Recipe for a Compound

Why it matters

Hydrogen and oxygen are invisible gases, yet when they meet with a spark, they lock together to form water. This is not a mixture; it is a new substance with fixed proportions.

Predict first

Why is water (H₂O) considered a compound and not a mixture of hydrogen and oxygen?

In a compound, elements lose their individual identities and combine in definite proportions to form a new substance with unique properties.

• It can be separated by filtration
• The elements are chemically combined in a fixed mass ratio — correct
• It retains the properties of both gases
• It is a liquid at room temperature

What you do

Fit hydrogen and oxygen together by valency — oxygen has 2 bonds, hydrogen has 1 — to build water, H2O.

Check yourself

Pure water from rain, a tap or a glacier is always H₂O. Which law does this show?

Pure water always has hydrogen and oxygen in the same fixed mass ratio, 1 : 8 — no matter where it comes from.

Hydrogen and oxygen combine in a fixed mass ratio of 1 : 8 to make water. If you spark 2 g of hydrogen with 8 g of oxygen, what happens?

The recipe is fixed: 1 g of hydrogen for every 8 g of oxygen. Whatever is extra simply stays behind, unreacted.

Hydrogen burns in oxygen with a pop and water appears. What does this tell you?

Water is nothing like the invisible gases it came from — a new substance formed, so this is chemical combination, not mixing.

• A compound always contains its elements in a fixed ratio by mass (law of constant proportion) — correct
• Elements can combine in any ratio they like
• Water from different sources is a different compound
• Only 1 g of hydrogen reacts — the extra 1 g is left over — correct
• All the hydrogen reacts, making hydrogen-rich water
• The combining ratio changes to 2 : 8
• The gases chemically combined into a new compound — correct
• The two gases just mixed together
• The hydrogen disappeared