The Air Hiding in Water
Dissolved gases keep ponds alive
What this lesson covers
Why it matters
A fish spends its entire life underwater and never once comes up for a breath. Yet it needs oxygen every second — oxygen it finds hidden inside the water itself, as invisible dissolved air.
Predict first
Warm a flask of tap water gently. Long before it boils, tiny bubbles cling to the glass and rise. What are they?
All natural water holds dissolved air. The solubility of a gas always falls as temperature rises, so gentle warming squeezes the air out — and boiling expels it almost completely.
- Dissolved air escaping — gases dissolve less as water warms — correct
- Hydrogen and oxygen from water molecules splitting apart
- Steam — water always bubbles when warmed
What you do
Every card is a clue about gases hiding in water. Sort each clue to the dissolved-gas story behind it.
Check yourself
How do fish survive without ever coming up for air?
Marine life breathes the oxygen of dissolved air — about 40 cm³ of dissolved oxygen in every litre of water. That is why aquatic life suffocates in poorly aerated water.
Ordinary air is about 21% oxygen, but the air dissolved in water is about 33% oxygen. Why the difference?
Both gases dissolve, but oxygen dissolves more readily. The result: dissolved air is roughly 33% oxygen, 66% nitrogen and 1% carbon dioxide — a richer mix than the atmosphere.
Boiled-and-cooled water tastes flat. How can you bring its taste back?
The pleasant taste of drinking water comes from dissolved air, carbon dioxide and minerals. Boiling drives the gases out; aeration — mixing the water with air — dissolves them back.
- They use the oxygen of the air dissolved in water for respiration — correct
- Their gills split water molecules into hydrogen and oxygen
- They store enough air to last from birth
- Oxygen is more soluble in water than nitrogen, so dissolved air is oxygen-richer — correct
- Fish absorb the nitrogen faster than the oxygen
- Nitrogen is too heavy to dissolve at all
- Aerate it — shake it with air so the gases dissolve back in — correct
- Boil it again, but for longer
- Nothing can — boiling turns water into a new compound