Solubility Curves

Each line on the graph represents one compound. For any point directly ON the curve, the solution is exactly saturated at that temperature. A point BELOW the curve represents an unsaturated solution (less solute than the maximum). A point ABOVE the curve represents a theoretical supersaturated solution — more solute than should normally be able to dissolve at that temperature.

Solubility data are almost always quoted per 100 g of water, but real problems rarely use exactly 100 g. Because solubility is a ratio, the maximum mass of solute scales in direct proportion to the mass of water: twice the water dissolves twice the solute, and half the water dissolves half. So multiply the solubility by the mass of water divided by 100 g: $m_{\text{max}}=S\times \frac{m_{\text{water}}}{100\text{ g}}$. At 40°C KCl's solubility is 40 g/100 g, so 250 g of water can hold at most 40 g × 250 g ÷ 100 g = 100 g of KCl. Going the other way, you can convert any actual mixture into a 'per 100 g' value — 30 g of solute in 60 g of water is the same ratio as 50 g per 100 g — and then compare it directly with the curve. Since 1 mL of water has a mass of about 1 g, 250 mL of water can be treated as 250 g.

Dissolved gases behave oppositely to dissolved solids. As water warms up, dissolved oxygen (${\text{O}}_2$) becomes noticeably less soluble — about 14.6 mg/L at 0°C, dropping to about 7.6 mg/L at 30°C. This happens because gas particles are already free and moving fast; heating the water gives gas molecules even more kinetic energy, letting them escape the liquid more easily and return to the gas phase above the surface. This is why a warm, flat soda has fewer bubbles than a cold one, and why warm lake water in summer can hold less dissolved oxygen — a real concern for fish and other aquatic life. Thermal pollution makes it worse: power plants and factories that draw cooling water from a lake or river return it several degrees warmer, and warm water also speeds up fish metabolism, so they need more oxygen just as less is available. Pressure matters too, but only for gases: the harder a gas presses on a liquid's surface, the more gas particles are pushed into solution. Soft drinks are sealed under high carbon dioxide pressure; opening the cap releases the pressure, and the extra dissolved ${\text{CO}}_2$ escapes as bubbles.

If you saturate water with a solid solute at a high temperature and then cool it, the solubility curve tells you exactly how much solute must come back out of solution (crystallize) — because the maximum amount that CAN stay dissolved drops as temperature drops. The particles that can no longer stay dissolved leave the solution and build onto crystal surfaces. Chemists use this deliberately to purify solids (recrystallization): dissolve an impure sample in the minimum amount of hot solvent, let it cool, and collect the crystals. Small amounts of impurities never reach their own saturation point, so they stay behind in the solution.