Does every substance have a triple point?

In one line

The rule

The triple point of a substance is the unique temperature and pressure at which its solid, liquid, and gas phases can all exist in equilibrium (balance).

Let's understand it simply

Imagine you have ice, water, and steam — all from the same cup of water — together in a closed box. If you set the box to just the right temperature and pressure, all three forms can exist together without changing into each other. That special spot is called the triple point.

But not every substance can do this. Some things, like carbon dioxide (CO₂), skip the liquid stage at normal pressures. That means you can’t always get all three forms together.

How it works

1. Every substance has its own melting, boiling, and freezing points.
2. The triple point is where the lines for solid, liquid, and gas meet on a phase diagram (a special graph).
3. For some substances, the solid changes directly to gas (sublimation) without ever becoming liquid at normal pressure.
4. If a substance never has all three forms together under any temperature and pressure, it has no triple point.

The story behind it

Scientists found the triple point while studying how substances change from solid to liquid to gas. The triple point of water is used to define the Kelvin temperature scale. It’s a key idea in thermodynamics (the study of heat and energy).

Where you see it in real life

• Water: In a lab, you can see ice, liquid water, and water vapor together at 0.01°C and 611.657 pascals.
• Carbon dioxide (CO₂): At normal pressure, dry ice (solid CO₂) turns straight into gas, so you don’t see all three forms together easily.
• Sulphur: Has a triple point, but at a much higher temperature than water.
• Iodine: Can show a triple point in special lab conditions.

Why it matters

• Used to calibrate (set) thermometers and temperature scales.
• Important in making and storing chemicals safely.
• Helps scientists understand how substances behave under different conditions.
• Used in physics and chemistry exams and higher studies.

Common mistakes

• Thinking every substance must have a triple point — some don’t!
• Mixing up the triple point with melting or boiling point — triple point needs all three forms together.
• Believing you can see the triple point easily at home — it often needs special lab equipment.

Remember it

Not every substance has a triple point — only those that can have solid, liquid, and gas together at some temperature and pressure.