Temperature and solid solubility

Solubility is the maximum amount of a solute that can dissolve in a fixed quantity of solvent (the textbook convention uses 100 mL of solvent, or of solution) at a particular temperature, before the solution becomes saturated — that is, before no more solute will dissolve no matter how much you stir. The temperature trend for solids is simple and worth stating precisely: as temperature rises, the solubility of most solid solutes in a liquid solvent also rises. A warmer solvent can accommodate a larger mass of dissolved solid before it reaches its saturation point. This is not automatic for every kind of solute — it is specifically the pattern seen for solids dissolving in liquids, and it is the opposite of what happens with gases, whose solubility in liquids generally falls as temperature rises (which is why a warm fizzy drink loses its bubbles faster than a cold one). A classroom activity makes the solid trend concrete: take three identical beakers, each with 100 mL of water at 20°C, 50°C and 70°C, and add baking soda to each, one spoon at a time with stirring, until no more dissolves. The beaker at 70°C accepts noticeably more baking soda before turning saturated than the beaker at 20°C, and the 50°C beaker falls in between. Plotted as a simple bar chart, the amount of solute dissolved climbs steadily as the starting temperature climbs. An important consequence follows directly: a solution saturated at a lower temperature can be made unsaturated simply by heating it, with no extra solvent added. The warmer solvent has more "room" (a higher solubility limit) for that same solute, so any solid still sitting undissolved at the bottom gets a fresh chance to dissolve.

There is no numeric formula for this trend at Class 8 level — no solubility curve, no graph reading, no calculation. The reasoning stays qualitative, and it rests on two ideas the chapter already builds. First, solubility is always tied to a particular temperature — it is not a single fixed number for a substance. Saying "the solubility of baking soda in water" without stating a temperature is incomplete, the same way saying "the volume of the box" without stating its dimensions would be. Every solubility value quoted in the chapter (and on any solubility table you might meet later) comes attached to a temperature. Second, heating a solvent changes its capacity to hold dissolved solid — it raises the amount needed to reach saturation. This is why the same 100 mL of water, tested at three different temperatures with the same solute, gives three different "maximum amount dissolved" readings, each one larger than the last as temperature climbs. The undissolved solid at the bottom of a saturated beaker is not "stuck" — it is simply solid that the cooler water could not accommodate; give the water more thermal energy and some (or all) of it dissolves. This single trend explains several everyday observations at once: tea dissolves sugar faster and in greater quantity than iced water does; a supersaturated sugar solution used to grow rock-candy crystals is made by dissolving sugar in hot water (near its higher-temperature solubility limit) and then cooling it, so the solution suddenly holds more dissolved sugar than the cooler water can sustain, forcing crystals to grow out of solution; and a jar of honey with sugar crystals at the bottom often clears up when gently warmed.

Question: A student dissolves baking soda in 100 mL of water at 20°C, stirring until no more dissolves — some powder remains undissolved at the bottom, so the solution is saturated. Without adding any more water, she heats the same beaker to 70°C. She then repeats the whole experiment with a fresh 100 mL of water and sugar instead of baking soda, comparing a beaker kept at 10°C with an identical beaker kept at 60°C, adding sugar one spoon at a time with stirring after each addition. (a) What happens to the undissolved baking soda on heating, and why? (b) In which sugar beaker does more sugar dissolve before saturation, and why?

Step 1. For part (a), identify what changed and what stayed the same. The amount of water (100 mL) and the identity of the solute (baking soda) are unchanged; only the temperature changed, from 20°C to 70°C.

Step 2. Apply the temperature trend. Since solid solubility increases with temperature, the water's capacity to dissolve baking soda is higher at 70°C than it was at 20°C.

Step 3. Conclude for part (a). Some or all of the previously undissolved baking soda now dissolves into the warmer water, because the water at 70°C can hold more dissolved baking soda before becoming saturated. The solution, saturated at 20°C, becomes unsaturated (or saturated with far less leftover solid) at 70°C.

Step 4. For part (b), again isolate the variable. Both beakers have the same 100 mL of water and the same solute (sugar); only the beaker temperature differs, 10°C versus 60°C.

Step 5. Apply the same trend and conclude. Since solid solubility rises with temperature, the 60°C beaker has a higher solubility limit for sugar than the 10°C beaker. More sugar therefore dissolves in the 60°C beaker before it reaches saturation, while the 10°C beaker reaches saturation — and starts showing undissolved sugar at the bottom — after fewer spoons have been added.

• Assuming all solubility rises with temperature — the "increases with temperature" trend is specifically for solid solutes; gas solubility in liquids generally decreases as temperature rises.
• Quoting solubility as a single fixed number for a substance, forgetting it must always be stated at a particular temperature.
• Confusing solubility (the maximum amount that can dissolve at that temperature) with the amount actually dissolved so far, which can be less (an unsaturated solution).
• Believing heating dissolves undissolved solid instantly — heating raises the solvent's capacity, but stirring and time are still needed for the solid to actually go into solution.

Ask the tutor: Want to explore why gas solubility behaves the opposite way, or try a few more saturated-versus-unsaturated practice questions?