4. TO DETERMINE THE MIXTURE MELTING POINT OF WAX BY THE COOLING CURVE METHOD.

APPARATUS REQUIRED

i) Hard glass test tube containing wax.

ii) Centigrade thermometer

iii) Stopwatch or clock

iv) Heating arrangement

THEORY

When a solid substance is heated, its temperature gradually increases and just begins to melt. The temperature at which the solid just begins to melt is called the melting point of the solid. This temperature remains constant until all the solid changes into liquid after which the temperature rises again. Conversely, the substance in the liquid state begins to solidify. The temperature at which solidification starts is some of the temperature at which the solid begins to melt. This temperature remains constant during the process of solidification.

When a substance in liquid form is allowed to cool, its temperature falls gradually. If we record the temperature of the substance at equal interval of time and plot the graph between time (along x-axis) and temperature (along y-axis) the nature of the graph of shown on next page 

OBSERVATION

Value of 10 smallest divisions of the thermometer = 10°C

Value of 1 smallest division of the thermometer = 1°C

Table: For finding the melting point of solid

No.of obs	Time in minute	Temperature in °C
1	0	83
2	1	80
3	2	75
4	3	71
5	4	67
6	5	64
7	6	61
8	7	58
9	8	56
10	9	56
11	10	55
12	11	55
13	12	55
14	13	55
15	14	55
16	15	54
17	16	54
18	17	54
19	18	54
20	19	54
21	20	54
22	21	54
23	22	54
24	23	54
25	24	54
26	25	54
27	26	54
28	27	54
29	28	54
30	29	53
31	30	53

RESULT

From the curve, the melting point of solid = 57°C

CONCLUSION

The melting point of a solid was determined by the cooling curve method.

PRECAUTION

1. The temperature and time should be recorded together.

2. The experimental wax should be pure.

3. Correction for the thermometer should be applied in order to get the correct temperature reacting

4. The watch should have precision of 1 second

5. The hard glass test tube should be clean.