Question

Which condition must exist in order for conduction to occur between two substances?

The molecules of the substances must be touching.
Both substances must be the same temperature.
At least one substance must be a liquid.
Most of the molecules must be slow moving.

Answer 1

The molecules of the substances must be touching.

Conduction transfers thermal energy by particle collisions or free-electron motion across a boundary, so direct contact (and a temperature difference) is required. The other choices are incorrect: equal temperatures give no net transfer; a liquid is not required (solids conduct); molecule speed being "slow" is irrelevant.