The Amazing World of Solutes, Solvents, and Solutions

Chapter at a glance

• A uniform mixture of two or more substances is called a solution.
• In a solution formed by a solid and a liquid, the solid is the solute and the liquid is the solvent; when two liquids mix, the component present in smaller amount is the solute.
• A saturated solution contains the maximum amount of solute that can dissolve at a given temperature; an unsaturated solution can dissolve more solute at that temperature.
• Solubility is the maximum amount of solute that dissolves in a fixed quantity of solvent at a particular temperature.
• For most solids, solubility increases with rise in temperature; for gases, solubility decreases with rise in temperature.
• Density is the mass per unit volume of a substance and is expressed as Density = Mass/Volume.
• Density generally decreases with increase in temperature; pressure affects density of gases more than that of liquids and solids.
• Objects float or sink in a liquid depending on their density relative to the liquid; ice floats on water because its density is lower than that of liquid water at 0 °C.

Definitions and laws

• “A uniform mixture, such as that of salt or sugar, and water, is called a solution.”
• “Whenever a solid is mixed with a liquid to form a solution, the solid component is called the solute, and the liquid component is called the solvent.”
• “When a solution is formed by mixing two liquids, … the substance present in smaller amount is called the solute, while the one in larger amount is called the solvent.”
• “The solution in which more solute can be dissolved at a given temperature, is called an unsaturated solution.”
• “When the solute stops dissolving and begins to settle at the bottom, the solution is called a saturated solution at that particular temperature.”
• “The amount of solute present in a fixed quantity of solution (or solvent) is termed as its concentration.”
• “The maximum amount of solute that dissolves in a fixed quantity of the solvent is called its solubility.”
• “Density is defined as the mass present in a unit volume of that substance.”
• “Density = Mass/Volume”
• SI unit of density: kilogram per cubic metre (kg/m³); other units used: g/mL, g/cm³.
• “Relative density of any substance with respect to water = Density of that substance / Density of water at that temperature” (a number without units).
• “The mass of 1 mL of water is close to 1 g at room temperature.”

Important diagrams and activities

• Fig. 9.1 (sugar/salt + water) and Fig. 9.2 (sand/sawdust + water): show uniform vs non-uniform mixtures.
• Fig. 9.3: magnified view of solute particles evenly distributed in solvent.
• Fig. 9.5 & 9.6: unsaturated and saturated solutions (undissolved solute settles).
• Activity 9.1 & 9.2: adding solute till saturation; effect of heating on solubility of baking soda.
• Fig. 9.7: heating setup to demonstrate temperature effect on solubility.
• Fig. 9.8: aquatic life supported by dissolved oxygen.
• Fig. 9.9: objects that float or sink.
• Fig. 9.12: using digital balance and tare to measure mass.
• Fig. 9.15–9.18: measuring cylinder, meniscus reading for volume of liquids.
• Activity 9.7 & Fig. 9.19: displacement method to find volume of irregular solids.
• Fig. 9.23: ice floating on water.
• Fig. 9.22: hot-air balloon (hot air less dense).

Common misconceptions and exam pitfalls

• Confusing saturated solution with concentrated solution or assuming a saturated solution cannot be made more concentrated by changing temperature.
• Stating that solubility of all substances increases with temperature (gases decrease).
• Writing units incorrectly (using kg/m³ for liquids in school experiments) or forgetting that relative density has no unit.
• Reading the top of the meniscus for water or forgetting to keep eyes level with the meniscus.
• Assuming density depends on shape/size instead of mass per unit volume; forgetting that 1 mL water ≈ 1 g.
• Thinking all objects heavier than water sink (density comparison, not absolute weight, decides floating/sinking).

Formula sheet