Composition and structure of water

Introduction to water

Water is the most common substance on Earth. It is so important that we use it for almost everything: cooking garri, bathing, and washing our clothes. In chemistry, we look at what water is actually made of and how it stays together.

Chemical composition of water

Think of water like a small building block. It is made by joining two different gases together: hydrogen and oxygen. For every one piece of oxygen, you need two pieces of hydrogen. This is why we write it as H_2O.

Simply put, water is a molecule formed when hydrogen and oxygen gases react and join together.

Ratio by mass and volume

If you look at the amount of space these gases take, there are two parts of hydrogen for every one part of oxygen. But if you weigh them, oxygen is much heavier. By mass, the ratio is 1:8 because oxygen atoms are heavier than hydrogen atoms.

Structure of water

Water molecules are not straight like a ruler. They are bent or V-shaped. The oxygen atom sits at the top, and the two hydrogen atoms sit at the bottom like the legs of a tripod. The angle between them is about 104.5 degrees.

Hydrogen bonding

In a bucket of water, the molecules stick to each other. This is because the oxygen in one molecule is a bit 'selfish' and pulls electrons toward itself, becoming slightly negative. It then attracts the slightly positive hydrogen of another molecule. This 'glue' is called a hydrogen bond.

This bond is the reason why water stays as a liquid and does not just fly away as gas like the air from a generator exhaust.

Physical properties of water

Pure water has no color, no smell, and no taste. It freezes at 0 degrees Celsius and boils at 100 degrees Celsius at normal pressure. If you add salt to water, it will take longer to boil; just like how your pot of soup boils at a higher temperature than plain water.

Testing for water

If you want to know if a clear liquid is water, you can use white Anhydrous Copper(II) Sulfate. If you drop water on it, it turns blue. You can also use blue Cobalt(II) Chloride paper, which turns pink when water touches it.

Worked Example: Composition by mass

Question: Calculate the percentage by mass of oxygen in H_2O. (Relative Atomic Masses: H = 1, O = 16).

1. Find the Total Molar Mass of H_2O: (2 x 1) + 16 = 18 g/mol.

2. Find the mass of Oxygen in the formula: 16 g.

3. Calculate the percentage: (Mass of Oxygen / Total Mass) x 100 = (16 / 18) x 100 = 88.89%.

4. Final Answer: Water is roughly 88.9% oxygen by mass.