How do you find the triple point of a substance in the lab?

In one line

The rule

The triple point of a substance is the unique set of temperature and pressure at which all three phases—solid, liquid, and gas—coexist in equilibrium.

For water, the triple point is at 0.01°C (273.16 K) and 611.657 pascals (0.00604 atm).

Let's understand it simply

Imagine you have ice, water, and water vapour in a sealed glass container. Usually, if you heat ice, it melts to water, and if you heat water, it turns to steam. But at one special temperature and pressure, all three can exist together—none of them fully takes over. This is the triple point. It’s like a cricket match where three teams are tied, and no one is winning or losing.

To find it in the lab, you carefully control both temperature and pressure until you see all three forms at once. It’s a delicate balance, like keeping a cycle upright without moving.

How it works

1. Set up a sealed chamber: Place a pure sample of the substance (like water) in a transparent, airtight cell.
2. Insert sensors: Put in a thermometer (for temperature) and a manometer or pressure gauge (for pressure).
3. Evacuate air: Remove all air from the chamber using a vacuum pump, so only the substance remains.
4. Adjust pressure: Slowly change the pressure inside the chamber using a vacuum pump or by adding/removing the substance’s vapour.
5. Control temperature: Use a cooling/heating system to reach near the expected triple point temperature.
6. Watch for all three phases: Look for solid (ice), liquid (water), and gas (vapour) to appear together. Adjust pressure and temperature very finely until all three are stable.
7. Record the values: Note the exact temperature and pressure where this happens. This is the triple point.

The story behind it

The concept of the triple point was clarified in the late 19th and early 20th centuries as scientists tried to define temperature and pressure standards. The triple point of water became the official definition for the Kelvin temperature scale. The International Committee for Weights and Measures chose it because it is reproducible and easy to measure with pure water. The image of all three phases together fascinated early chemists and physicists, showing the deep connection between temperature, pressure, and matter.

Where you see it in real life

• Calibration of thermometers: Labs use the triple point of water to set accurate temperature scales.
• Pressure cookers: The way water boils at different pressures is related to its phase diagram and triple point.
• Meteorology: Understanding snow, rain, and vapour in clouds depends on phase changes and triple points.
• Industrial gas production: Companies use phase diagrams to separate gases by adjusting temperature and pressure.
• Cryogenics: Scientists use triple points to handle and store substances like oxygen and nitrogen at very low temperatures.
• Space research: On planets with thin atmospheres, substances can pass directly from solid to gas (sublimation), skipping the liquid phase—triple points explain why.

Why it matters

• Standard for temperature: The triple point of water is used to define the Kelvin scale, the base for all temperature measurements.
• Designing equipment: Engineers use triple point data to build freezers, heaters, and pressure vessels.
• Understanding phase changes: It helps explain why ice can sublimate in a freezer or why frost forms.
• Advanced research: Triple points are key in physics, chemistry, and even planetary science.

Worked example

Let’s say you want to find the triple point of water in the lab:

Given: Pure water sample
Step 1: Place water in a sealed, transparent cell.
Step 2: Insert thermometer and pressure gauge.
Step 3: Remove air using a vacuum pump.
Step 4: Cool the cell to just below 0°C, form some ice.
Step 5: Slowly increase temperature while watching pressure.
Step 6: Adjust pressure to about 611 Pa.
Step 7: Observe when ice, water, and vapour are all present.
Step 8: Record temperature (should be 0.01°C) and pressure (611 Pa).

Common mistakes

• Not using pure substance: Impurities change the triple point. Always use pure samples.
• Ignoring air in the chamber: Air changes the pressure reading. Remove all air first.
• Rushing the process: Changing temperature or pressure too quickly can skip the triple point. Adjust slowly.
• Misreading the phases: Sometimes, small droplets or fog can be mistaken for liquid or vapour. Look carefully.

Remember it

Triple point = all three phases together, at one unique temperature and pressure.