Notes Chapter 9 Science Curiosity Class 8 CBSE Board

Notes For All Chapters – Science Curiosity Class 8

The Amazing World of Solutes, Solvents, and Solutions

1. Mixtures: Uniform and Non-uniform

• When sugar or salt dissolves in water → a uniform mixture is formed (solution).
• When chalk powder, sand, or sawdust is mixed with water → a non-uniform mixture is formed.
• Uniform mixtures = Solutions (components not seen separately).
• Non-uniform mixtures = Components remain separate and visible.

2. Solute, Solvent, and Solution

1. Solute: The substance that dissolves (smaller quantity).
2.Solvent: The substance in which solute dissolves (larger quantity).
3. Solution: Uniform mixture of solute and solvent.

• Example: Salt in water → Salt = solute, Water = solvent.
• Example: Sugar syrup (Gulab Jamun chashni) → Sugar = solute, Water = solvent.
• Example: Air → Nitrogen (largest amount) = solvent, Oxygen, Carbon dioxide, Argon = solutes.

3. How Much Solute Can Dissolve in Solvent?

• At first, solute dissolves completely.
• After a stage, no more solute dissolves → saturation point reached.

Types of Solutions

• Unsaturated Solution → More solute can dissolve at that temperature.
• Saturated Solution → No more solute can dissolve at that temperature.
• Concentrated Solution → Contains large amount of solute.
• Dilute Solution → Contains small amount of solute.

Solubility

• Maximum solute that dissolves in a fixed amount of solvent at a particular temperature.

4. Effect of Temperature on Solubility (Solids in Liquids)

• Generally, solubility increases with temperature.
• Example: Baking soda dissolves more in hot water than in cold water.
• A solution saturated at lower temperature may become unsaturated if heated.

5. Solubility of Gases in Liquids

• Many gases dissolve in water (e.g., oxygen).
• Dissolved oxygen is necessary for aquatic life.

Effect of Temperature:

• As temperature increases → solubility of gases decreases.
• Cold water dissolves more oxygen → supports aquatic life.
• Warm water → less oxygen dissolved → harmful for aquatic life.

6. Floating and Sinking in Water

• Some objects float (oil, husk), some sink (rice, sand).
• Common belief: lighter objects float, heavier objects sink.
• Actual reason → depends on density of the substance.

7. Density

• Definition: Mass per unit volume.
• Formula:

• Units:
  • SI unit → kg/m³.
  • Common units → g/mL, g/cm³.

Relative Density

• Comparison with water:

• No unit. Example: Aluminium has relative density = 2.7 (2.7 times denser than water).

8. Measuring Mass

• Measured using a balance (digital or traditional).
• Mass = quantity of matter.

Note: Mass ≠ Weight.

• Mass → amount of matter (kg, g).
• Weight → force of gravity on object (Newton, N).

9. Measuring Volume

For liquids: Measured using a measuring cylinder (5 mL, 10 mL, 50 mL, 100 mL etc.).

• Read volume at bottom of meniscus (curved surface).
• For coloured liquids → read at top of meniscus.

For regular solids (cuboid, box, dice):Volume=l×w×h

For irregular solids (stone, key):

• Use water displacement method in a measuring cylinder.
• Volume of water displaced = Volume of object.

10. Density Calculation

• Once mass and volume are known → Density = Mass ÷ Volume.
• Example: Mass = 16.4 g, Volume = 5 cm³ → Density = 3.28 g/cm³.

11. Effect of Temperature on Density

• Heating → particles spread apart → volume increases → density decreases.
• Cooling → particles come closer → volume decreases → density increases.
• Example: Hot air rises because it is less dense. (Hot air balloon principle).

12. Effect of Pressure on Density

• Gases: Increase in pressure → particles come closer → density increases.
• Liquids and Solids: Hardly affected by pressure because particles are already close.

13. Why Does Ice Float on Water?

• Water has highest density at 4°C.
• When water freezes at 0°C → it expands → occupies more volume → density decreases.
• Ice becomes lighter than water → floats.
• Floating ice keeps water below it warm enough for aquatic life to survive.

14. Examples and Applications

• Egg in water: Egg sinks in normal water, but floats if salt is added (water becomes denser).
• Oil packets: 1 L oil weighs less than 1 kg → oil is less dense than water.
• Bamboo rafts: Bamboo is hollow and light → floats easily.
• Earth’s layers: Density increases from crust to inner core.

15. Snapshots (Summary Points)

• Solution = Uniform mixture.
• Solute = Dissolving substance.
• Solvent = Medium in which solute dissolves.
• Saturated solution = Cannot dissolve more solute.
• Unsaturated solution = Can dissolve more solute.
• Solubility of solids ↑ with temperature.
• Solubility of gases ↓ with temperature.
• Density = Mass ÷ Volume.
• Density ↓ when temperature ↑.
• Pressure affects gases most.
• Ice floats on water because it is less dense.