The answer is: to prevent leakage of the device, affecting the gas Collect; close the water stop clip, from the instrument A into the appropriate amount of water to form a liquid column, if you see the liquid level in the instrument A does not fall, then the device is well sealed.
(3) this device to produce oxygen, we must use hydrogen peroxide and manganese dioxide reaction, the reaction equation is: 2H2O2
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(4) Change the test tube into a conical flask or a wide-mouth flask to produce more CO2; when conducting the experiment in Figure 2, the solid drug should be placed at a, this device can control the reaction at any time; CO2 does not support combustion, so when checking the fullness of a lit match placed in the mouth of the cylinder, the match is extinguished, indicating that it is full;
Therefore, the answer is: to change the test tube into a conical flask or a wide-mouth flask; a.? ? The reaction can be controlled to start or stop at any time; the lit match is placed at the mouth of the gas collecting cylinder and the match goes out, indicating that it is full.
(5) the combustion of carbon to produce CO2, the reactants have gas participation, it is difficult to collect pure CO2 gas, or combustion conditions, it is difficult to control it in a closed environment, and therefore can not be collected. Therefore, the answer is: there are gases in the reactants, it is difficult to collect pure CO2 gas.