Q1Multiple choice
Which of the following statements is true for pure substances?
(a) (i) and (ii)
(b) (i) and (iii)
(c) (iii) and (iv)
(d) (ii) and (iii)
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Worked solution
The answer is b) (i) and (iii)
NCERT Exemplar · Class 9 · Science
Chapter 2: Is Matter around Us Pure?
Practice each question, then open the answer or worked solution.
Multiple-choice Questions
9 questionsQ2Multiple choice
Rusting of an article made up of iron is called
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Correct option: C
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Worked solution
The answer is c) corrosion, and it is a chemical change.
Explanation :
Rusting of iron is corrosion, and it’s a chemical change because rust is a chemical compound called hydrated iron oxide Fe2O3. nH2O, iron(III), which is different from elemental iron.
Physical changes are those changes in which substance identity is not changed, and it can get back to its original form. For example, water freezing to ice can be melted back to waste, whereas chemical changes are those in which the original substance identity is changed, and they cannot be restored to their original form.
On adding solid solutes to the solvent, some solute dissolves, and their concentration increases in the solution. This process is known as dissolution.
Crystallisation is a process in which solute particles in a solution collide with the solid solute particles to get separated out of the solution.
Q3Multiple choice
A mixture of sulphur and carbon disulphide is
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Correct option: A
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Worked solution
The answer is a) heterogeneous and shows the Tyndall effect.
Explanation:
A mixture of sulphur and carbon disulphide is a heterogeneous colloid and shows the Tyndall effect because, in a colloidal solution, the particles are big enough to scatter light. The scattering of light by colloidal particles is known as the Tyndall effect. Colloids are actually heterogeneous in nature though they appear to be homogeneous.
Q4Multiple choice
Tincture of iodine has antiseptic properties. This solution is made by dissolving
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Correct option: D
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Worked solution
The answer is (d) iodine in alcohol
Explanation:
The tincture is prepared by using 2-7% elemental iodine and either potassium iodide or sodium dissolved in alcohol. Since alcohol is a good solvent and iodine does not dissolve in water answer should be alcohol.
Q5Multiple choice
Which of the following are homogeneous in nature?
a. (i) and (iii)
b. (ii) and (iv)
c. (i) and (iv)
d. (iii) and (iv)
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Worked solution
The answer is (c) (i) and (iv)
Explanation
Air and ice are homogeneous mixtures because their elements are not visible and cannot be distinguished from one another.
Q6Multiple choice
Which of the following are physical changes?
(a) (i), (ii) and (iii)
(b) (i), (ii) and (iv)
(c) (i), (iii) and (iv)
(d) (ii), (iii) and (iv)
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Worked solution
The answer is (c) (i), (iii) and (iv)
Explanation
Rusting of iron is a chemical process where iron reacts with water and oxygen to produce iron oxide, whereas other processes are physical changes.
Q7Multiple choice
Which of the following are chemical changes?
(a) (i) and (ii)
(b) (ii) and (iii)
(c) (iii) and (iv)
(d) (i) and (iv)
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Worked solution
The answer is (a) (i) and (ii)
Explanation
Decaying of wood and burning of wood, because there will be a change of chemical composition and wood cannot be restored to its original form.
Sawing of wood and hammering of nails into a piece of wood are physical processes where the chemical composition of compounds is not changed.
Q8Multiple choice
Two substances, A and B, were made to react to form a third substance, A2B, according to the following reaction 2A + B → A2B. Which of the following statements concerning this reaction are incorrect?
(a) (i), (ii) and (iii),
(b) (ii), (iii) and (iv)
(c) (i), (iii) and (iv)
(d) (ii), (iii) and (iv)
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Worked solution
The answer is (c) (i), (iii) and (iv)
Explanation:
A2B is a compound made up of two elements, A and B, in a fixed ratio. The properties of a compound (For example, A2B) are entirely different from those of its constituent elements (i,e.g A and B). The composition of a compound is fixed.
Q9Multiple choice
Two chemical species, X and Y, combine together to form a product P which contains both X and Y X + Y → P. X and Y cannot be broken down into simpler substances by simple chemical reactions. Which of the following concerning the species X, Y and P are correct?
(a) (i), (ii) and (iii),
(b) (i), (ii) and (iv)
(c) (ii), (iii) and (iv)
(d) (i), (iii) and (iv)
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Worked solution
The answer is (d) (i), (iii) and (iv)
Here, X and Y cannot be further broken down into simpler substances. Hence, X and Y are elements, and P can be broken down into its elements, P is a compound with a fixed composition.