(2)52.0; 30.0

(3)0.94

(4) Students may have two views on this issue:

① Xiaohua can conclude that the sample oil is gutter oil by comparing the calculation results.

Because the density of sample oil calculated by Xiaohua is within the density range of waste oil.

② Xiaohua can't draw the conclusion that the sample oil is waste oil by comparing the calculation results.

Because the density of high-quality salad oil is not much different from that of gutter oil, and there are errors in the experimental process, it is impossible to conclude that the sample oil is gutter oil.

(5) In the same time, the heat released by the two resistance wires is equal;

Comparison of control variables or the same conditions;

Changes in readings of two thermometers.