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The Amazing World of Solutes, Solvents, and Solutions – Class 8 Science notes, important questions and MCQs

NCERT Class 8 Science, chapter 9. Notes, key terms, important questions with answers and MCQs.

Chapter overview

A solution forms when a solute dissolves evenly in a solvent. How much dissolves depends on the substances, temperature and stirring.

Solutions

Solution = solute + solvent (salt + water). Water is called the universal solvent.
Saturated solution: no more solute can dissolve at that temperature.
Solubility of most solids increases with temperature; solubility of gases decreases with temperature (fish need cool, oxygen-rich water).

Concentration and examples

Concentration = amount of solute in a given amount of solution (dilute or concentrated).
Mass % = solute / solution × 100.
Solutions can be solid (alloys like brass), liquid (sugar syrup) or gas (air).

Key terms

Solute: substance that dissolves
Solvent: substance in which the solute dissolves
Saturated solution: solution that cannot dissolve more solute at that temperature
Solubility: maximum amount of solute that dissolves in a fixed amount of solvent at a given temperature

Formulas

Mass % = (mass of solute / mass of solution) × 100

Important questions with answers

What is a saturated solution?2 marks

A solution in which no more solute can dissolve at that temperature; extra solute stays undissolved.

Why is water called the universal solvent?3 marks

Water can dissolve a very large number of substances (salts, sugar, many gases), more than most other liquids.

Describe an activity to show that the solubility of a substance changes with temperature, and explain why fish can find it hard to breathe in warm water.5 marks

Take 50 mL of water at room temperature and add salt or sugar spoon by spoon, stirring, until no more dissolves; count the spoons. Repeat with 50 mL of hot water. More spoons dissolve in hot water, showing solubility of solids usually increases with temperature. Gases behave the opposite way: less oxygen dissolves in warm water, so fish get less oxygen and may struggle to breathe.

Quick revision

Water is the universal solvent.
Heating dissolves more sugar.
Warm water holds less dissolved oxygen.

Exam tips

Solution mass = solute mass + solvent mass.
Stirring and powdering make dissolving faster but do not change solubility.

Practice MCQs

In salt water, the solvent is
1. salt
2. water
3. both
4. neither
Show answer
Answer: (b) water. Water dissolves salt.
A solution that cannot dissolve more solute is
1. dilute
2. saturated
3. unsaturated
4. concentrated only
Show answer
Answer: (b) saturated. Saturated.
10 g sugar in 90 g water gives a mass % of
1. 90%
2. 11.1%
3. 9%
4. 10%
Show answer
Answer: (d) 10%. 10/100 × 100.
Brass is an example of a
1. liquid solution
2. solid solution
3. gaseous solution
4. suspension
Show answer
Answer: (b) solid solution. Alloy.
In salt water, the solvent is
1. salt
2. water
3. both
4. neither
Show answer
Answer: (b) water. Water dissolves salt.
Why do fish find it hard to live in very warm water?
1. warm water has more salt
2. fish dislike light
3. warm water is heavier
4. less oxygen dissolves in warm water
Show answer
Answer: (d) less oxygen dissolves in warm water. Gas solubility falls with temperature.
Powdering sugar before adding to water
1. increases solubility
2. changes its taste
3. makes it insoluble
4. makes it dissolve faster
Show answer
Answer: (d) makes it dissolve faster. More surface area.
More sugar dissolves in hot water than in cold water because
1. hot water is heavier
2. solubility usually increases with temperature
3. sugar melts
4. cold water has no space
Show answer
Answer: (b) solubility usually increases with temperature. Solubility rises with temperature.

All chapters of Class 8 Science

Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.