"Casing experiment" is a smaller glass instrument into another glass instrument, assembled to complete the original need for more instruments to carry out the experiment.

(1) sodium bicarbonate is easy to decompose by heat, the small test tube for sodium bicarbonate, indicating that the temperature can be low decomposition, and sodium carbonate is heated to a high temperature, sodium carbonate does not decompose, so the purpose of the experiment is to explore (or prove) sodium bicarbonate is unstable, easy to split into decomposition of heat to produce CO2 and H2O, while sodium carbonate is stable,

The answer is: explore (or prove) sodium bicarbonate is unstable, easy to split into decomposition of heat to produce CO2 and H2O, while sodium carbonate is stable. (2) before the start of the experiment, microthermal test tube, indicating that the device is not leakage phenomenon is A, B catheter end are bubbles, after cooling are liquid column back, after the end of the experiment, to prevent the backflow of water, the test tube exploded, the operation of the problem requires special consideration is to prevent the backsiphonage,

So the answer is: A, B catheter end are bubbles produced, cooling are liquid column back; prevent backsiphonage;

(3) 2NaHCO3

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Na2CO3+CO2↑+H2O, then the solid is Na2CO3 or Na2CO3, NaHCO3 after heating for a period of time,

If it decomposes completely, there is only sodium carbonate, and from the fact that Na2CO3+HCl=NaCl+NaHCO3, NaHCO3+HCl=NaCl+CO2↑+ H2O can be seen, the beginning of the gas is not generated, and the generation of gas and not generate the gas consumes the same acid, image C is consistent with;

If partially decomposed, the solid Na2CO3, NaHCO3, NaHCO3, NaHCO3 + HCl = NaCl + CO2 ↑ + H2O, Na2CO3 + 2HCl = 2NaCl + CO2 ↑ + H2O, it can be seen that the beginning of the gas is not generated, but the generation of gas consumes the acid, the beginning of the gas is not generated, but the generation of gas consumes the same acid. gas, but the acid consumed to produce gas is greater than the acid consumed when no gas is produced, the graph B is consistent,

So the answer is: B, C.