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Ans. (a) Electric motor is a device which converts electrical energy into mechanical energy. An electric motor works on the principle that when a coil carrying current is held in a magnetic field, it experiences
a force which rotates the coil. The rotational direction of the coil is given by Fleming’s Left hand rule.

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.

Science 2008 Question Papers Class X
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