Solubility of gases

Gases form true solutions too. When a gas such as oxygen or carbon dioxide dissolves in a liquid such as water, the gas particles spread evenly throughout the liquid, just as sugar or salt particles do when a solid dissolves. The result is a single uniform mixture — you cannot see the dissolved gas, and it behaves like part of the liquid, even though it is still a gas chemically.

The temperature rule is reversed from solids. For solid solutes, warming the liquid usually lets it hold more dissolved solid — think of sugar dissolving faster and in larger amounts in hot tea. For gaseous solutes, the opposite happens: gas solubility generally decreases as temperature increases, and correspondingly increases as temperature falls. Cold water can hold more dissolved gas than warm water can.

Why the direction matters more than a number. At this level there is no formula, graph, or table of solubility values to calculate — the syllabus expects you to know and apply the direction of the trend confidently: colder means more dissolved gas, warmer means less. This qualitative rule is enough to explain a wide range of everyday and ecological situations.

Two everyday consequences worth holding onto. First, carbonated drinks (dissolved carbon dioxide) fizz out faster and go flat sooner when warm, because the warmth is actively pushing the gas back out of solution. Second, and more seriously, aquatic ecosystems depend on this trend: dissolved oxygen is essential for fish and other water-dwelling animals to breathe, and anything that warms a water body — sunlight, a heatwave, or hot water discharged from a factory — reduces how much oxygen that water can hold, putting aquatic life under stress.

Explain, in physical terms, why raising the temperature of a liquid drives dissolved gas particles back out into the surrounding air while a solid dissolved in the same liquid is generally encouraged to stay dissolved or dissolve further as the liquid gets hotter, and why this qualitative difference is treated as a fixed rule rather than something you need to calculate at this stage.

Step 1. Picture dissolved gas particles as tiny, fast-moving molecules sitting in the spaces between the liquid's own molecules, held there loosely rather than bonded strongly. Unlike a dissolved solid, which usually breaks apart into ions or molecules that fit more comfortably among the liquid's particles as energy is added, a dissolved gas is already a free, energetic particle whose natural tendency is to escape into open space.

Step 2. When you heat the liquid, you give all its particles — including the dissolved gas particles — more kinetic energy, so they move faster and more energetically. For a dissolved gas, this extra energy makes it easier for the gas particles to break free of the liquid's surface and re-enter the air above it, which is why warming pushes gas out of solution.

Step 3. For a dissolved solid, that same extra energy usually helps the liquid's molecules pull apart more of the solid's structure and keep more of it dissolved, which is why heating generally increases solid solubility — the opposite response to the same cause.

Step 4. Because gas particles are escaping into the air rather than being pulled apart from a rigid structure, cooling the liquid does the reverse: it slows the gas particles down, making it harder for them to escape, so more gas stays dissolved. This is exactly why refrigerated or naturally cold water holds more dissolved oxygen, and why a chilled fizzy drink keeps its carbon dioxide far longer than a warm one.

Step 5. At the Class 8 level, this behaviour is captured as a qualitative rule — solubility of gases decreases with rising temperature — without needing a numeric solubility value, a graph, or a formula; recognising and correctly applying the direction of the trend is what is expected.

Question: A student has two closed jars of water that have been sitting undisturbed: Jar A at room temperature (about 25 degrees Celsius) and Jar B taken straight from a refrigerator (about 5 degrees Celsius). Both jars were exposed to air and allowed to dissolve oxygen before being sealed. Which jar is likely to contain more dissolved oxygen? Explain using the temperature-solubility trend for gases.

Step 1. Identify the gas being dissolved: oxygen, entering the water from the surrounding air before each jar was sealed.

Step 2. Recall the trend: gas solubility in a liquid decreases as temperature increases, and correspondingly increases as temperature decreases.

Step 3. Compare the two jars: Jar B is colder (5 degrees Celsius) than Jar A (25 degrees Celsius).

Step 4. Apply the trend: since colder water has a higher solubility for gases, Jar B is expected to hold more dissolved oxygen than Jar A.

Step 5. Check against real life: this matches the everyday observation that cold water bodies generally support higher levels of dissolved oxygen, which is one reason cold, well-oxygenated streams can support more sensitive fish species than warm, still ponds.

Competitive stretch

Step 1. Now extend the reasoning: a pond that used to support healthy fish starts showing stressed and dying fish after a nearby factory begins releasing warm water into it, even though no new chemical pollutant is detected.

Step 2. The key change is a rise in the pond's water temperature, not the addition of any toxic substance.

Step 3. As the pond water warms, its capacity to hold dissolved oxygen decreases, so less oxygen remains available even though nothing was "added" to poison the water.

Step 4. With reduced dissolved oxygen, fish struggle to breathe — this explains the stress and deaths as thermal pollution reducing gas solubility, not chemical pollution.

• Applying the "solubility increases with temperature" rule learnt for solids to gases as well — gases follow the opposite trend, with solubility decreasing as temperature rises.
• Assuming dissolved gases do not form "real" solutions the way solids do — a gas dissolved evenly in a liquid is just as much a true, uniform solution as salt dissolved in water.
• Thinking that boiling or heating water is a good way to keep gases dissolved in it — heating actively drives dissolved gases out, which is why boiled water often tastes flat.
• Overlooking the ecological angle — a temperature rise in a lake or river is not just "warmer water," it directly means less dissolved oxygen for the organisms living in it.

Ask the tutor: Want to see how this same trend explains why deep, cold ocean water is often richer in dissolved gases than warm surface water?