals have the six-sided form. The small figured crystals of ice which are formed in clouds are better known as a snowflake.
5. Process of transition of substance of liquid state in firm is called hardening or crystallization, and transition from firm in liquid — melting. Melting of ice in a popular speech is called thawing, and its crystallization — freezing.
6. All bodies when heating extend, and when cooling clench. However the distance between water molecules in a crystal of ice is a little more, than in liquid. Therefore ice when freezing extends so the water left to freeze in a bottle, can break off it, having turned into ice. For the same reason the ice formed in water in the winter always floats from above, but does not fall by a bottom.
7. Water freezes at a temperature of 0 degrees on Celsius scale. It will be more correct to tell, however, that the zero mark in a scale of Celsius was established on ice thawing temperature, and Celsius accepted water boiling temperature to equal 100 degrees.