Class 9 Science Chapter 2 Case Based Questions - Is Matter Around Us Pure??

(I) Read the given passage and answer the questions that follow based on the passage and related studied concepts.
A pure substance consist of single type of particles. Mixture consist of more than one kind of pure form of matter. Mixtures can be separated by physical methods but pure substances especially compounds cannot be separated into chemical constituents by physical methods. Pure substance has same composition throughout. Soil and soft drinks are mixtures. Mixtures can be separated by various methods depending upon nature of substance present in it. Solution is a homogeneous mixture.

Q1: Name the process by which pure NaCl can be obtained from salt solution.
Ans: The process to obtain pure NaCl from a salt solution is known as crystallisation.

Q2: What are alloys-compounds or mixtures?
Ans: Alloys are homogeneous mixtures made from two or more metals or a combination of a metal and a non-metal. They cannot be separated into their individual components through physical methods. Key points about alloys:

• They retain the properties of their constituent materials.
• Alloys can have varying compositions.
• For example, brass is an alloy consisting of approximately 30% zinc and 70% copper.

Q3: What is size of particles in solution?
Ans: The size of particles in a solution is typically less than 1 nm (10^-9 metres).

• These particles are so small that they cannot be seen with the naked eye.
• Due to their tiny size, they do not scatter light, making the path of light invisible in a solution.
• Particles in a solution cannot be separated by filtration.
• When left undisturbed, the particles do not settle, indicating that a solution is stable.

Q4: What is solute and solvent in cold drinks?
Ans: In cold drinks, the components are classified as follows:

Solutes: These are substances that dissolve in the solvent. In cold drinks, common solutes include:

• Carbon dioxide (CO₂) gas
• Sugar
• Preservatives

Solvent: This is the substance that dissolves the solutes. In cold drinks, the solvent is: Water

(II) Study the table showing melting points and boiling points of certain compounds. Answer the questions based on the table and related studied concepts
Q1: Which one of the given substances may be separated by using a water condenser?
Ans: Pentane can be separated using a water condenser because:

• The temperature of the water condenser is 25°C.
• The boiling point of pentane is 36°C.

Since the boiling point of pentane is higher than the temperature of the condenser, it can be effectively condensed and separated.

Q2: How can gases be separated that are present in air?
Ans: The separation of gases present in air can be achieved through fractional distillation of liquid air.

Q3: If sample of urea has melting point 129°C, then:
(a) It is impure
(b) It is pure
(c) It can not be predicted
(d) The compound is not urea
Ans: (a) It is impure

Q4: Which of the following will have boiling point 100°C?
(a) Distilled water
(b) Sea water
(c) River water
(d) Well water
Ans: (a) Distilled water will have boiling point 100°C.

(III) A homogeneous mixture of two or more substances is called a true solution. A solution has a solute and a solvent as its component. The particles of a true solution are smaller than 1nm in diameter. A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remain suspended throughout the bulk of the medium. A colloid is a mixture that is actually heterogeneous but appears to be homogeneous as the particles are uniformly spread throughout the solution.
Read the given passage carefully and give the answer of the following questions:

Q1. The correct sequence which describes the true solution, suspension and colloidal solution in order of their increasing stability is:

(a) colloidal solution < true solution < suspension
(b) suspension < colloidal solution < true solution
(c) colloidal solution < suspension < true solution
(d) true solution < colloidal solution < suspension

​Ans: (b) suspension < colloidal solution < true solution

Suspensions are least stable, colloids are moderately stable, and true solutions are the most stable.

Q2. Which type of mixture can be separated by filtration?
(a) Colloid 
(b)True solution
(c) Suspension 
(d) All of these

Ans: (c) Suspension

Suspension particles are large enough to be trapped by filter paper.

Q3. Colloids are classified on the basis of:

(a) nature of dispersed phase and dispersion medium
(b) phase of solution
(c) temperature of solution
(d) none of the above

Ans: (a) nature of dispersed phase and dispersion medium

Colloids are classified by the nature of the dispersed phase and the dispersion medium.

Q4. Component present in larger amount in solution is known as:

(a) Dispersed phase
(b) Solvent
(c) Solute
(d) None of these

Ans: (b) Solvent

The solvent is the component present in the greater amount in a solution.

Q5. The teacher instructed three students Ali, Aisha and Arvind respectively to prepare a 50% (volume by volume) solution of NaOH. Ali dissolves 50 mL of NaOH in 100 mL of water, dissolves 100 mL of NaOH in 500 g of water while Arvind dissolves 50 mL of NaOH in water to make 100 mL of solution. Which one of them has made the desired solution?

(a) Ali
(b) Aisha
(c) Arvind
(d) All of these

Ans: (c) Arvind

Volume by volume percentage of a solution = Volume of solute / Volume of solution × 100

For Ali,
Volume by volume % = 50 / 150 × 100 = 33.33%

For Aisha,
Volume by volume % = 100 / 500 × 100 = 20%

(∴ 500 g = 500 mL)

For Arvind,
Volume by volume % = 50 / 100 × 100 = 50%

Hence, Arvind has made the desired solution.