Home Chemistry Substance Water


What is water molecule?

Water molecule, chemical formula H2O, made by oxygen and hydrogen atom, existing in our earth environment as a crystalline solid, liquid, or gaseous form. It is a tasteless, colorless liquid that contains a v-shaped structure.

Shape of water molecule, chemical formula H2O, formed by oxygen and hydrogen existing in solid (ice), liquid, gas form

Water is an essential liquid that has the ability to dissolve many inorganic and organic compounds or chemical elements. Liquid and solid states of water arise due to the existence of hydrogen bonding.

The molecular weight of H2O is 18 gm mol−1 with a melting point of 0 °C and a boiling point of 100 °C. Water has an amphiprotic substance and properties to act as an acid or base.

Importance of water in human life

  • Water is very important for human life or living organism.
  • It made our majority of body weight.
  • It involves the major biological function of our body.
  • Every part of our body uses water to regulate body temperature.
  • It helps to flush out waste from our body and control human brain function.

The availability of hygienically safe and pure drinking water for home uses is a big problem all over the world. Therefore, we used many techniques for its conservation.

Sewage disposal, industrial waste, sulfuric acid, nitric acid, fertilizer, insecticides, and detergents are the major pollutant that causes water pollution in our environment

Structure of water molecule

The valance shell electronic configuration of the central oxygen atom in the water molecule is 2s2 2px2 2py1 2pz1. All these four atomic orbitals hybridize to form four equivalent sp3 tetrahedral hybrid orbitals.

Two of these four hybridized orbitals contain singly occupied electrons. These two orbitals overlap with two 1s-orbital of the hydrogen atom to form two covalent bond. The remaining two orbitals contain two electron pairs.

Due to the presence of electron pairs on the central atom, H2O is a v-shape molecule with an H-O-H bond angle of 105.5°.

Properties of water

  • H2O molecules are polar.
  • It is a universal solvent. It dissove most of the polar and ionic substances.
  • The heat capacity of water is very high. It takes a large amount of energy for rising one degree of temperature.
  • Liquid H2O molecules are denser than ice because ice crystals contain huge empty space.
  • It has cohesive and adhesive properties. Cohesive properties of water responsible for surface tension.

Physical properties

A small quantity of water appears colorless. A large quantity of molecules is a blue color liquid due to the atomic absorption of red light.

It appears in two forms, hard and soft. Present-day, the hardness or softness and pH scale level of water change due to the formation of acid rain and dissolving materials. It is harmful effects on the biological systems of plants and animals.

Properties of water
Molecular formula H2O
Molar mass 18 g mol−1
Melting point 0.00 °C or 32.00 °F or 273.15 K
Boiling point 99.98 °C or 211.96 °F or 373.13 K
Density (g/ml) Liquid Solid
At 0 °C: 0.9998396
At 0 °C: 0.9970474
At 0 °C: 0.9167
Conjugate acid-base pair Acid Base
hydronium ion (H3O+) hydroxide ion (OH)
Heat capacity 75.385 ± 0.05 J mol−1 K−1
Crystal structure hexagonal
Dipole moment 1.8546 D

Polarity of H2O molecule

The electronegativity and electron affinity of oxygen is much greater than that of hydrogen. Therefore, electric polarization arises due to gaining a partial positive charge on hydrogen and a partial negative charge on the oxygen atom.

The polarity of the H2O molecule comes from two facts,

  1. Dipole moment due to oxygen-hydrogen electronegativity differences.
  2. Presence of two-electron pairs in the central oxygen atom with v-shaped structure.

Conductivity of pure water

The pure solution is not a good conductor of electricity. It slightly dissociates and behaves like a weak electrolyte. The specific conductance of pure water at 25 °C has been found to be 0.58 × 10−7 ohm−1 cm−1.

Structure of ice

The crystal structure of ice represented the different types of complex structures in chemistry. The ice molecule formed nine complex crystal lattices are established according to the different ranges of temperature and pressure.

In ordinary forms, it freezes at 0°C and 1 atm pressure with puckered hexagonal structure. In this structure, each H2O unit is surrounded tetrahedrally by the oxygen atoms of four other H2O molecules through hydrogen bonding extended over the entire structure.

What happens when ice melts into water?

In ice crystals, there are huge empty spaces. Each oxygen atom has two near and two distant neighbors.

  • At a very low density and very low temperature of about −120 °C, the cubic crystal lattice of ice is observed. The experimental zero-point entropy of ice is 3.4 joule mol−1.
  • When the number of H2O molecules is brought together, the positive end of one dipole attacks the negative end of the other dipole. Therefore, H2O molecules are associated together to form the cluster (H2O)n,
  • In the gas phase of the water molecule, no such hydrogen bonding or association is found.

Acid and base in H2O solution

The protons unite with a water molecule to produce H3O+ or hydronium ions. The acid-base neutralization process in water involves the diffusion of the proton to the base.

The chemical equilibrium reaction of H2O,
H2O → H+ + OH

The ionic product,
kw = k [H2O] = [H+] [OH]

The ionic product is constantly equal to 10−14. The concentration of hydrogen and hydroxyl ions in H2O solution is inversely proportional to each other.

  • It functions as an acid in presence of a base stronger than itself. For example,
    NH3 + H2O → NH4+ + OH
  • In presence of acids, it behaves like a base by accepting a proton. For example,
    HSO4 + H2O → H3O+ + SO4−2

Hard and soft water

The universal solvent water is categorized into two types, hard and soft, according to dissolved minerals. Some of these minerals are beneficial for our health.

What is hard water?

It contains common calcium and magnesium salt. These ions produce insoluble precipitates with shop material. Such types of solutions are termed hard water. Calcium and sodium bicarbonate makes the solution temporarily hard.

When the solutions contain soluble sulfates of calcium or magnesium which are not precipitated by boiling are called permanent hard water.

What is soft water?

On boiling, the temporary hardness of the water can remove but the permanent hardness can not be removed by the easy process.

  • Hard solutions are made soft by passing them through beds of insoluble aluminum sodium silicates or ion exchange resin (zeolites).
  • A useful regent like EDTA or ethylene diamine tetra-acetic acid or Calgon (sodium hexametaphosphate) is used in removing the hardness of water to make a soft solution.