Water Hydrogen Bond, the Unique Property of Water

Although the entire water molecule is neutral, it has an uneven charge distribution. The oxygen atom will take electrons from the hydrogen atom, giving itself partial negative charges, while the hydrogen atom exhibits partial positive charges. The hydrogen atom will be attracted to the oxygen atom in another water molecule to form a hydrogen bond. It is not too weak force whose strength is between Van der Waals force and covalent bond. The hydrogen bond gives water properties that other solvent does not have.

Strong surface tension

Due to hydrogen bond, water molecules are subjected to stronger inward forces at the interface between liquid and gas, which gives water a strong surface tension. Some small insects can walk on the surface of water.

Water has a high specific heat

The irregular thermal motion of the particles will be accelerated by the absorption of heat. Water molecules have to break hydrogen bonds before they can accelerate their motion. This consumes some extra energy, and water molecules kinetic energy does not increase as much as expected. Compared to other liquids, water absorbs more heat by raising the same temperature and releases more heat by lowering the same temperature. It is essential for maintaining a moderate suitable temperature for life. For example, the ocean absorbs heat from the sun while its temperature increases by only a few degrees during the day; at night, the ocean releases the heat it absorbed during the day to prevent the fast dropping temperature. This keeps the Earth's temperature from changing drastically (the greenhouse effect is another reason).

Since hydrogen bond enhances the force between water molecules, more energy is consumed when water molecules enter the air. The evaporation of water carries away a large amount of heat. In summer or during intense exercise, people take away a lot of heat by sweat evaporation, so that their body temperature drops rapidly to normal levels.

Ice floats on the water surface.

For the same substance, solid is denser than liquid. However, liquid water is denser because its molecular thermal movement is more intense and water molecules overcome hydrogen bonds to stay closer. When temperature is below its freezing point, the hydrogen bond pushes the adjacent water molecules further apart due to the reduced thermal motion, forming a regular hexagon. This causes ice to be more dense than water, so it can float on the surface. This property is essential for life. If ice sinks to the bottom,it will freeze the benthos and destroy the ecosystem. Heat from the sun can be blocked by the water above, and the ice in the bottom may not fully melt during the summer. In the long run, there will be more and more ice in rivers, lakes and oceans, and eventually life will not survive.

Water is a good solvent

Negatively charged oxygen atoms of water attract cations and positively charged hydrogen atoms of water attract anions. Ions are easily stripped off by attraction and dissolved in water. ionic compounds such as NaCl, MgCl₂ are easily dissolved in water. Although sugar is not composed of ions, it has hydrophilic groups -OH or -CHO. The uneven distributed charge of these groups attract water molecules to make them easily soluble in water. Even some large molecules like proteins can dissolve in water if they have hydrophilic regions or ionic regions.

Because it dissolves polar molecules and ions easily, water has become an ideal medium for various chemical reactions in life, and nutrient or waste in cells are transported through water

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