Final answer:
When a gas expands, it has a positive sign (+), and when a gas contracts, it has a negative sign (-). When water boils, it expands with a positive sign (+) and there is no change in temperature.
Step-by-step explanation:
When a gas expands, it has a positive sign (+). This is because expanding means the gas is increasing in volume, which is a positive change. When a gas contracts, it has a negative sign (-). This is because contracting means the gas is decreasing in volume, which is a negative change.
For the process of boiling water, the sign of expansion is positive (+) because the water is changing from a liquid to a gas, which involves an increase in volume. The sign of change in temperature during boiling is zero, meaning there is no change in temperature during this phase transition.