Saturated steam is in dynamic balance with liquid or a solid body of the same chemical composition. Pressure of saturated steam depends on other indicators of steam: for example, the temperature dependence of pressure of saturated steam allows to judge substance boiling temperature.
It is required to you
- - vessel;
- - mercury;
- - pipette;
- - water;
- - alcohol;
- - tubes;
- - air.
Instruction
1. The quantity of the molecules which are taking off in one second from unit of area of a surface of liquid directly depends on temperature of this liquid. At the same time the quantity of the molecules returning from steam to liquid is caused by concentration of steam and speed of the thermal movement of its molecules. So, concentration of molecules of steam at balance of steam and liquid depends on equilibrium temperature.
2. As vapor pressure is in dependence with its temperature and concentration, a conclusion arises: pressure of saturated steam depends only on temperature. With increase in temperature pressure of saturated steam and also its density increases, at same liquid density because of thermal expansion decreases.
3. Pressure of saturated steam of different liquids at an identical temperature can strongly differ. The made experiment will help to make sure of it.
4. In a vessel in which there is a mercury lower several barometric tubes. The tube and will serve as a barometer. Fill b tube by means of the pipette with water, into a tube with enter alcohol, and into d tube – air.
5. Watch the events. So, in b tube a part of water in "Torricellian emptiness" will very quickly evaporate, and the rest will accumulate over mercury in the form of liquid (it is sign that over mercury there is a saturated water vapor).
6. Compare height of a mercury column in a barometer to mercury height in tubes of b, c and d. The difference between mercury column height in each of three tubes and height of a mercury column in a barometer will also be an indicator of pressure of saturated steam of this liquid. The made experiment proves that the largest pressure in this case saturated steam of air, and the smallest – water vapor has.
7. If temperature in the closed vessel reaches critical value (Tkr) for substance which is in it, then density of its liquid and steam become identical. The subsequent increase in temperature leads to disappearance of physical differences between liquid and saturated steam.