What this chapter is about
This chapter introduces you to solutions — the mixtures we use every day without even thinking about them. When you stir sugar into water or add salt to dal, you are making a solution. Understanding how substances dissolve helps explain cooking, cleaning, medicines, and many industrial processes.
You will learn what solutes and solvents are, how solutions differ from other mixtures, and what makes some solutions stronger or weaker than others. The chapter also covers saturation — the point where no more solute can dissolve — and factors that affect how much of a substance dissolves in a liquid.
By the end, you should be able to identify solutions around you, describe them using correct terms, and predict how temperature or stirring changes how fast something dissolves.
Key ideas
- A solution is a uniform mixture of two or more substances where the particles are so small they cannot be seen, even under a microscope.
- The substance that dissolves is called the solute; the substance in which it dissolves is called the solvent. In a glass of lemonade, sugar and lemon juice are solutes, and water is the solvent.
- Water is called the universal solvent because it dissolves more substances than any other common liquid.
- A solution that contains a small amount of solute compared to solvent is called dilute; one with a large amount of solute is called concentrated.
- When no more solute can dissolve at a given temperature, the solution is saturated. If you add more solute, it settles at the bottom.
- Solubility is the maximum amount of solute (in grams) that dissolves in 100 grams of solvent at a particular temperature.
- Temperature usually increases solubility of solids in liquids: hot water dissolves more sugar than cold water.
- Stirring or crushing the solute speeds up dissolving but does not change the total amount that can dissolve.
Formulas and facts to remember
1. Solution = Solute + Solvent. The solute is usually present in a smaller amount. 2. Solubility is measured in grams of solute per 100 g of solvent at a fixed temperature. 3. Increasing temperature generally increases solubility of solid solutes in water. 4. Gases dissolve better in cold liquids than in hot liquids — this is why a cold soft drink is fizzier. 5. A saturated solution at a higher temperature may become supersaturated when cooled, and extra solute can suddenly crystallise out. 6. Not all liquids mix: oil does not dissolve in water because water molecules attract each other more strongly than they attract oil molecules. 7. Common solvents besides water include alcohol, kerosene, and acetone (nail-polish remover).
Worked examples
### Example 1: Identifying solute and solvent Rina adds 2 teaspoons of salt to 500 mL of water to make saline for gargling.
Step 1: Identify what dissolves — salt dissolves, so salt is the solute. Step 2: Identify what it dissolves in — water is present in a larger amount and does the dissolving, so water is the solvent. Answer: Solute = salt; Solvent = water.
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### Example 2: Comparing dilute and concentrated solutions Arun makes two glasses of sharbat. Glass A has 1 spoon of syrup in 200 mL water. Glass B has 4 spoons of syrup in 200 mL water.
Step 1: Both glasses have the same amount of solvent (water). Step 2: Glass B has more solute (syrup) than Glass A. Answer: Glass A is dilute; Glass B is concentrated.
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### Example 3: Using solubility data At 20 °C, the solubility of common salt in water is about 36 g per 100 g of water. Meera has 50 g of water. How much salt can she dissolve at 20 °C?
Step 1: Solubility tells us 36 g dissolves in 100 g water. Step 2: For 50 g water (half of 100 g), maximum salt = 36 ÷ 2 = 18 g. Answer: Meera can dissolve up to 18 g of salt before the solution becomes saturated.
Common mistakes
- Thinking the solvent must always be water → Many liquids act as solvents; nail-polish dissolves in acetone, not water.
- Believing stirring increases solubility → Stirring only speeds up dissolving; it does not let more solute dissolve than the maximum for that temperature.
- Confusing saturated with concentrated → A saturated solution has reached its limit; a concentrated solution simply has a lot of solute but may still dissolve more.
- Assuming all substances dissolve in water → Some substances like sand or wax do not dissolve in water at all.
- Forgetting that gas solubility behaves opposite to solids → Gases dissolve less as temperature rises, which is why warm soda goes flat quickly.
Quick revision
- Solution = Solute + Solvent; solute is usually the smaller component.
- Water is the universal solvent; it dissolves many, but not all, substances.
- Dilute means less solute; concentrated means more solute; saturated means maximum solute.
- Heating usually helps solids dissolve faster and in greater amounts.
- Gases dissolve better in cold liquids.
- Solubility is measured in grams of solute per 100 g of solvent at a given temperature.