Which one is a chemical change? (a) Evaporation of a liquid (b) Breaking of a brick (c) Burning of a candle (d) Moving of a wheel Chemical changes are (a) mostly irreversible (b) always reversible (c) always irreversible (d) mostly reversible Which one is not a physical change? (a) Dissolution of salt in water (b) Tearing a piece of paper (c) Melting of wax (d) Digestion of food During the dissolution of urea, heat (a) is evolved (b) is absorbed (c) does not change (d) none of these
Answer (1)
Let's go through each of the questions step-by-step.
Which one is a chemical change?
(a) Evaporation of a liquid : This is a physical change because it involves a change in state from liquid to gas without altering the chemical composition.
(b) Breaking of a brick : This is a physical change because breaking a brick just changes its shape and size but not its chemical composition.
(c) Burning of a candle : This is a chemical change because it involves combustion, which changes the wax to new substances like carbon dioxide and water.
(d) Moving of a wheel : This is a physical change because it involves movement without any chemical reactions.
Chosen option: (c) Burning of a candle is a chemical change.
Chemical changes are:
(a) mostly irreversible : Many chemical changes convert substances into new products that cannot be easily changed back to the original reactants.
(b) always reversible : This is not true as many chemical reactions are not easily reversed.
(c) always irreversible : While many are irreversible, there are exceptions, so this is not always true.
(d) mostly reversible : This is incorrect as most chemical changes tend to be irreversible rather than reversible.
Chosen option: (a) mostly irreversible
Which one is not a physical change?
(a) Dissolution of salt in water : This is a physical change as the salt can be recovered by evaporation.
(b) Tearing a piece of paper : Physical change as it only alters the shape of the paper.
(c) Melting of wax : This is a physical change as wax can solidify back into its original form.
(d) Digestion of food : This is a chemical change because food is broken down into different substances by the digestive enzymes.
Chosen option: (d) Digestion of food is not a physical change.
During the dissolution of urea, heat:
(a) is evolved : This is incorrect as urea dissolution is an endothermic process.
(b) is absorbed : Correct, because the process requires heat, making it endothermic.
(c) does not change : Incorrect as there is a change in heat.
(d) none of these : Not applicable as option (b) is correct.
Chosen option: (b) is absorbed
