After the test, if the alcohol lamp is turned off first, when the heating is stopped, the temperature of the test tube will drop, resulting in the pressure in the test tube being less than the external pressure, so that cold water will flow back into the hot test tube. (According to pV=nRT, the gas volume in the test tube is constant, which leads to the decrease of air pressure when the temperature drops, so there is a pressure difference between inside and outside, so water is pumped into the test tube by atmospheric pressure).
Because the inner wall of the test tube expands with heat and contracts with cold, the test tube suddenly contracts, causing the test tube to burst. Therefore, after the experiment of heating potassium chlorate, the catheter should be unplugged first, and then the alcohol lamp should be extinguished.
Extended data:
About the air tightness test before the experiment
Assemble the equipment to form a closed system. Put one end of the catheter below the liquid level of the beaker and tighten the test tube plug. Then cover the test tube with your hand and observe whether there are bubbles coming out of the guide nozzle of the beaker. When you release your hand, water will flow back to the end of the catheter to form a water column, indicating good air tightness.
Matters needing attention in the experiment of oxygen production by heating potassium chlorate
1, when making oxygen with potassium chlorate, put a small ball of cotton at the mouth of the test tube to prevent potassium permanganate powder from entering the catheter when heating.
2. The container containing oxygen should cover the glass and put it on the table. Because the density of oxygen is greater than that of air, forward discharge can reduce gas escape.
3, stop heating, should first take out the catheter from the water, and then put out the alcohol lamp. If the alcohol lamp is turned off first, the temperature and pressure in the test tube will be reduced, the water in the sink will be sucked into the boiling test tube, and the test tube will burst.
4. In order to prevent the water in the medicine from turning into steam after heating, it will liquefy and flow back to the bottom of the test tube when it is cold, so that the test tube will burst, and the mouth of the test tube should be slightly inclined downward.
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