9.3 Solubility of Gases
Solids dissolve in water — but do gases? And if so, does temperature change things the same way? The answer holds the secret to life underwater.
Gases Dissolve Too Many gases, including oxygen, dissolve in water — though oxygen only to a small extent. Tiny as that amount is, this dissolved oxygen is what keeps aquatic life alive: the fish, plants and other creatures in a river or pond breathe the oxygen dissolved in their water. Since the gas spreads evenly, gas-in-water is a uniform mixture — a solution.
The opposite rule
Temperature and Gas Solubility Here gases behave opposite to solids: the solubility of a gas generally decreases as temperature rises. So cold water holds more dissolved oxygen than warm water.
| Solids | Gases | |
|---|---|---|
| Solubility as temperature rises | increases | decreases |
Why This Matters for Fish Because cold water dissolves more oxygen, cool rivers and lakes can support more aquatic life. When water warms up (in summer, or from pollution and warm run-off), it holds less oxygen — which can leave fish struggling to breathe. A small physics rule with big consequences for ecosystems (a theme we’ll return to in Chapter 12).
Don’t Mix Up the Two Rules A common exam trap: “Oxygen is more soluble in hot water.” False! That’s the rule for solids. For gases, it’s the reverse — more gas dissolves in cold water. Keep the two straight: solids like it hot, gases like it cold.