For effective heat transfer, the specific heats of two substances should ideally be:

For effective heat transfer between two substances, having their specific heats be as similar as possible facilitates the transfer process. When two materials have closely matched specific heats, they can exchange thermal energy more efficiently without one substance significantly lagging behind the other in temperature response. This means that when one substance heats up, the other can heat at a comparable rate, helping to maintain a consistent temperature gradient that is essential for effective convection.

If the specific heats of the two substances were identical, the heat transfer might occur, but there would be less incentive for energy exchange, as the thermal response would be nearly the same. On the other hand, if the specific heats are vastly different, the substance with the lower specific heat would heat up or cool down much faster, leading to inefficient energy transfer and potentially significant temperature imbalances, which could inhibit convection currents.

Therefore, striving for specific heats that are as similar as possible enhances the effectiveness of heat transfer through convection, thereby optimizing the overall thermal interaction between the two materials.