Biochemistry

Physical & Chemical Properties of Water

water-molecule

Core Concepts

In this tutorial on the properties of water, you will learn about the physical and chemical properties of water. You will also learn about the structure of a water molecule.

Topics Covered in Other Articles

Polarity of Water

Electronegativity

Solvent v.s. Solute

Specific Heat

Density

Vocabulary

Density- Mass per unit volume

Specific Heat Capacity- Amount of energy required to raise the temperature of 1 kg of a material by 1 C

Heat of Vaporization- Amount of energy required to transform a quantity of a liquid into a gas

Polar Molecule- A molecule with a partially negative charged end and a partially positive charged end

Electronegativity- Tendency for an atom to attract shared electrons in a chemical bond

Introduction to Properties of Water

Water (H20) is the “universal solvent” and the most abundant surface on Earth. It is also the only common substance to exist as a solid, liquid, and gas naturally. Water molecules form hydrogen bonds and are extremely polar. The five main properties of water are its high polarity, high specific heat, high heat of vaporization, low density as a solid, and attraction to other polar molecules.

Polarity and Structure

One oxygen atom and two hydrogen atoms make a water molecule. It has a bent molecular geometry with the oxygen having two lone pairs of electrons. The difference in electronegativity between the oxygen and hydrogens causes the oxygen to have a partial negative charge and the hydrogen to have a partial positive charge. This difference in charge causes polarity. The partial positive charge of the hydrogen of one water molecule attracts the partial negative charge of the oxygen of another water molecule. This attraction is called hydrogen bonding. 

Hydrogen bonding is weaker than the covalent bonds between the oxygen and hydrogens of the same molecule but causes many of water’s unique properties. For example, more energy is required to break hydrogen bonds, so water has a higher melting and boiling point.

Universal Solvent

Water is the solvent of life. Hydrophilic substances are those that dissolve in water, while hydrophobic substances do not mix well with water. Substances can dissolve in water if they can match or overcome the hydrogen bonding between water molecules. If they cannot, the substance forms a precipitate. Acids, alcohols, and salts are rather soluble in water while fats and oils are hydrophobic. 

An ionic solute dissolved in water becomes separated. For example, NaCl separates into Na+ cations and Cl- anions surrounded by water molecules. Water is amphoteric, meaning it can act as either an acid or a base depending on the solution. It can produce both H+ and OH- ions. 

Specific Heat Capacity

Water has a high specific heat of 4184J/(kg x K) at 20 C and high heat of vaporization because of hydrogen bonding. This allows bodies of water to have minimal fluctuations in temperature to regulate climate.

Water’s high heat of vaporization allows humans to use sweat to cool off. Sweat is mostly made of water. It absorbs excess body heat as it evaporates. This process is known as evaporative cooling.

Density

Water’s density is 1 gram per cubic cm. This is used to define the gram. Instead of undergoing thermal expansion, the density rises with temperature to a peak of 3.98 C and then decreases. Negative thermal expansion is the increase in density between 32 and 39.16 F. As a result, ice is less dense than water, which has a decrease in density by about 10%. This is why bodies of water may have a layer of ice on the surface but contain liquid underneath. This allows fish and marine life to survive under the ice. High specific heat keeps the temperature of the water relatively stable through the winter so that marine life can survive.

Salt content lowers the freezing point of the ocean by almost 2 C. Ice still floats on the ocean, but the ice is near salt-free with a similar density to ice on bodies of freshwater. The salt adds to the salinity and density of the remaining water which sinks by convection. This process is called brine rejection. 

Compressibility

Compressibility is a result of pressure and temperature. The compressibility of water is so low that is often assumed to be incompressible. Low compressibility allows water in deep oceans with high pressure to only decrease by 1.8% in volume.

Electrical Conductivity

Pure water is a good insulator, but deionized water is never completely free of ions. Water undergoes a process called autoionization as a liquid. This means that two water molecules can form one hydroxide anion (OH-) and one hydronium cation (H30+).

Cohesion and Adhesion

Hydrogen bonds between water molecules are constantly breaking and reforming with other water molecules. Cohesion is the ability of water molecules to stick together. Water’s polarity also gives water high adhesion: the ability to stick to other surfaces. The adhesive forces are stronger than cohesive forces.

Strong cohesion and adhesion cause water to exhibit capillary action. Capillary action is the process of liquid flowing through a narrow space without and often against gravity. Water adheres to the walls of plants’ roots and rises up into the plant. Porous materials such as water also exhibit capillary action. Trees can transport water through capillary action over 100 meters.

Surface Tension

The hydrogen bonding also causes water to have a high surface tension of 71.99mN/m at 25. The surface tension is high enough for insects to walk on water. Surface tension is a result of water’s cohesive properties. Water droplets and water rising above the rim of a glass show water’s high surface tension. Learn more about the properties of water here.