## PRACTICAL-PHARMACEUTICAL ENGINEERING-I-FACTORS AFFECTING RATE OF EVAPORATION

**Object –** To see the effect of following factors on rate of evaporation –

- Concentration
- Surface area
- Thickness/Viscosity

**Reference –** Subrahmanyam C.V.S. etal, Laboratory manual of Pharmaceutical engineering – unit operations, Vallabh publications, Delhi, II edition, 2011, 116-120.

**Materials required – **

**Chemicals –** Glycerine, sodium chloride

**Glasswares and apparatus –** Water bath, beaker, graph paper, measuring cylinder

**Theory – **Change of water to water vapor or liquid to vapor phase is called evaporation. Rate of evaporation is effected by many factors like surface area, concentration and thickness/viscosity. As surface area increases, the rate of evaporation increases because number of molecules are more for getting the heat. As concentration increases, the rate of evaporation decreases and as viscosity increases, the rate of evaporation decreases.** **

**Procedure –**

**Effect of concentration –**- Three beakers were taken of different capacity.
- 25 ml water was taken in each beaker.
- All beakers were kept over boiling water bath for 30 minutes.
- After 30 minutes, the volume of water left was measured.
- Quantity of water evaporated was then calculated.
- Then rate of evaporation was calculated.
- Surface area of each beaker was calculated.
- A graph was plotted between rate of evaporation and surface area.

**Observation – **

S.NO. |
BEAKER |
RADIUS |
VOLUME OF WATER TAKEN |
VOLUME OF WATER LEFT AFTER HEATING |
QUANTITY OF WATER EVAPORATED |

1 | 50 ml | 2.0 cm | 25 ml | ||

2 | 150 ml | 2.75 cm | 25 ml | ||

3 | 250 ml | 3.25 cm | 25 ml | ||

4 | 500 ml | 4 cm | 25 ml |

** **

**Calculation – **

Rate of evaporation = Quantity of water evaporated / Time of heating in minutes.

Let for 50 ml beaker, amount of water evaporated = 1 ml, then rate of evaporation = 1/30 ml/min

Surface area of beaker = πr^{2}

- If radius is 2 cm for 50 ml beaker, then area = 3.14 x 2 x 2 = 12.56 sq.cm.
- If radius is 2.75 cm for 150 ml beaker, then area = 3.14 x 2.75 x 2.75 = 23.74 sq.cm.
- If radius is 3.25 cm for 250 ml beaker, then area = 3.14 x 3.25 x 3.25 = 33.16 sq.cm.
- If radius is 4 cm for 500 ml beaker, then area = 3.14 x 4 x 4 = 50.24 sq.cm.

**Effect of Thickness/Viscosity-**- In same capacity beakers, solutions of different ratios of glycerine and water were prepared as given below

S.NO. |
GLYCERINE |
WATER |
CONCENTRATION |

1 |
5 |
45 |
10% |

2 |
10 |
40 |
20% |

3 |
15 |
35 |
30% |

4 |
20 |
30 |
40% |

- The above solutions were kept on boiling water bath for 30 minutes.
- After 30 minutes, volume of water left in all the beakers were measured.
- Quantity of water evaporated was then calculated.
- Then rate of evaporation was calculated.
- A graph was plotted between rate of evaporation and viscosity.

**Observation – **

S.NO. |
TYPE OF SOLUTION |
VOLUME OF SOLUTION |
VOLUME OF WATER LEFT AFTER HEATING |
QUANTITY OF WATER EVAPORATED |

1 | 10% | 50 ml | ||

2 | 20% | 50 ml | ||

3 | 30% | 50 ml | ||

4 | 40% | 50 ml |

**Calculation – **

Rate of evaporation = Quantity of water evaporated / Time of heating in minutes.

**Effect of concentration –**- In same capacity beakers, sodium chloride solution of different concentrations were prepared as given below

S.NO. | SODIUM CHLORIDE | WATER | CONCENTRATION |

1 | 5 | 100 | 5% |

2 | 10 | 100 | 10% |

3 | 15 | 100 | 15% |

4 | 20 | 100 | 20% |

- The above solutions were kept on boiling water bath for 30 minutes.
- After 30 minutes, volume of water left in all the beakers were measured.
- Quantity of water evaporated was then calculated.
- Then rate of evaporation was calculated.
- A graph was plotted between rate of evaporation and concentration.

**Observation – **

S.NO. |
TYPE OF SOLUTION |
VOLUME OF SOLUTION |
VOLUME OF WATER LEFT AFTER HEATING |
QUANTITY OF WATER EVAPORATED |

1 | 5% | 100 ml | ||

2 | 10% | 100 ml | ||

3 | 15% | 100 ml | ||

4 | 20% | 100 ml |

**Calculation – **

Rate of evaporation = Quantity of water evaporated / Time of heating in minutes.

**Result –** After performing the experiment, graphs were plotted. From the obtained graph, it was concluded that-

- As surface area increases, rate of evaporation increases.
- As viscosity increases, rate of evaporation decreases.

- As concentration increases, rate of evaporation decreases.

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