When hydrogen is bonded to an electronegative atom such as oxygen, nitrogen or fluorine, a special or unique type of attraction is present among the molecules of such compounds. The hydrogen of one molecule is attracted by the electronegative atom of the adjacent molecule. Such type of bonding is shown by dotted lines for hydrogen fluoride and water in Fig.5.8.
The strength of hydrogen bonding varies from about 4 kJ mol-1 to 25 kJ mol-1 in various substances. This energy is much less than that required breaking one mole of an ionic or a covalent substance as you can see from Table 5.3.

H_____F-----H_____F-----H_____F

The existence of water in liquid state is because of hydrogen bonding. Hydrogen bonding is also responsible for the low density of ice as compared to water. In ice, hydrogen bonding gives an ordered arrangement of water molecules which has a lot of free space in between them. Since ice is lighter than water, it floats on water and provides an insulating layer over water which is very important for the survival of aquatic life.
Hydrogen bonding also explains the miscibility of alcohol in water in all proportions. Glucose which contains six-OH groups makes hydrogen bonds with water molecules and hence is very soluble in water. More than 80 g of glucose dissolves in 100 mL of water.
In proteins, hydrogen bonding is responsible for their helical structure