Question

which best describes why heat pumps are reversible?
they can heat and cool an indoor space.
they involve a metal expanding or contracting against another.
they combust and can also add coolant.
they allow a piston to move up and down.

Explanation:

Step1: Analyze each option

• Option 1: Heat pumps can reverse their operation. In one mode, they can transfer heat from the outside to the inside (heating the indoor space), and in another mode, they can transfer heat from the inside to the outside (cooling the indoor space). This shows reversibility.
• Option 2: A metal expanding or contracting against another is related to bimetallic strips (used in thermostats, for example), not the reversibility of heat pumps.
• Option 3: Combusting is related to combustion engines (like in cars for power generation from fuel), and adding coolant is a one - way process in some cooling systems (not the key to heat pump reversibility).
• Option 4: A piston moving up and down is a characteristic of reciprocating engines (e.g., in internal combustion engines), not relevant to heat pump reversibility.

Answer:

They can heat and cool an indoor space.