The Amazing World of Solutes, Solvents, and Solutions

1. Uniform and non-uniform mixtures

You may have made an Oral Rehydration Solution (ORS) at home — sugar and salt stirred into water. No matter which sip you take, it tastes the same. Why? Because the sugar and salt spread out evenly throughout the water.

When salt or sugar mixes with water, the components are evenly distributed and you cannot see them separately — this is a uniform mixture. But when chalk powder, sand, or sawdust is mixed with water, the bits stay visible and settle or float — this is a non-uniform mixture.

• Uniform mixtures (salt + water, sugar + water): components evenly spread, not visible separately. These are called solutions.
• Non-uniform mixtures (sand + water, chalk + water, sawdust + water): components visible to the naked eye or with a magnifier; they settle or float.

2. Solute, solvent and solution (Section 9.1)

A uniform mixture such as salt-and-water is called a solution. When a solid mixes into a liquid to form a solution:

• The solid component is the solute — it dissolves.
• The liquid component is the solvent — it does the dissolving.

When two liquids mix, it is not always obvious which dissolves which. The rule then is: the substance present in the smaller amount is the solute, and the one in the larger amount is the solvent.

Water is called a good ("universal") solvent because it dissolves so many substances. Why do sugar and salt dissolve in water but not in oil? Because they are attracted to and held by water particles, not by oil particles. (Air is also a uniform mixture of gases — so a mixture of gases can also be a solution.)

3. How much solute can a solvent dissolve? (Section 9.2)

Activity 9.1 — keep adding salt to half a glass of water, one spoon at a time, stirring each time. At first the salt dissolves completely. After a few spoons, a stage comes when the added salt stops dissolving and settles at the bottom. The water has reached its limit — it cannot dissolve any more salt at that temperature.

• Unsaturated solution: more solute can still be dissolved at that temperature (the salt keeps disappearing).
• Saturated solution: the solute stops dissolving and begins to settle — no more can dissolve at that temperature.

This answers the "too much sugar in tea" puzzle: once the tea is saturated, extra sugar just sits at the bottom. To dissolve it, you can heat the tea (raising temperature usually increases solubility).

4. Concentration: dilute vs concentrated

The amount of solute present in a fixed quantity of solution (or solvent) is its concentration. By comparing concentrations:

• Dilute solution: less solute (e.g. 1 spoon of salt in a glass of water).
• Concentrated solution: more solute (e.g. 4 spoons of salt in the same water).

Dilute and concentrated are relative terms — always a comparison. NCERT reflect: which is more concentrated — 2 spoons of salt in 100 mL of water, or 4 spoons in 50 mL? The second one: more solute packed into less solvent, so it is the more concentrated solution.

5. Solubility — the key definition

Solubility is the maximum amount of solute that can be dissolved in a fixed quantity (usually 100 mL) of a solvent at a particular temperature. The phrase "at a particular temperature" matters, because temperature changes solubility (next section).

6. Effect of temperature on solubility of solids (Section 9.2.1)

Activity 9.2 (demonstration) — take about 50 mL of water at 20 °C and add baking soda (sodium hydrogen carbonate), stirring, until some stays undissolved at the bottom (a saturated solution at 20 °C). Then:

• Heat to 50 °C: the undissolved baking soda dissolves. Add more until some again stays undissolved.
• Heat further to 70 °C: the leftover baking soda dissolves again.

Inference: water at 70 °C dissolves more baking soda than at 50 °C, which dissolves more than at 20 °C. For most solids, solubility increases as temperature rises. So a saturated solution can become unsaturated if it is heated — it can then take in more solute.

7. Solubility of gases (Section 9.3)

Many gases, including oxygen, dissolve in water — only to a small extent, but this dissolved oxygen is what keeps fishes, aquatic plants and other water organisms alive. Gases dissolved in water form a uniform mixture (a solution), since they spread evenly.

Temperature effect — opposite to solids: the solubility of gases decreases as temperature rises. So:

• Cold water holds more dissolved oxygen — good for aquatic life.
• Warm water holds less oxygen — when water heats up, dissolved oxygen escapes.

Remember the contrast: solids → more soluble when hot; gases → less soluble when hot.

8. Float or sink, and density (Sections 9.4 & 9.5)

Some objects float in water (a wooden stick, husk) and some sink (an iron rod, rice). A wooden stick and an iron rod can be the same size, yet the iron feels much heavier — because of a property called density, the "heaviness" packed into a given space.

Density is the mass present in a unit volume of a substance:

• Generally, objects that are less dense than the liquid float, and those denser than the liquid sink. (Density is not the only factor, but it is the main one.)
• Density does not depend on shape or size — a small and a large iron block have the same density.
• SI unit = kilogram per cubic metre (kg/m³). For liquids we often use g/mL or g/cm³.
• The mass of 1 mL of water ≈ 1 g at room temperature, so 100 mL of water ≈ 100 g.

9. Measuring mass and volume (density practical)

To find density you must measure mass and volume.

• Mass — measured with a balance (e.g. a digital weighing balance). Zero/"tare" it, place a watch glass, tare again, then place the object and read the mass. Mass (matter in an object, in g or kg) is not the same as weight (the pull of gravity, in newtons).
• Volume of liquids — use a measuring cylinder (tall, narrow, with markings). Read at the bottom of the curved surface, the meniscus, with your eye at its level. A 100 mL cylinder is often the best choice — accurate enough and lets you measure in one step.
• Volume of a regular solid (cuboid) — Volume = length × width × height. E.g. 25 cm × 18 cm × 2 cm = 900 cm³.
• Volume of an irregular solid (Activity 9.7, "displacement method") — note the water level in a measuring cylinder (say 50 mL), gently lower the object on a thread, note the new level (say 55 mL). Volume of object = final − initial = 5 mL = 5 cm³ (for solids, mL = cm³).

10. Effect of temperature and pressure on density (Sections 9.5.2 & 9.5.3)

• Temperature: generally, heating decreases density (and cooling increases it). On heating, particles spread out, so the volume increases while the mass stays the same; since Density = Mass/Volume, density falls. This is why a hot air balloon rises — the warm air inside is less dense than the cooler air around it.
• Pressure: mainly affects gases — higher pressure squeezes gas particles closer, so volume drops and density rises. Liquids and solids are nearly incompressible, so pressure has very little effect on their density.

11. Common mistakes to avoid

• Calling a sand–water mixture a "solution" — it is non-uniform, so it is not a solution.
• Thinking the larger amount is always the solvent: when a solid dissolves in a liquid, the solid is the solute even if there is a lot of it (Gulab Jamun syrup).
• Saying gas solubility increases with heat — it is the opposite (gases escape from warm water).
• Confusing saturated and unsaturated: an unsaturated solution can still dissolve more solute; a saturated one cannot.
• Forgetting "at a particular temperature" in the definition of solubility / saturated solution.
• Mixing up Density = Mass/Volume (not Volume/Mass), and forgetting that for solids 1 mL = 1 cm³.

12. Quick revision checklist

• Solution = uniform mixture; solute dissolves in solvent; solute is the smaller amount (but a solid is always the solute).
• Saturated = no more dissolves; unsaturated = more can dissolve. Solubility = max solute in fixed solvent at a given temperature.
• Heat ↑ solubility of solids; heat ↓ solubility of gases (cold water = more oxygen).
• Density = Mass/Volume. Denser than liquid → sinks; less dense → floats.
• Heating → volume ↑ → density ↓ (hot air balloon, floating ice). Pressure mainly affects gases.
• Measure mass with a balance, liquid volume with a measuring cylinder (read the meniscus), irregular solids by water displacement.