Final answer:
The difference in temperature between two substances determines the rate of heat transfer, with heat moving from the hotter to the cooler substance until they reach the same temperature.
Step-by-step explanation:
The main factor that causes heat transfer between two substances is the temperature difference between them. When two substances are brought into contact, heat flows from the substance with the higher temperature to the substance with the lower temperature. This will continue until thermal equilibrium is reached and both substances are at the same temperature. The rate of heat transfer can be quantitatively expressed and is directly proportional to the surface area, the temperature difference, and the thermal conductivity of the materials. It is inversely proportional to the thickness of the material through which heat is being transferred.
Therefore, the heat transfer rate between two substances is indeed determined by the difference in their temperatures, with higher temperature differences generally leading to faster heat transfer rates.