Question

Classify the following as physical or chemical properties – a) The composition of a sample of steel is: 98% iron, 1.5% carbon and 0.5% other elements. b) Zinc dissolves in hydrochloric acid with the evolution of hydrogen gas. c) Metallic sodium is soft enough to be cut with a knife.d)Most metal oxides from alkalis on interacting with water .

Answer 1

Physical change involves change in one or more physical properties of matter.There is no change in the chemical composition and property of the matter.It is generally reversible.

Whereas, a chemical change involves change in the chemical composition and chemical properties of the matter.It involves formation of a new kind of matter and the process is irreversible.
(a)Chemical composition of a substance is regarded as its chemical property.

(b) it is a chemical reaction.HCl reacts with Zn to evolve a gas (H₂). Reaction is shown below :
Zn + 2 HCl → ZnCl ₂ + H ₂

(d) metal oxides dissolving in water to form alkalies :
When a metal oxide reacts with water, it results in the formation of an alkali. Metal oxide has different chemical properties from that of base . Since there is a change in the properties of the matter thus it is a chemical reaction.
For eg.
K₂O + H₂O$\to$2 KOH

(c)Metallic sodium is soft enough to be cut with a knife
when sodium is cut with a knife, only its size change. There is no change in the chemical properties of sodium. Thus it is a physical change.

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