Core Concepts
In this tutorial, you will learn about the melting point, freezing point, and boiling point of water, as well as some important concepts adjacent to them.
Vocabulary
Melting point: a temperature where a substance can change from its solid state to a liquid state.
Freezing point: the temperature at which a substance undergoes a phase change from a liquid state to a solid-state.
Boiling point: the temperature at which a liquid undergoes a phase change and turns into a vapor.
What is water?
Water is a molecule consisting of three atoms, one oxygen atom and two hydrogen atoms. Due to a force called hydrogen bonding, water molecules adhere to one another. Water exists in a gaseous, liquid, and solid state depending on its temperature. On earth, water exists in all three states of matter: solid, liquid, and gas (sometimes called vapour).
Melting Point of Water
Melting is a state change of a solid into a liquid when heat is applied. As the temperature rises, the molecules move faster. Once they have reached a specific temperature, they break free from their rigid crystalline structure and begin to move more freely, now in the liquid state. In a pure crystalline solid, this process occurs at a fixed temperature. The melting point of water is about 0°C (32°F) for pure water at sea level (normal elevation). At other elevations the melting point will change due to different ambient pressure. These changes are discussed further in a section below.
Boiling Point of Water
Boiling is the process when a liquid changes states and turns into a gas. As temperature increases, a molecule will gain enough energy to become a gas, where intermolecular distances become much larger compared to the liquid state. The boiling point of water is 100ºC (212°F), whereas the boiling point of salt water is about 102ºC. The boiling point of water will also change at non-standard pressures. Impurities in water, like salt, modify the intermolecular interactions between water molecules that result in a modified boiling point.
Boiling at High Elevations
The standard boiling temperature of water only applies at standard pressures, that is at sea level. As you move to higher elevations (lower pressure) the boiling temperature will decrease.
For example, although water normally boils at 100°C (212°F), on Mount Everest (elevation about 27,000 feet) water boils at 68°C (154°F).
What if we try to boil water deep in the ocean? In some of the deepest parts of the ocean, water will stay in its liquid state until a temperature of 400°C (750°F). Here the pressure is much higher than at normal sea level, and this makes it so more energy (higher temperature) is needed to boil the water.
Fun Facts
- Water in its solid form floats instead of sinking when sitting in liquid water. This is different from most other solids.
- Gas can be turned into a liquid or even a solid through pressure.
- The boiling point and freezing point can differ depending on how much salt is in the water.
- At higher altitudes, the boiling point of water is lower.