CBSE Guess > Papers > Question Papers > Class X > 2008 >Science > Science Theory

Q.16. (a) What is a homologous series? Give one example.
(b) What will happen if ethanoic acid reacts with ethanol in the presence of an acid as a catalyst? Name the reaction. Write the chemical equation for this reaction.
(c) Why are soaps ineffective in hard water?

Ans. (a) A homologous series is a group of organic compounds having similar structures and similar chemical properties in which the successive compounds differ by C_nH_2n2 group. The various organic compounds of a homologous series are called homologues. Example : alkane is a homologous series and the general formula of the homologous series of alkanes is CnH2n + 2.
CH₄ – Methane C₂H₆– Ethane
C₃H₈ – Propane C₄H₁₀ – Butane {differ by CH₂ group}
C₅H₁₂– Pentane
The difference in the molecular masses of any two adjacent homologues is 14 u. The members of a homologous series show a gradual change in their physical properties with increase in molecular
mass.
(b) When ethanoic acid reacts with ethanol in the presence of a few drops of a concentrated acid as a catalyst, a sweet smelling ester called ethyl-ethanoate is formed.

(c) Soaps are ineffective in hard water because

(i) The formation of lather is necessary for removing dirt from clothes during the washing of clothes. Soap does not give lather with hard water as it reacts with the calcium and magnesium ions present in
hard water to form insoluble precipitates of calcium and magnesium salts of fatty acids.

(ii) The scum (or precipitate) formed by the action of hard water on soap sticks to the clothes being washed and it interferes with the cleaning ability of soap. This makes the cleaning of clothes difficult.

or

Complete the following equations

(i)

(ii)
(iii)

Science 2008 Question Papers Class X
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