The most important factor that determines whether water is a solid, liquid, or gas is temperature. As temperature increases, the average kinetic energy of molecules also increases. This increased kinetic energy causes the molecules to move faster and break away from each other, resulting in a change in the physical state of the substance.

Here's how temperature affects the state of water:

1. Solid (Ice): At temperatures below 0°C (32°F), water molecules have low kinetic energy and are held together by strong hydrogen bonds. They form a rigid crystalline structure, resulting in the solid state known as ice.

2. Liquid (Water): As the temperature rises above 0°C (32°F), the average kinetic energy of water molecules increases. This weakens the hydrogen bonds, causing the molecules to move more freely. At room temperature, water is in its liquid state, where molecules are close together but not rigidly fixed in place.

3. Gas (Water Vapor): When water is heated to 100°C (212°F) at sea level, it reaches its boiling point. At this temperature, the kinetic energy of water molecules becomes high enough to overcome the hydrogen bonds completely. The molecules escape from the liquid and transform into a gaseous state, known as water vapor.

Therefore, temperature plays a critical role in determining the physical state of water, causing it to change from solid (ice) to liquid (water) and eventually to gas (water vapor) as temperature increases.