What Temperature Does a Kettle Boil To?

Most of us grow up with the simple rule that water boils at 100°C. It’s a fact of life, just like water being wet, or that dawn follows dusk.

Yet, anyone who has watched a kettle closely may have noticed something puzzling. Bubbling often begins when the temperature gauge reads far lower, sometimes as low as 60°C.

To the casual observer, this looks like boiling, but the kettle insists otherwise.

So is the water truly boiling at 100°C, or does the kettle reach its boiling point earlier? To understand what is really happening, we need to look at the scientific definition of boiling and the factors that influence it. Keep reading to learn more…

What temperature does a kettle boil to? Here’s what you need to know:

• Water boils at 100°C under standard atmospheric pressure, though precise values are slightly lower.

• The boiling point varies with altitude, dropping by about 1°C for every 300 metres above sea level.

• Impurities raise the boiling point slightly, though tap water changes are negligible.

• Bubbling below 100°C in kettles is caused by localised boiling at the element and sensor placement.

What is the boiling point of water?

Let’s put our science hats on for a moment.

Boiling is not simply the appearance of bubbles. Scientifically, the boiling point is defined as the temperature at which the vapour pressure of a liquid equals the surrounding environmental pressure. 

At the molecular level, boiling is the result of kinetic energy. As water is heated, molecules move faster.

When their energy is sufficient to overcome intermolecular forces, they escape into the vapour phase. This process occurs throughout the liquid once the vapour pressure equals atmospheric pressure.

The International Union of Pure and Applied Chemistry (IUPAC) defines the standard boiling point at one bar (100 kPa), which is 99.61°C (or 211.3°F for our neighbours across the ocean). 

These values are so close to 100°C that the rounded figure has become the accepted shorthand.

Once water reaches its boiling point, additional heat does not raise the temperature further. Instead, the energy goes into breaking intermolecular bonds, allowing molecules to escape into the vapour phase. This energy is known as the latent heat of vaporisation. It is why boiling water remains at a constant temperature until all of it has turned to steam.

So all of this is a long-winded way of saying that the boiling point of water is 100°C? 

Well, not quite. In perfect conditions, yes water will boil at 100°C, but what about the factors that influence it?

What can influence the true boiling temperature of water?

Interestingly, the boiling point of water varies with atmospheric pressure, which itself changes with altitude. At higher elevations, the air pressure is lower, so water boils at a lower temperature.

At sea level, under standard atmospheric pressure (101.325 kPa or 1 atm), the boiling point of pure water is very close to 100°C. In fact, with scientific precision, it is measured at 99.97°C (211.95°F). 

Head up to 1,905 metres - the height of Mont Chiran in France (Ben Nevis is 1,345m for content) - water boils at 93.4°C (200.1°F).

Keep going to the peak of the world, good old Mount Everest (which is 8,848 metres) and water will boil at just 71°C (160°F).

That’s quite a difference, and yes, we know that there isn’t a huge number of people looking to get a brew from the top of Everest. But what’s important here is that the boiling point of water is not fixed.

As a general rule, the boiling point drops by about 1°C for every 300 metres above sea level. This explains why cooking times must be adjusted at altitude. Pasta or rice cooked in boiling water will never reach the same temperature as at sea level, so it takes longer to soften.

And it's not the only thing that can affect the boiling point of water. Another is the purity of the water used.

Pure water is rarely encountered outside the laboratory. Tap water contains dissolved minerals and salts, which can slightly alter its boiling behaviour. 

The presence of non‑volatile solutes raises the boiling point, a phenomenon known as boiling point elevation. Salt water, for instance, boils at a higher temperature than pure water.

In everyday kitchen use, however, the effect is minimal. The impurities in potable water are not concentrated enough to change the boiling point by more than a fraction of a degree.

Lastly, there’s the container used to boil water.

Most kettles are open systems, which vents steam freely. In a sealed container, however, the pressure above the liquid can be increased, which raises the boiling point. 

This principle is used in pressure cookers, where water can reach temperatures well above 100°C, speeding up cooking dramatically.

What’s going on in the kettle boiling point?

So we’ve established that unless you’re attempting to boil laboratory-grade pure water in a sealed kettle on top of Earth’s largest mountain, you’ll probably get the standard boiling point of 99.61°C.

So, what’s going on in the kettle, and why does it appear to boil faster than it should?

To answer that, we need to look at localised boiling. The bubbling seen in a kettle before the water reaches 100°C is caused by localised boiling. 

This occurs when the heating element transfers energy directly to the water in contact with it. This small volume of water can reach the boiling point quickly, forming bubbles that rise through the cooler bulk liquid. 

The overall temperature of the kettle may still be below 100°C, but the localised boiling creates the impression that the entire kettle is boiling.

This can cause discrepancies with the placement of sensors in your kettle. Many kettles with built‑in thermometers measure the temperature of the bulk water or the outer wall, rather than the hottest water at the element. 

As a result, the displayed temperature may lag behind the true boiling point at the element. It is common to see vigorous bubbling when the gauge reads 90°C, even though the water at the element has reached 100°C.

And speaking of bubbles, let’s put our science hats back on for a moment and talk about nucleation sites.

Bubbles form at nucleation sites, which are tiny imperfections or particles in the kettle. In very smooth containers, water can be superheated above its boiling point without forming bubbles, until a disturbance triggers rapid boiling. This is why microwaved water in a smooth glass can sometimes erupt suddenly when disturbed.

This can be further exacerbated by the kettle’s shut‑off mechanisms. Basic electric kettles use a bimetallic switch triggered by steam. 

The kettle continues heating until enough steam is produced to activate the switch, at which point the kettle turns off. This ensures that the water has reached boiling, even if the displayed temperature is lower. 

More advanced kettles with digital controls can be set to stop heating at specific temperatures, useful for brewing teas or coffees that require water below boiling.

Electric kettles are designed to maximise heat transfer. The element is in direct contact with the water, to provide efficient heating. The wide base of pyramid kettles allows for even distribution of heat, while jug kettles use a more upright design. Both rely on convection currents to circulate water and equalise temperature.

The everyday implications of boiling water

We’ve delved into the science behind boiling water, but what does this mean for you? Well, actually, a lot.

Different drinks benefit from different water temperatures. Black tea is usually brewed with water close to boiling, while green tea is best at 70–80°C to avoid bitterness. 

Coffee brewing methods also vary. Filtered coffee typically uses water around 92–96°C, while espresso machines rely on pressurised water at similar temperatures.

For those living or travelling at altitude, the lower boiling point of water has a big impact on cooking. Foods that rely on boiling water, such as pasta or rice, take longer to cook. In these instances, pressure cookers are often used to compensate, raising the boiling point by increasing pressure.

Lastly, it's important to remember that understanding boiling behaviour also has safety implications. Steam can cause severe burns, and water at 90°C is still hot enough to scald. The apparent paradox of bubbling below 100°C should not lead to underestimating the danger of hot water.

Find your new kettle at Morphy Richards

Now that we’ve learned everything about the boiling point of kettle water, why not have a look at our range?

When you buy direct from Morphy Richards, you’ll benefit from a three-year warranty (with an extra year of warranty for free when you register your heater with us).

We also offer free shipping and flexible payment options that let you spread the cost of your new portable air conditioner.

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For more home appliance buying guides, inspiration and recipes, explore the Morphy Richards blog…

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