Chapter 2: Is Matter Around Us Pure? Case Study Questions | science Class 9 CBSE

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In a classroom activity, students were asked to mix sugar in water. They observed that sugar dissolved completely and the mixture appeared uniform throughout. Another group mixed chalk powder in water, which settled at the bottom after a while.

Question 1: Q1. What type of mixtures were formed in each case?

Sugar and water formed a homogeneous mixture (solution).
Chalk and water formed a heterogeneous mixture (suspension).
Homogeneous mixtures have uniform composition; heterogeneous ones do not.

Question 2: Q2. Why did the chalk powder settle down?

Chalk is insoluble and forms larger particles.
It does not form a stable mixture with water.
Gravity pulls undissolved particles down in suspensions.

Question 3: Q3. How can these mixtures be separated?

Sugar solution cannot be separated by filtration.
Evaporation is used to get sugar back from the solution.
Chalk-water suspension can be separated by filtration.

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Case Set 2

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Riya observed that when light was passed through a solution of copper sulphate, the path of light was not visible. However, when light passed through a mixture of milk and water, the beam of light became clearly visible.

Question 1: Q1. What phenomenon is responsible for the visibility of the light beam?

The Tyndall effect causes the scattering of light.
It occurs when light passes through a colloid.
Colloidal particles are large enough to scatter light.

Question 2: Q2. Which mixture is a colloid in this case?

Milk and water formed a colloidal solution.
Colloids are heterogeneous mixtures that appear uniform.
Copper sulphate solution is a true solution, not a colloid.

Question 3: Q3. What are the key features of colloids?

Particles do not settle on standing.
They scatter light and show Tyndall effect.
Cannot be separated by filtration.

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Case Set 3

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A student dissolved 36g of sodium chloride in 100g of water at 293 K until no more salt could dissolve. On heating the mixture, more salt dissolved. Upon cooling, some crystals settled at the bottom.

Question 1: Q1. What is the term for the original solution at 293 K?

It is called a saturated solution.
A saturated solution holds the maximum solute at a given temperature.
Any additional solute remains undissolved.

Question 2: Q2. Why did more salt dissolve on heating?

Solubility of most solids increases with temperature.
Heating provides energy to break solute-solvent interactions.
The solution becomes unsaturated at a higher temperature.

Question 3: Q3. Why did crystals form again after cooling?

Cooling reduces solubility of salt in water.
Excess salt becomes insoluble at lower temperature.
It precipitates out as crystals.

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Case Set 4

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Neha mixed iron filings and sulphur powder. Her friend Raj heated the same mixture until it turned into a black compound that did not attract a magnet. Both were asked to identify the type of changes.

Question 1: Q1. What change occurred in Neha’s experiment?

Physical mixing of iron and sulphur occurred.
No new substance was formed.
It was a physical change.

Question 2: Q2. What happened in Raj’s experiment?

A chemical reaction occurred on heating.
Iron sulphide was formed with new properties.
It was a chemical change.

Question 3: Q3. How can we differentiate mixtures and compounds?

Mixtures retain properties of components.
Compounds have fixed composition and new properties.
Mixtures can be separated physically; compounds cannot.

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Case Set 5

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Students were shown brass (a mixture of copper and zinc) and mercury. They were asked whether these are pure substances or mixtures. Their responses varied.

Question 1: Q1. Is brass a pure substance?

Scientifically, brass is a mixture (an alloy).
It consists of multiple elements: copper and zinc.
It is homogeneous but not chemically pure.

Question 2: Q2. What kind of mixture is an alloy?

It is a solid solution — a homogeneous mixture.
Alloys show properties of constituent elements.
They cannot be separated by physical methods easily.

Question 3: Q3. Why is mercury considered a pure substance?

Mercury contains only one type of atom.
It has uniform chemical composition.
It is classified as a metal element.

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Chapter sections

Chapter 2: Is Matter Around Us Pure? | Case Study Questions

Passage

Question 1: Q1. What type of mixtures were formed in each case?

Question 2: Q2. Why did the chalk powder settle down?

Question 3: Q3. How can these mixtures be separated?

Passage

Question 1: Q1. What phenomenon is responsible for the visibility of the light beam?

Question 2: Q2. Which mixture is a colloid in this case?

Question 3: Q3. What are the key features of colloids?

Passage

Question 1: Q1. What is the term for the original solution at 293 K?

Question 2: Q2. Why did more salt dissolve on heating?

Question 3: Q3. Why did crystals form again after cooling?

Passage

Question 1: Q1. What change occurred in Neha’s experiment?

Question 2: Q2. What happened in Raj’s experiment?

Question 3: Q3. How can we differentiate mixtures and compounds?

Passage

Question 1: Q1. Is brass a pure substance?

Question 2: Q2. What kind of mixture is an alloy?

Question 3: Q3. Why is mercury considered a pure substance?

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