Page 20 - Chemistry - XI

5. Now, start heating the side arm of the Thiele’s tube slowly and note the temperature when solid starts
melting (t °C).
1
6. Remove the burner and allow the water to cool.
7. Note the temperature (t °C) when solid starts reappearing.
2
8. Take the average of the two readings.

S. Temperature when solid starts Temperature when solidifi cation Average = t+ t 2
1
No. melting (t ) starts (t ) 2
1
2
1.
2.

Result
The melting point of given solid is _____________ °C.

Precautions

1. Crystalline substance should be fi nely powdered and then fi lled in the capillary tube by tapping.
2. The lower end of the capillary tube and that of the thermometer should be at the same level.
3. Water should be heated very slowly.
4. Temperature should be noted carefully.

5. While heating slowly, water should be stirred well so that the temperature is uniform throughout.
6. The open end of the capillary tube must be kept above the liquid level.

VIVA VOCE

Q1. Defi ne melting point.
Ans. It is the temperature at which vapour pressure of solid is equal to that of liquid, i.e. both co-exist.

Q2. How do we determine purity of solids?
Ans. It is determined with the help of its melting point. Pure solids have fi xed melting point.
Q3. Explain why does a solid melt.
Ans. When we heat a solid, force of attraction between its particles decreases and kinetic energy of
particles increases, and it changes to liquid at a temperature at which vapour pressure of solid
and liquid is equal.

Q4. What is the eff ect of impurities on melting point of a solid?
Ans. Impurities decrease the melting point of solid.

Q5. What is meant by sharp melting point?
Ans. When solid melts entirely within a range of 1°C, it is called sharp melting point. Crystalline solids
have sharp melting point.

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