Most solids dissolve MORE in hot water, but gases dissolve LESS in hot water and MORE in cold water. Gas solubility also goes UP when pressure goes up (like a sealed soda bottle). Memory hook: "Solids love heat, gases love cold and squeeze." This matters for NEET because one line about temperature or pressure decides the answer in Assertion-Reason and "correct statement" questions.
Left: with rising temperature, most solids dissolve more (blue up) while gases dissolve less (red down). Right: raising the pressure forces more gas to dissolve in the liquid (Henry's law), as in a sealed soda bottle.
Your doubts, answered
Do solids dissolve more in hot or cold water?
Most solids dissolve MORE in hot water. NCERT says the dissolving of most salts is endothermic (it takes in heat, so the dissolution enthalpy is positive), so raising the temperature helps them dissolve. That is why sugar and most salts dissolve faster and in larger amounts in hot water. Note: this is the general rule, not a fixed law for every solid.
Why do gases dissolve LESS in hot water?
Dissolving a gas in water gives out heat (it is exothermic). When you heat the water, you push this equilibrium backward, so the gas escapes. That is why boiling water loses its dissolved air as small bubbles before it even boils. So gas solubility goes DOWN as temperature goes UP, and UP as temperature goes DOWN. This is the exact opposite of most solids.
Why are aquatic animals more comfortable in cold water?
Fish and other aquatic species breathe the oxygen dissolved in the water. Since gas solubility increases as temperature falls, cold water holds MORE dissolved oxygen than warm water. So cold water has more oxygen to breathe, and that is why NCERT says aquatic species are more comfortable in cold water. This exact line was tested in NEET 2026.
How does pressure change gas solubility?
For gases, higher pressure means MORE gas dissolves. This is Henry's law: the amount of gas dissolved is proportional to the pressure of that gas above the liquid. A soda bottle is sealed under high CO2 pressure, so a lot of CO2 stays dissolved. When you open it, pressure drops, so CO2 fizzes out and the drink slowly goes flat. Pressure barely affects solid solubility, but strongly affects gas solubility.
Does pressure affect the solubility of solids too?
No, not much. Solids and liquids are nearly incompressible, so squeezing them changes their solubility very little. Pressure is important only for GASES dissolving in liquids. A common NEET trap is a statement saying 'solubility of a gas increases with DECREASE in pressure' — that is false; it increases with an INCREASE in pressure.
⚠️ The NEET trap ✗ Gas solubility increases when you HEAT the water, just like solids do. ✓ Gas solubility DECREASES when you heat the water (dissolving a gas is exothermic), and it increases when you cool it. Only most SOLIDS dissolve more on heating. 🧠 NEET loves the reversal: solids and gases behave OPPOSITE with temperature. In the 2026 'correct statements' question, statement (C) 'aquatic species are more comfortable in cold water' is TRUE for exactly this reason, while (D) 'gas solubility increases with DECREASE in pressure' is FALSE. Read temperature vs pressure words very carefully.
Real NEET questions
NEET 2023 Phase 1
Assertion (A): Helium is used to dilute oxygen in diving apparatus. Reason (R): Helium has high solubility in O2. Choose the correct answer.
A · Both A and R are true and R is the correct explanation of A
B · Both A and R are true and R is NOT the correct explanation of A ✓
C · A is true but R is false
D · A is false but R is true
Solution: The Assertion is true: helium really is mixed with oxygen for deep-sea divers. Divers face very high pressure, and by Henry's law a highly soluble gas like nitrogen would dissolve into the blood and then bubble out on the way up, causing 'the bends'. Helium is used because it has LOW solubility in blood, so it avoids this. The Reason as worded talks about high solubility in O2, which does not explain the use. The official NEET key marks option (B) — both statements taken as true but R not the correct explanation of A. Lesson: gas solubility under pressure (Henry's law) is the real reason behind diving mixtures.
NEET 2024
The Henry's law constant (KH) values of three gases (A, B, C) in water are 145, 2 x 10^-5 and 35 kbar, respectively. The solubility of these gases in water follows the order:
A · B > C > A ✓
B · A > C > B
C · A > B > C
D · B > A > C
Solution: Henry's law is p = KH x, where x is the mole fraction (solubility) of the gas. Rearranged, x = p / KH, so at the same pressure a SMALLER KH means MORE gas dissolves. The KH values in order are A (145) > C (35) > B (2 x 10^-5). Since solubility is the inverse of KH, the solubility order flips to B > C > A. Answer: (A). Remember: big KH means the gas barely dissolves; small KH means it dissolves a lot.
NEET 2026
Identify the correct statements: (C) Aquatic species are more comfortable in cold water. (D) The solubility of a gas increases with decrease in pressure. (Two statements shown from the full question.)
A · A, B and C only ✓
B · A and B only
C · A and C only
D · A, D and E only
Solution: Focus on the gas-solubility statements. (C) is TRUE: gas solubility rises as temperature falls, so cold water holds more dissolved oxygen, and aquatic species are more comfortable in it — this is straight from NCERT. (D) is FALSE: gas solubility increases with an INCREASE in pressure (Henry's law), not a decrease. With the arithmetic statements (A) and (B) also correct, the right choice is 'A, B and C only', option (A). The trap is entirely in the direction words: cold = more gas, high pressure = more gas.
Solved Solutions NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What is the one-line rule for solubility with temperature?
Most solids dissolve more in hot water, but gases dissolve more in cold water. They behave in opposite ways with temperature.
Why does soda water go flat after opening?
The bottle is sealed under high CO2 pressure, so a lot of CO2 stays dissolved. Opening it drops the pressure, so CO2 escapes and the drink goes flat over time.
Is pressure important for dissolving solids?
No. Pressure has almost no effect on solids or liquids because they cannot be compressed much. Pressure matters mainly for dissolving gases in liquids.
Which gas law connects pressure and gas solubility?
Henry's law. It says the amount of a gas dissolved in a liquid is proportional to the pressure of that gas above the liquid. Higher pressure means more dissolved gas.
Why does boiling water lose its dissolved gases?
Dissolving a gas gives out heat (exothermic). Heating pushes the equilibrium the other way, so dissolved air comes out as tiny bubbles even before the water boils.
