Solubility & Henry's Law (NTCH05/02) Big idea: How much dissolves, and why it changes Solubility tells us the maximum amount of a solute that dissolves in a given amount of solvent at a specified temperature and pressure. Think of it as the 'capacity' of the solvent to hold the solute at given conditions. Changing temperature or pressure can shift this capacity. We will focus on gases dissolving in liquids (like CO2 in soda, O2 in water for fish), and also recall simple rules for solids in liquids ('like dissolves like'). Maximum amount of solute that dissolves in a given amount of solvent at specified temperature and pressure to form a saturated solution. Solubility Saturated solution A solution that holds the maximum possible amount of solute at given T and P; it is in equilibrium with undissolved solute. Unsaturated solution A solution containing less solute than its solubility at given T and P; it can dissolve more solute. A metastable solution containing more solute than the solubility limit at that T and P; often prepared by dissolving at higher T and then cooling gently. Unstable to seeding/shaking. Supersaturated solution A solution formed when a gas dissolves in a liquid solvent (e.g., CO2 in water). Gas–liquid solution Partial pressure The pressure a gas would exert if it alone occupied the container at the same temperature. In air, for O2, P O 2 ≈ 0.21 bar at sea level. At constant temperature, the partial pressure of a gas over a solution is directly proportional to its mole fraction in the solution (for gases that do not react with the solvent). Henry's Law Henry's constant ( K H ) Proportionality constant in Henry's Law (units often kbar for p = K H x). Larger K H means the gas is less soluble at a given pressure. Dissolution The process by which solute particles disperse uniformly among solvent particles to form a solution. 'Like dissolves like' — polarity rule of thumb Polar solvents (like water) dissolve polar or ionic solutes well due to strong ion–dipole or dipole–dipole interactions. Non-polar solvents (like hexane) dissolve non-polar solutes (like oils) via dispersion forces. This is a quick predictor for solid-in-liquid solubility at a given temperature. Saturated, unsaturated, supersaturated — visualising the limit If you keep adding solute to a solvent at fixed T and P, initially everything dissolves (unsaturated). At some point, extra solute no longer dissolves — you've reached saturation and an equilibrium sets between dissolved and undissolved solute. If you dissolve more solute at higher T and then cool quietly, you can get a supersaturated solution — it looks clear but is unstable, and a small seed crystal triggers rapid crystallisation. Effect of temperature on solubility Solids in liquids: For many ionic solids (e.g., potassium nitrate, KNO3), solubility increases with temperature — heating helps the solid break into ions and disperse. Some show only a slight increase (sodium chloride, NaCl). A famous exception is sodium sulfate (Na2SO4): its solubility increases up to about 32–33 ° C due to formation of the decahydrate (Glauber’s salt), then decreases (retrograde solubility) beyond this point. Gases in liquids: Solubility of gases decreases as temperature increases. As T , gas molecules have higher kinetic energy and escape the liquid more easily, so fewer remain dissolved. This is why warm water holds less dissolved oxygen than cold water — important for fish and aquaculture. Plot of solubility (g solute per 100 g water) vs temperature (0–100 C) for four salts: KNO3 (steep increase), NaCl (gentle increase), CaCl2 (moderate increase), Na2SO4 with a clear maximum at ~32 C and drop thereafter. Clean 2D graph, color-coded curves with legends. Vector textbook style. gpt-image-2 Representative solubility–temperature curves: KNO3 (sharp rise), NaCl (slight rise), CaCl2 (rise), and Na2SO4 showing a maximum around 32–33 ° C then decreasing. 2026-05-26T17:04:27.187Z Solid in liquid Usually increases KNO3, NaCl, CaCl2; exception: Na2SO4 peaks near 32–33 ° C Heating helps ions separate and disperse; hydrate formation/decomposition can cause anomalies Gas in liquid Decreases O2 in water, CO2 in soda Higher T means higher KE; gas escapes the liquid more easily Qualitative effect of temperature on solubility Case System Typical trend with T Example(s) Why (intuitive) Effect of pressure — crucial for gases For solids and liquids, pressure has negligible effect on solubility because they are nearly incompressible. For gases, pressure is the main control knob: higher partial pressure of the gas above the liquid pushes more gas molecules into solution until equilibrium is reached. Higher partial pressure of a gas above a liquid drives more gas molecules into solution; lowering the partial pressure does the opposite. Henry's Law — the linear link between pressure and dissolved amount At a fixed temperature, for a gas that does not react with the solvent, the equilibrium partial pressure of the gas above the solution is directly proportional to its mole fraction dissolved in the liquid. The proportionality constant is Henry’s constant ( K H ). Convention used in NCERT: larger K H means the gas is less soluble. Also, K H increases with temperature for most gases in water — so gases become less soluble as you heat. Here, P A is the partial pressure of gas A (often in bar), x A is its mole fraction in the liquid, and K H (often in kbar) depends on temperature and the gas–solvent pair. Henry's Law (NCERT convention) Alternate concentration form C A is concentration (e.g., mol L -1 ); K H ' is a Henry-type constant with units that match the concentration–pressure relation. Be careful which form and units you use. Graph of partial pressure P A (y-axis, 0–5 bar) vs mole fraction x A (x-axis, 0–0.003) showing a straight line through origin. Slope labeled K H . Clean axes, units indicated, neutral vector style, no extra text. gpt-image-2 Henry’s Law plot: a straight line through the origin when P A is plotted against x A at constant temperature. 2026-05-26T17:04:27.359Z CO2 1.67 Highest among listed O2 34.86 Lower than CO2, higher than N2/He H2 71.18 Low N2 76.48 Low (slightly less soluble than H2) He 144.97 Very low (least soluble among listed) Henry's constants ( K H ) for gases in water at 298 K (NCERT; units kbar for P A = K H x A ) Entry Gas K H (kbar) Relative solubility (inverse of K H) Bigger K H means LOWER solubility (for P A = K H x A ). Temperature up (T ) usually makes K H go up and the gas less soluble. Keep pressure units consistent with K H (kbar with kbar; bar with bar). neet-alert Henry's Law constant K H itself is the solubility of a gas. K H is the proportionality linking P A and x A ( K H = P A / x A ). A higher K H means, at the same pressure, a smaller mole fraction dissolved — i.e., lower solubility. Henry's Law holds for gases that do not react chemically with the solvent and works best at low to moderate pressures and suitable temperatures. Ammonia (NH3) and HCl react with water; they deviate strongly. Henry's Law applies to all gases in all conditions. Warm soda holds more CO2, so it should fizz less when opened. Gas solubility decreases as temperature increases. Cold soda holds more CO2. When you open a bottle, the pressure drop causes CO2 to come out — warm soda loses CO2 even faster because its solubility is already lower. Quick numericals — how to calculate dissolved gas Workflow for P A = K H x A : pick consistent units, then x A = P A / K H . If you need moles or concentration, connect x A to total moles of solution. Example 1 (CO2 in a sealed drink): At 298 K, suppose P C O2 = 2.5 bar above the liquid. With K H (CO2) = 1.67 kbar = 1670 bar, x C O2 = 2.5 / 1670 ≈ 1.50 10 -3 . Example 2 (O2 in water exposed to air): At 298 K, air has P O 2 ≈ 0.21 bar = 0.00021 kbar. With K H (O2) = 34.86 kbar, x O 2 = 0.00021 / 34.86 ≈ 6.0 10 -6 . This very small mole fraction matches the idea that O2 is only sparingly soluble in water. Steps to avoid mistakes Check which Henry form is used: P A = K H x A or C A = K H ' P A . Match units: if K H is in kbar, convert P A to kbar. Identify the gas: larger K H means smaller solubility at the same P A . State temperature: K H depends strongly on T. If asked for mass of gas dissolved, compute moles from x A and then multiply by molar mass. Real-life applications Carbonated drinks: CO2 is dissolved at high pressure during bottling; when opened, pressure drops, solubility falls, and CO2 escapes as fizz — a direct Henry's Law effect. Scuba diving: At depth, total pressure increases, so more N2 dissolves in blood and tissues. If a diver ascends too quickly, pressure drops rapidly and N2 comes out as bubbles, causing decompression sickness ('the bends'). Using a helium–oxygen mix reduces the N2 partial pressure, reducing risk. Fish-farming (aquaculture): Colder water holds more dissolved O2; aeration techniques and temperature control maintain healthy O2 levels for fish. Blood-gas analysis: Dissolved O2 in plasma follows Henry’s Law and is measured as arterial oxygen tension (PaO2). Most O2 transport is via chemical binding to haemoglobin (not described by Henry’s Law), which greatly increases oxygen-carrying capacity. Sealed soda: high CO2 pressure means high dissolved CO2. Opening reduces pressure, so CO2 rushes out as bubbles. Diving and bends: At depth, more N2 dissolves. Rapid ascent causes N2 bubbles to form in tissues and blood — painful and dangerous. clinical Decompression sickness ('the bends'): Prevent with slow, staged ascent or decompression stops. Breathing He–O2 mixes reduces nitrogen partial pressure and hence its dissolved amount (Henry’s Law), lowering bubble risk. Only the dissolved fraction of O2 in plasma follows Henry’s Law and is reported as PaO2. The majority of O2 is carried bound to haemoglobin, which is a separate chemical equilibrium. remember KH high, gas shy — larger K H means the gas is shy to dissolve. Practice check (what you should be able to do) gpt-image-2 Diagram of aquarium cross-section with air stone bubbling. Labels: colder water more O2 solubility, increased local P O 2 at bubble–water interface. Vector style, clean labels, no extra text in the figure. 2026-05-26T17:04:28.865Z Putting it together: schematic of an aquarium with air stone — cold water holds more O2; higher aeration increases partial pressure at the interface for better dissolution.