What happens if you change the pressure or temperature away from the triple point?

In one line

The rule

The triple point of a substance is the unique combination of temperature and pressure where all three states—solid, liquid, and gas—can exist in equilibrium (balance) together.

If you move away from the triple point by changing pressure or temperature, only certain states can exist together, or just one state will be stable.

Let's understand it simply

Imagine you are making tea in a pressure cooker. At a special pressure and temperature, you might see ice, water, and steam all together. That’s the triple point. But if you turn up the flame (increase temperature) or tighten the lid (increase pressure), you change this balance.

If you heat more, the ice melts or the water boils away. If you change the pressure, maybe only ice and water, or only water and steam, can stay together. The triple point is like a magic spot—move away, and the magic is gone.

How it works

1. At the triple point, solid, liquid, and gas are all in balance.
2. Increase temperature (at constant pressure):

• Solid melts to liquid, or liquid boils to gas.
• You lose the balance—only two or one state left.

1. Decrease temperature:

• Gas condenses to liquid, or liquid freezes to solid.
• Again, only two or one state left.

1. Increase pressure (at constant temperature):

• Gas can turn to liquid or solid.
• Only two or one state can exist.

1. Decrease pressure:

• Solid or liquid can turn to gas (sublimation or boiling).
• Only two or one state can exist.

The story behind it

The idea of the triple point came from scientists studying how water and other substances change state. In the 19th century, scientists like James Thomson and Thomas Andrews explored phase changes. The triple point is important because it gives a fixed reference for temperature and pressure. For example, the triple point of water is used to define the Kelvin temperature scale.

Where you see it in real life

• Pressure cooker in the kitchen: If you change the weight on the lid (pressure), water boils at a different temperature. You never see ice, water, and steam together unless at the triple point.
• Mountaineering: At high altitudes (low pressure), water boils at a lower temperature. Ice and water may exist, but not all three states.
• Dry ice (solid CO₂): At normal pressure, dry ice turns directly from solid to gas (sublimation), skipping the liquid state. Only at the triple point can all three states exist together.
• Weather balloons: As they rise, pressure drops and water vapor can freeze directly, showing how pressure and temperature affect state changes.
• Laboratories: Scientists use the triple point of water to calibrate thermometers.
• Steam engines: Engineers use phase diagrams to avoid unwanted state changes in the machinery.

Why it matters

• Thermometers: The triple point of water is used to set the Kelvin scale, a standard for temperature worldwide.
• Industrial processes: Factories need to control pressure and temperature to keep substances in the right state (like making ice cream or steel).
• Weather prediction: Knowing how water changes state helps meteorologists predict rain, snow, or fog.
• Chemistry and physics exams: Understanding phase changes is key for scoring well in board exams and competitive tests.

Worked example

Let’s say you have water at its triple point (0.01°C, 611 Pa pressure). What happens if you increase the temperature but keep the pressure the same?

Given: Water at triple point (0.01°C, 611 Pa)
Step 1: Increase temperature, keep pressure constant
Step 2: Solid (ice) melts to liquid, then liquid boils to gas
Step 3: Only liquid and gas can exist together at this pressure and higher temperature

Common mistakes

• Thinking all three states can exist at any temperature and pressure: Only at the triple point can this happen.
• Forgetting that pressure and temperature work together: Changing one without the other can lead to different results.
• Assuming the triple point is the same for all substances: Each substance has its own unique triple point.
• Mixing up boiling and sublimation: At low pressure, solids can turn directly to gas (sublimation), not liquid.

Remember it

Only at the triple point can solid, liquid, and gas all exist together—move away, and only one or two states remain.