Understanding the Change in Water State
When we heat water, it undergoes a physical change due to the increase in temperature. This process involves the transformation of water from its liquid state to a gaseous state. Let’s delve into this concept.

Water's States
- **Solid State (Ice)**: When water freezes at 0°C (32°F), it turns into ice, which is its solid form.
- **Liquid State (Water)**: At room temperature, water exists in its liquid form, which is the state we commonly encounter in daily life.
- **Gaseous State (Water Vapor)**: When water is heated to its boiling point (100°C or 212°F), it transitions from liquid to gas, forming water vapor.

Heating Water
- **Temperature Increase**: As heat is applied, the temperature of the water rises.
- **Molecules in Motion**: The molecules in water start moving faster as the energy from the heat increases.
- **Boiling Point**: Once the temperature reaches the boiling point, the water molecules gain enough energy to break free from the liquid state and escape into the air as gas.

Conclusion
In summary, when water is heated, it changes into water vapor, which is a gaseous form of water. Understanding this transformation is crucial in comprehending the water cycle and the properties of different states of matter. Hence, when asked what water changes into upon heating, the correct answer is water vapor (option C).