Chemistry
Give a reason why —
(a) Heating a platinum wire is a reversible change but, heating a magnesium wire is an irreversible change.
(b) Addition of zinc nitrate to water is a physical change but, addition of zinc to dilute nitric acid is a chemical change.
(c) Rusting of iron is a chemical change but, magnetization of iron is a physical change.
Phy & Chem Changes
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Answer
(a) Heating a platinum wire is a temporary change that gets reversed on cooling the wire. Moreover, the composition and properties of platinum does not change. Hence, it is a reversible change. On burning magnesium wire, a new product [magnesium oxide] is formed. Hence, we cannot recover magnesium back, so it is an irreversible change.
(b) On adding zinc nitrate to water, it dissolves and no new product is formed. The reaction is reversible, hence it is a physical change.
On reacting zinc with dil. nitric acid, hydrogen gas is released and zinc nitrate is formed. The change is irreversible hence it is a chemical change.
(c) Rusting of iron is a chemical change because a new product iron oxide [rust] is formed after the reaction and the reaction is irreversible.
Magnetization of iron is a physical change because on hammering, the iron nail gets demagnetized and we get the original product back. The reaction is reversible.
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Related Questions
State which of the following terms connected with change of state of matter i.e.
(a) Melting
(b) Vaporisation
(c) Condensation
(d) Freezing
pertain to
(i) Conversion of a liquid into a vapour or gas.
(ii) Conversion of a liquid into a solid.
(iii) Conversion of a solid into a liquid on heating.
Potassium chloride is added to water and stirred. A salt solution is obtained which is then boiled leaving behind a residue
(a) Is the above change physical or chemical?
(b) Name the residue which remains behind after the salt solution is boiled.
(c) Is the change reversible or not reversible?
(d) Are the composition and properties of the original substance altered.
(e) Give a reason why the above experimentation would not be possible, if calcium carbonate is taken, in place of potassium chloride.
Select the chemical changes from the following list of changes -
(a) Vaporisation of water into water vapour
(b) Boiling of milk
(c) Respiration in mammals
(d) Rotting of eggs
(e) Drying of a fruit
(f) The carbon cycle
(g) Occurrence of lightening
(h) Breaking of glass
(i) Butter turning rancid
(j) Glowing of an electric bulb
(k) Crystallisation of a salt from its solution
(l) Change of seasons
(m) Preparation of carbon dioxide from calcium carbonate & dilute hydrochloric acid
Select the correct answer from the choice in bracket to complete each sentence:
A change in which a substance retains its identity is a …………… [physical/chemical] change.
Matter is added or removed during a …………… [chemical/physical] change.
Energy required for completion of a physical change is …………… [released/absorbed] when the change is reversed.
Dissolution of lead nitrate in water is deemed as a …………… [physical/chemical] change.
A change in which matter undergoes a change but the total mass of substance is unaltered is a …………… [physical/chemical] change.