Why do you have to put the upper limit of the mercury ball with the lower limit of the distillation

When heated, you can see that the liquid in the distillation flask is gradually boiling and the vapor is gradually rising. The thermometer reading also rose slightly. When the tip of the vapor reaches the mercury ball of the thermometer, the thermometer reading rises dramatically.

At this time, the flame of the gas lamp should be appropriately reduced or the voltage of the heating furnace or electric heating jacket should be appropriately reduced, so that the heating speed is slightly slowed down, and the top of the vapor stays in place, so that the upper part of the bottleneck and the thermometer are heated, so that the temperature of the droplets and the vapor on the mercury ball can reach equilibrium. Then increase the flame slightly and distill. Control the heating temperature and adjust the distillation speed, usually 1 2 drops per second.

Throughout the distillation process, the mercury balls of the thermometer should be so that there are often condensed droplets. The temperature at this point is the temperature at which the liquid and vapor are in equilibrium, and the thermometer reading is the boiling point of the liquid (distillate). The flame heated during distillation should not be too large, otherwise it will cause overheating in the neck of the distillation flask, so that the vapor of a part of the liquid will be directly affected by the heat of the flame, so that the boiling point read by the thermometer will be high; On the other hand, distillation should not be carried out too slowly, otherwise the boiling point read on the thermometer will be low or irregular, because the mercury ball of the thermometer cannot be adequately infiltrated by the distillate vapor.

The lower limit at the branch nozzle is equivalent to the position where the steam goes out, so as to ensure that the measured temperature is the temperature at which the steam is distilled. This allows the steam temperature to be accurately known and the heating can be controlled.

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In the ordinary distillation process, the upper part of the mercury ball of the thermometer should be at the same level as the lower part of the distillation branch, so that the mercury ball of the thermometer is exactly wrapped by the vapor of the distilled substance, and the temperature displayed by the thermometer is the exact temperature of the vapor, that is, the boiling point of the distilled fraction.

If the thermometer is too high, the thermometer will show a lower temperature, lower than the exact temperature of the vapor and therefore below the boiling point of the distillate.

If the thermometer is too low, the thermometer will show a higher temperature than the exact temperature of the vapor and therefore above the boiling point of the distillate.

Whether it is too low or too high, it is not conducive to the collection of pure fractions, for example, to evaporate ethanol from the mixture, it is known that the boiling point of ethanol is 78 degrees Celsius, then it should be collected when the thermometer is 78 degrees, if the thermometer position is incorrect and the thermometer is not the exact temperature of the vapor, then the collected fraction is likely to be impure.

A During the distillation operation, the thermometer measures the temperature of the components to flow out, and the mercury ball of the thermometer should be close to the branch nozzle of the distillation flask, so a is correct;

b When heating and evaporating, the glass rod should be continuously stirred to make the liquid state heated evenly, so B is wrong;

c During the dispensing operation, in order to avoid pollution, the lower liquid in the separating funnel is released from the lower mouth, and then the upper liquid is poured out from the upper mouth, so C is correct;

D makes the cooling water in the condenser enter and exit from the lower mouth and the upper mouth, and the condensation effect is the best, so D is correct and B is selected