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Practice each question, then open the answer or worked solution.
Which of the following is a physical change?
Ans: (d) Melting of wax
Explanation: A physical change occurs when the physical form of a substance changes but not its chemical content. In most cases, physical alterations can be reversed. Only the conversion of ice into water is an example of a physical change that can be reversed when we look at the possibilities provided.
Which of the following is a chemical change?
Ans: (b) Cooking of vegetables
Explanation: A chemical reaction produces a chemical change. When matter changes shapes but not chemical identity, this is referred to as physical transformation. A physical alteration simply impacts physical properties, such as shape, size, and so on. If the chemical makeup changes, it is a chemical change; if the chemical composition remains constant, it is a physical change. Physical changes include the twinkling of stars, the chopping of fruits, and the boiling of water.
A chemical change may involve –
Ans: (d) Any or all of the above
Explanation: The chemical change could result in a change in colour, temperature, or the evolution of a gas, among other things.
Which of the following is/are true when milk changes into curd?
(i) Its state is changed from liquid to semi solid.
(ii) It changes colour.
(iii) It changes taste.
(iv) The change cannot be reversed.
Choose the correct option from below :
Ans: (c) (i), (iii) and (iv) are correct
Explanation: The physical state of curd differs from that of milk. The taste of milk and curd is also distinct. The chemical reaction that converts milk to curd is irreversible.
A man painted his main gate made up of iron, to
(i) Prevent it from rusting.
(ii) Protect it from the sun.
(iii) Make it look beautiful.
(iv) Make it dust free.
Which of the above statement(s) is/are correct?
Ans: (d) (i) and (iii)
Explanation: It is vital to paint iron to keep it from rusting and to make it seem nice.
Iron pillar near the Qutub Minar in Delhi is famous for the following facts. Which of these facts is responsible for its long stability?
Ans: (d) It has not rusted after such a long period
Explanation: The 'iron pillar' is notable for not rusting after such a long time.
Galvanisation is a process used to prevent the rusting of which of the following?
Ans: (a) Iron
Explanation: 'Galvanisation' is a process used to keep 'iron' from rusting. In this technique, we put a layer of 'zinc' to the surface of the iron.
Paheli’s mother made a concentrated sugar syrup by dissolving sugar in hot water. On cooling, crystals of sugar got separated. This indicates a –
Ans: (a) physical change that can be reversed.
Explanation: The presented circumstance denotes physical change that is reversible.
Which of the following statements is incorrect for a chemical reaction?
Ans: (a) Heat may be given out but never absorbed.
Explanation: Depending on the nature of the reaction, heat can be given or absorbed in a chemical reaction.
Two drops of dilute sulphuric acid were added to 1 g of copper sulphate powder and then a small amount of hot water was added to dissolve it (step I). On cooling, beautiful blue coloured crystals got separated (step II). Step I and step II are:
Ans: (c) Both physical changes.
Explanation: A chemical change occurs when 'copper sulphate powder' dissolves in water in the presence of 'sulphuric acid.' While cooling the obtained hot solution, the crystals of 'copper sulphate' will separate.