HOTS Questions Is Matter around us Pure - Science Class 9 PDF Download

Q1: Which of the following is a colloidal solution?  
(a) Starch solution
(b) Copper sulphate solution 
(c) Chalk powder in water
(d) Kerosene oil and water
Ans: (a)

A colloidal solution consists of tiny particles suspended in a continuous medium. The particle size in a colloid is between that of a true solution and a suspension, meaning the particles do not settle out upon standing. Starch solution is a colloid because the starch particles are small enough to remain suspended in water without settling.

Incorrect Options

• Copper sulphate solution is a true solution, not a colloid.
• Chalk powder in water is a suspension, as the chalk particles settle down when left undisturbed.
• Kerosene oil and water form an immiscible liquid mixture, not a colloid.

Q2: Which of the following is a physical change?  
(a) Burning of Natural Gas
(b) Rusting of iron
(c) Freezing of water 
(d) Digestion of food
Ans: (c)

Physical change involves a change in the physical state or appearance of a substance without forming a new substance.

Freezing of water is a physical change because it changes from liquid to solid without changing its chemical composition.

Incorrect Options

• (a) Burning of natural gas - chemical change (new substances like CO₂ and water are formed).
• (b) Rusting of iron - chemical change (formation of iron oxide).
• (d) Digestion of food - chemical change (complex food broken down into simpler substances).

Q3: Which of the following statements about mixtures is true? 
(a) Homogeneous mixtures can have variable composition
(b) Homogeneous mixtures always have fixed composition
(c) Heterogeneous mixtures have fixed composition
(d) Salt solution is a heterogeneous mixture

Ans: (a)

Homogeneous mixtures (also called solutions) have a uniform composition throughout, but the proportions of their components can vary. For example, a sugar-water solution can have different amounts of sugar dissolved in water.
Incorrect Options:

• Heterogeneous mixtures do not have a uniform composition, and their components are visibly distinct (e.g., a mixture of sand and iron filings).
• A salt solution is a homogeneous mixture, not a heterogeneous one.
• Homogeneous mixtures do not always have a fixed composition, as their components can be mixed in varying proportions.

Q4: What determines whether a solution is dilute, concentrated, or saturated? 
(a) The temperature of the solution
(b) The relative proportion of solute and solvent
(c) The type of solute used
(d) The colour of the solution
Ans: (b)

A solution is classified as dilute, concentrated, or saturated based on how much solute is dissolved in a given amount of solvent.

1. Dilute solution: Small amount of solute in a large amount of solvent.

2. Concentrated solution: Large amount of solute in a smaller amount of solvent.

3. Saturated solution: Maximum amount of solute has been dissolved; no more can dissolve at a given temperature.
Incorrect options:

• (a) Temperature affects solubility but does not alone determine the category.
• (c) Type of solute affects rate or solubility but not classification directly.
• (d) Colour is not a reliable indicator of concentration.

Q5: Who was the first scientist to define an element as a basic form of matter that cannot be broken down further by chemical reactions? 
(a) Antoine Lavoisier
(b) Robert Boyle 
(c) John Dalton
(d) Isaac Newton
Ans: (a)

Antoine Laurent Lavoisier (1743-94), a French chemist, was the first to establish an experimentally useful definition of an element. He defined an element as a basic form of matter that cannot be broken down into simpler substances by chemical reactions.
Incorrect Options:
(b) Robert Boyle was the first scientist to use the term element in 1661. 
(c) John Dalton is known for the atomic theory, not the first definition of an element. 
(d) Isaac Newton contributed to physics and mathematics, not the classification of elements.

Q6: 40 g of common salt is dissolved in 320 g of water. The mass percentage of salt is  
(a) 11.1% 
(b) 12.5% 
(c) 15% 
(d) 10% 
Ans: (a)

Mass of common salt (solute) = 40g
Mass of water (solvent) = 320g
Mass of solution = Mass of Solute + Mass of Solvent = 320 + 40 = 360g
Concentration of solution = mass of solute / mass of solution = 40/360 x 100 = 11.11%

Q7: The particles of suspension  
(a) can't be seen with the naked eye 
(b) can't be seen with the help of a powerful microscope 
(c) can be seen with the naked eye 
(d) can't be seen with an electron microscope

Ans: (c)

The particles of suspension:
- Can be seen with the naked eye. Suspensions are heterogeneous mixtures in which solid particles are dispersed in a liquid or gas. These particles are usually large enough to be visible to the naked eye.
- Cannot be seen with a powerful microscope. While the particles in a suspension may not be visible under a powerful microscope, they can still be seen without the aid of any magnification.
- Cannot be seen with an electron microscope. Electron microscopes are capable of magnifying objects to a much higher degree than light microscopes, but even they may not be able to visualize the particles in a suspension due to their size and distribution.
- Are larger than the particles in a solution. In contrast to solutions, where the solute particles are evenly distributed and too small to be seen, the particles in a suspension are larger and can settle over time due to gravity.
- Can be separated by filtration or settling. Since the particles in a suspension are larger and more visible, they can be separated from the liquid or gas phase through processes such as filtration or allowing them to settle and then decanting the liquid above.

Q8: Which of the following statements is true about compounds? 
(a) The compound is homogeneous and has a fixed composition
(b) The compound is heterogeneous and has fixed composition
(c) Compound is heterogeneous
(d) Compound is heterogeneous and has no fixed composition
Ans: (a)

• A compound is a pure substance made of two or more elements chemically combined in a fixed ratio.
• It is homogeneous, meaning it has a uniform composition throughout.
• Example: Water (H₂O) always has 2 hydrogen atoms and 1 oxygen atom in each molecule.
• Incorrect options: (b), (c), and (d) are incorrect because they describe heterogeneous mixtures, not compounds.

Q9:  The particles of a colloidal solution can be separated by  
(a) evaporation 
(b) filtration 
(c) centrifugation 
(d) distillation
Ans: (c)

• Centrifugation is a process used to separate particles of a colloidal solution.
• In this method, the mixture is spun at high speeds, which causes the denser particles to move outward and separate from the rest of the solution.
• Filtration is not effective because colloidal particles are too small to be captured by ordinary filter paper.
• Evaporation and distillation are not suitable methods for separating colloidal particles.

Q10: Which of the following statements is always true when a substance undergoes a physical change 
(a) It changes colour
(b) A new substance is formed
(c) It boils
(d) Its composition remains the same
Ans: (d)

• In a physical change, the substance does not form a new substance; only its physical properties like state, shape, size, or appearance may change.
• The chemical composition remains the same.

Incorrect options:

• (a) It changes colour - not always true; some physical changes do not involve any color change.
• (b) A new substance is formed - this is true for chemical changes, not physical ones.
• (c) It boils - boiling is one example of a physical change but not true for all physical changes.