The chemistry of water

Four emergent properties of water

Cohesion

Temperature moderation - high specific heat capacity

Expansion upon freezing

Universal solvent

Cohesion (of water)

Water molecules sticking to each other via hydrogen bonds

Adhesion (of water)

Water clinging to other polar/charged substances e.g. Walls of plant cells

Surface tension

A water measure of how hard it is to stretch or break the surface of a liquid; high in due to hydrogen bonding

How does water rise to the top of tall trees?

Evaporation from leaves pulls water upwards; cohesion between water molecules and adhesion to cell wall keep water in a column

Thermal energy

The total kinetic energy of all the molecules in a body of matter; depends on volume

Temperature

The average kinetic energy of molecules in a substance, regardless of volume

Heat

Thermal energy in transfer from a warmer object to a cooler one

Specific heat capacity

The amount of heat that must be absorbed or lost for 1g of a substance to change its temperature by 1°C

Why does water have a high specific heat capacity?

Heat must first break hydrogen bonds before water can move faster (raise temp) , so water resists temperature change

Why is water's high specific heat important for life?

Moderates temperature fluctuations in bodies of water, the atmosphere, and organisms

Heat of vaporization

The amount of neat needed to convert 1g of a liquid into a gas

Evaporative cooling

As the highest-energy molecules evaporate from a liquid's surface, the remaining liquid cools down

Why does ice float on liquid water?

Hydrogen bonds lock water molecules into a spaced-out crystal lattice, making ice less dense than liquid water

Solution

A homogenous mixture of two or more substances

Solvent vs solute

Solvent: dissolving agent

Solute: substance being dissolved

Aqueous solution

A solution where water is the solvent

Hydrophilic substance

A substance that has an affinity for water

Hydrophobic substance

A substance that doesn't have an affinity for water

Molecular mass

The sum of the masses of all atoms in a molecule

Mole

An exact number of particles: 6.02 × 10²³ (Avogadro's number).

Molarity

The number of moles of solute per litre of solution

Acid

A substance that increases the H⁺ concentration of a solution

Base

A substance that reduces the H⁺ concentration of a solution (directly or indirectly)

Dissociation of water

Water molecules occasionally split into a hydrogen ion (H⁺) and a hydroxide ion (OH⁻); the H⁺ joins another water molecule to form hydronium (H3O⁺)

pH formula

pH = –log[H⁺]

Strong acid/base

One that dissociates completely in water (e.g. HCl, NaOH)

Weak acid/base

One that only partially/reversibly dissociates in water (e.g. carbonic acid, ammonia)

Buffer

A substance that minimises changes in H⁺/OH⁻ concentration by accepting or donating hydrogen ions

Main buffer system in human blood

The carbonic acid–bicarbonate buffer, which keeps blood pH close to 7.4.