Extra 20 long-answer questions from the chapter:2 "Is Matter Around Us Pure" for Class 9 CBSE Science:
1. What is a mixture? How is it different from a pure substance?
Answer:
A mixture is a combination of two or more substances in any proportion, where each retains its own properties.
Differences between Mixture and Pure Substance:
Composition:
A mixture has a
variable composition
, whereas a pure substance has a
fixed composition
.
Separation:
Mixtures can be separated by
physical methods
, but pure substances require
chemical methods
.
Properties:
Mixtures retain the properties of their components, whereas pure substances have
fixed properties
.
Examples:
Mixtures –
Air, Saltwater
; Pure substances –
Iron, Water (H₂O)
.
Homogeneity:
Pure substances are always homogeneous, whereas mixtures can be
homogeneous or heterogeneous
.
Melting & Boiling Points:
Mixtures do not have a
sharp melting or boiling point
, but pure substances do.
2. Differentiate between homogeneous and heterogeneous mixtures with examples.
Answer:
|
Feature |
Homogeneous Mixture |
Heterogeneous Mixture |
|---|---|---|
|
Definition |
A mixture that has a uniform composition throughout |
A mixture where the composition is not uniform |
|
Visibility |
The components are not visible separately |
The components are visible separately |
|
Examples |
Air, Salt solution, Brass |
Oil and water, Sand and iron filings |
|
Number of Phases |
Single phase |
Two or more phases |
|
Separation |
Difficult to separate by physical means |
Can be separated easily |
|
Tyndall Effect |
Does not show Tyndall effect |
Shows Tyndall effect if colloidal |
3. What are colloids? Explain their properties with examples.
Answer:
A colloid is a mixture where one substance is dispersed in another without dissolving.
Properties of Colloids:
Heterogeneous Nature:
Colloids appear homogeneous but are actually
heterogeneous
.
Tyndall Effect:
Colloids scatter light, making the beam visible. Example:
Fog, Milk
.
Particle Size:
Particles are
larger than a solution
but
smaller than a suspension
.
Stability:
Colloidal particles
do not settle down
on standing.
Filterability:
Cannot be separated by ordinary
filtration
but can be separated by
ultrafiltration
.
Examples:
Starch solution, Gel, Blood, Butter.
4. Define solution, solute, and solvent with examples.
Answer:
Solution:
A homogeneous mixture of two or more substances. Example:
Salt solution
.
Solute:
The substance
dissolved
in the solution. Example:
Salt in water
.
Solvent:
The substance in which the solute dissolves. Example:
Water in salt solution
.
Properties of Solutions:
Homogeneous in nature.
Solute particles are
too small
to be seen.
Does not show
Tyndall effect
.
Cannot be separated by
filtration
.
Has a
fixed boiling point
.
5. What is the Tyndall effect? Give two examples.
Answer:
The Tyndall effect is the scattering of light by particles in a colloid or suspension.
Examples:
Sunlight passing through mist or fog.
Beam of light in a dusty room.
Properties of Tyndall Effect:
Observed in
colloids and suspensions
but not in solutions.
Light is
scattered in all directions
.
The
colour
of the scattered light depends on
particle size
.
Used in the
study of colloids
.
Helps in
distinguishing true solutions from colloids
.
6. What is sedimentation and decantation? Give examples.
Answer:
Sedimentation:
The process in which
heavier particles
settle down in a liquid. Example:
Mud settling in water
.
Decantation:
The process of
pouring off the clear liquid
after sedimentation. Example:
Removing water from settled mud
.
Differences:
|
Feature |
Sedimentation |
Decantation |
|---|---|---|
|
Process |
Solid particles settle down |
Clear liquid is poured out |
|
Use |
Separates solid from liquid |
Further purification |
|
Example |
Sand in water |
Oil and water separation |
7. Explain fractional distillation with an example.
Answer:
Fractional distillation is used to separate liquids with close boiling points.
Example: Separation of petroleum into diesel, petrol, and kerosene.
Steps in Fractional Distillation:
Heat the mixture in a
fractionating column
.
The liquid with the
lowest boiling point evaporates first
.
Vapours
condense at different heights and are collected separately.
Used in
separating alcohol from water
.
8. How can a mixture of salt and water be separated?
Answer:
By evaporation, the water is removed, leaving salt behind.
Steps:
Heat the saltwater solution.
Water evaporates, leaving salt crystals.
The salt is collected after drying.
9. How can a mixture of iron and sulphur be separated?
Answer:
By using a magnet, iron is attracted, while sulphur remains behind.
Steps:
Pass a magnet over the mixture.
Iron filings get attracted.
Sulphur
remains uncollected.
10. What is chromatography? Give an example.
Answer:
Chromatography is a technique to separate coloured substances.
Example:
Separating dyes from ink.
Identifying drug components.
Steps:
Place a drop of ink on filter paper.
Dip the paper in water.
The different
colours
separate at different heights.
11. What are alloys? How are they different from pure metals? Give examples.
Answer:
Alloys are homogeneous mixtures of two or more metals or a metal and a non-metal.
Differences Between Alloys and Pure Metals:
Composition:
Alloys contain
two or more elements
, while pure metals contain
only one element
.
Properties:
Alloys often have
improved strength, corrosion resistance, and hardness
compared to pure metals.
Melting Point:
Alloys have
variable melting points
, whereas pure metals have
fixed melting points
.
Conductivity:
Some alloys have
lower electrical conductivity
than pure metals.
Uses:
Alloys are widely used in
construction,
jewellery
, and industries
.
Examples:
Brass
(Copper + Zinc)
Bronze
(Copper + Tin)
Steel
(Iron + Carbon)
12. What are suspensions? List their properties with examples.
Answer:
A suspension is a heterogeneous mixture where solid particles are dispersed in a liquid or gas but do not dissolve.
Properties of Suspensions:
Heterogeneous:
The composition is
non-uniform
throughout.
Particle Size:
The solid particles are
large enough to be seen
by the naked eye.
Settling:
Particles
settle down
if left undisturbed.
Filtration:
Suspensions can be
separated by filtration
.
Tyndall Effect:
Suspensions show the
Tyndall effect
as the particles scatter light.
Examples:
Muddy water
Chalk powder in water
Flour mixed in water
13. Explain the method of separating immiscible liquids using a separating funnel.
Answer:
A separating funnel is used to separate two immiscible liquids that do not mix, such as oil and water.
Steps for Separation:
Pour the mixture into a
separating funnel
.
Let the mixture stand
undisturbed
until layers form.
Open the stopcock to
allow the denser liquid (e.g., water) to drain out
.
Close the stopcock and collect the
lighter liquid (e.g., oil) separately
.
Used in separating
kerosene from water
.
The separation occurs due to
difference in densities
.
14. What is crystallization? How is it different from evaporation?
Answer:
Crystallization is a purification technique used to separate a pure solid from its solution.
Steps of Crystallization:
Prepare a
saturated solution
of the substance.
Heat it to remove
impurities
.
Cool the solution slowly to allow
crystals to form
.
Filter and dry the crystals.
Differences Between Crystallization and Evaporation:
|
Feature |
Crystallization |
Evaporation |
|---|---|---|
|
Process |
Forms pure solid crystals |
Removes liquid |
|
Purity |
Gives pure substance |
May contain impurities |
|
Cooling |
Required for crystal formation |
Not needed |
|
Example |
Salt from sea water |
Sugar syrup thickening |
15. Why does the boiling point of a solution increase when a solute is added?
Answer:
When a solute (like salt or sugar) is dissolved in a solvent (like water), the boiling point of the solution increases due to:
Higher Intermolecular Forces:
Solute particles create
stronger attractions
, making it harder for solvent molecules to escape.
Colligative Properties:
Boiling point elevation is a
colligative property
, meaning it depends on the number of solute particles.
Increased Energy Requirement:
More heat is needed to break the liquid’s surface tension.
Example:
Saltwater boils at a higher temperature than pure water.
Used in Cooking:
Adding salt to boiling water increases cooking efficiency.
Industrial Applications:
Used in
food preservation and antifreeze solutions
.
16. How can we differentiate between a colloid, solution, and suspension?
Answer:
|
Feature |
Solution |
Colloid |
Suspension |
|---|---|---|---|
|
Nature |
Homogeneous |
Appears homogeneous but is heterogeneous |
Heterogeneous |
|
Particle Size |
Very small (<1 nm) |
Medium (1-1000 nm) |
Large (>1000 nm) |
|
Tyndall Effect |
No |
Yes |
Yes |
|
Filtration |
Cannot be filtered |
Cannot be filtered |
Can be filtered |
|
Example |
Sugar in water |
Milk, Fog |
Muddy water |
17. What is fractional crystallization? How is it useful?
Answer:
Fractional crystallization is used to separate two or more solids from a mixture based on different solubilities.
Steps:
Dissolve the mixture in a solvent.
Heat the solution to dissolve all components.
Cool the solution
gradually
so that one solid crystallizes first.
Filter out the solid and repeat the process for other components.
Uses:
Separation of
sugar from rock salt
.
Purification of
impure solids
.
Used in the
pharmaceutical industry
.
Used in
metal refining
.
18. What are emulsions? Give examples.
Answer:
An emulsion is a colloid in which both the dispersed phase and dispersion medium are liquids.
Examples of Emulsions:
Milk
– Fat droplets in water.
Mayonnaise
– Oil in water.
Butter
– Water in fat.
Properties of Emulsions:
Heterogeneous in nature.
Tyndall effect observed.
Requires emulsifying agents (like soap) to remain stable.
Used in food, cosmetics, and medicines.
19. Explain the principle of simple distillation.
Answer:
Simple distillation is used to separate liquids with different boiling points.
Principle: The liquid with the lower boiling point evaporates first, leaving the other behind.
Steps:
Heat the mixture in a
distillation flask
.
The component with the
lower boiling point turns into vapor
first.
The vapor passes through a
condenser
, where it cools and turns into liquid.
The pure liquid is collected separately.
Uses:
Purification of water
.
Separation of alcohol from water
.
Extraction of essential oils
.
20. How does temperature affect the solubility of solids and gases in liquids?
Answer:
Solids in Liquids:
Higher temperature → More solubility.
Example:
More sugar dissolves in hot water than cold water.
Gases in Liquids:
Higher temperature → Less solubility.
Example:
Cold drinks lose fizz when kept in warm places.
Other Factors Affecting Solubility:
Pressure:
Increased pressure increases gas solubility in liquids.
Nature of Solute & Solvent:
Like dissolves like (polar dissolves in polar).
Agitation (Stirring):
Speeds up solubility.
