Final answer:
Water turns from liquid to gas at 100 °C, which is its boiling point under standard atmospheric pressure.
Step-by-step explanation:
Water transitions from a liquid to a gas at its boiling point, which is 100 °C (212 °F) at standard atmospheric pressure. A boiling point is defined as the temperature at which a liquid's vapor pressure equals the surrounding atmospheric pressure, which causes the liquid to start forming gas bubbles and turn into vapor. Therefore, when water reaches its boiling point, it undergoes a phase change from liquid to gas.