Hi Holidayers,

A science question. This is for Speedy’s Jnr’s science assignment, where he needed to add salt to water to find whether the boiling point increased.

He chose to use 2 cups of water, boil it and take a temperature measurement, add a teaspoon of salt, take another measurement etc. until 4 teaspoons of salt were added. Results were as expected, with the temperature gradually increasing. He has almost completed writing his report on this, so all is good.

However, I expected the boiling point of the water to be 100 degC and to increase from there. What he found was that the temperature was only 98.60 degC and increased at 4 tsp salt to 99.92 degC. What is likely to have happened here? Speedy Jnr says that only the water bubbling at the bottom of the pot would measure at 100 degC and that the remainder is slightly cooler. My guess is that it has something to do with a digital thermometer that needs calibrating or impurities in the tap water.

Speedy said:

Hi Holidayers,

A science question. This is for Speedy’s Jnr’s science assignment, where he needed to add salt to water to find whether the boiling point increased.

He chose to use 2 cups of water, boil it and take a temperature measurement, add a teaspoon of salt, take another measurement etc. until 4 teaspoons of salt were added. Results were as expected, with the temperature gradually increasing. He has almost completed writing his report on this, so all is good.

However, I expected the boiling point of the water to be 100 degC and to increase from there. What he found was that the temperature was only 98.60 degC and increased at 4 tsp salt to 99.92 degC. What is likely to have happened here? Speedy Jnr says that only the water bubbling at the bottom of the pot would measure at 100 degC and that the remainder is slightly cooler. My guess is that it has something to do with a digital thermometer that needs calibrating or impurities in the tap water.

how high above sea level are you? and your guesses sound pretty good too.

ChrispenEvan said:

Speedy said:

Hi Holidayers,

A science question. This is for Speedy’s Jnr’s science assignment, where he needed to add salt to water to find whether the boiling point increased.

He chose to use 2 cups of water, boil it and take a temperature measurement, add a teaspoon of salt, take another measurement etc. until 4 teaspoons of salt were added. Results were as expected, with the temperature gradually increasing. He has almost completed writing his report on this, so all is good.

However, I expected the boiling point of the water to be 100 degC and to increase from there. What he found was that the temperature was only 98.60 degC and increased at 4 tsp salt to 99.92 degC. What is likely to have happened here? Speedy Jnr says that only the water bubbling at the bottom of the pot would measure at 100 degC and that the remainder is slightly cooler. My guess is that it has something to do with a digital thermometer that needs calibrating or impurities in the tap water.

how high above sea level are you? and your guesses sound pretty good too.

About 100m. Did you mean his guesses or mine?

Dodgy thermometer or low air pressure…

The 100 degC applies to pure water at STP.

Speedy said:

ChrispenEvan said:

Speedy said:

Hi Holidayers,

A science question. This is for Speedy’s Jnr’s science assignment, where he needed to add salt to water to find whether the boiling point increased.

He chose to use 2 cups of water, boil it and take a temperature measurement, add a teaspoon of salt, take another measurement etc. until 4 teaspoons of salt were added. Results were as expected, with the temperature gradually increasing. He has almost completed writing his report on this, so all is good.

However, I expected the boiling point of the water to be 100 degC and to increase from there. What he found was that the temperature was only 98.60 degC and increased at 4 tsp salt to 99.92 degC. What is likely to have happened here? Speedy Jnr says that only the water bubbling at the bottom of the pot would measure at 100 degC and that the remainder is slightly cooler. My guess is that it has something to do with a digital thermometer that needs calibrating or impurities in the tap water.

how high above sea level are you? and your guesses sound pretty good too.

About 100m. Did you mean his guesses or mine?

both lots really.

Speedy said:

Hi Holidayers,

A science question. This is for Speedy’s Jnr’s science assignment, where he needed to add salt to water to find whether the boiling point increased.

He chose to use 2 cups of water, boil it and take a temperature measurement, add a teaspoon of salt, take another measurement etc. until 4 teaspoons of salt were added. Results were as expected, with the temperature gradually increasing. He has almost completed writing his report on this, so all is good.

However, I expected the boiling point of the water to be 100 degC and to increase from there. What he found was that the temperature was only 98.60 degC and increased at 4 tsp salt to 99.92 degC. What is likely to have happened here? Speedy Jnr says that only the water bubbling at the bottom of the pot would measure at 100 degC and that the remainder is slightly cooler. My guess is that it has something to do with a digital thermometer that needs calibrating or impurities in the tap water.

Atmospheric pressure can alter the BP. https://socratic.org/questions/how-do-atmospheric-pressure-and-elevation-affect-boiling-point

Air pressure should have occurred to me straight away, but it didn’t and it would not for a very long time.

Thanks for your replies.

Speedy (and Speedy Jnr)

Tamb said:

Speedy said:

Hi Holidayers,

A science question. This is for Speedy’s Jnr’s science assignment, where he needed to add salt to water to find whether the boiling point increased.

He chose to use 2 cups of water, boil it and take a temperature measurement, add a teaspoon of salt, take another measurement etc. until 4 teaspoons of salt were added. Results were as expected, with the temperature gradually increasing. He has almost completed writing his report on this, so all is good.

However, I expected the boiling point of the water to be 100 degC and to increase from there. What he found was that the temperature was only 98.60 degC and increased at 4 tsp salt to 99.92 degC. What is likely to have happened here? Speedy Jnr says that only the water bubbling at the bottom of the pot would measure at 100 degC and that the remainder is slightly cooler. My guess is that it has something to do with a digital thermometer that needs calibrating or impurities in the tap water.

Atmospheric pressure can alter the BP. https://socratic.org/questions/how-do-atmospheric-pressure-and-elevation-affect-boiling-point

That in itself would make for interesting experiments going to various elevations and testing it

Speedy said:

Air pressure should have occurred to me straight away, but it didn’t and it would not for a very long time.

Thanks for your replies.

Speedy (and Speedy Jnr)

No worries.

Ian said:

The 100 degC applies to pure water at S T P.

There are two conventions regarding the standard boiling point of water: The normal boiling point is 99.97 °C (211.9 °F) at a pressure of 1 atm (i.e., 101.325 kPa). The IUPAC recommended standard boiling point of water at a standard pressure of 100 kPa (1 bar) is 99.61 °C (211.3 °F).

Cymek said:

Tamb said:

Speedy said:

Hi Holidayers,

A science question. This is for Speedy’s Jnr’s science assignment, where he needed to add salt to water to find whether the boiling point increased.

He chose to use 2 cups of water, boil it and take a temperature measurement, add a teaspoon of salt, take another measurement etc. until 4 teaspoons of salt were added. Results were as expected, with the temperature gradually increasing. He has almost completed writing his report on this, so all is good.

However, I expected the boiling point of the water to be 100 degC and to increase from there. What he found was that the temperature was only 98.60 degC and increased at 4 tsp salt to 99.92 degC. What is likely to have happened here? Speedy Jnr says that only the water bubbling at the bottom of the pot would measure at 100 degC and that the remainder is slightly cooler. My guess is that it has something to do with a digital thermometer that needs calibrating or impurities in the tap water.

Atmospheric pressure can alter the BP. https://socratic.org/questions/how-do-atmospheric-pressure-and-elevation-affect-boiling-point

That in itself would make for interesting experiments going to various elevations and testing it

I think that experiment was done in Victorian times.

Tamb said:

Cymek said:

Tamb said:

Atmospheric pressure can alter the BP. https://socratic.org/questions/how-do-atmospheric-pressure-and-elevation-affect-boiling-point

That in itself would make for interesting experiments going to various elevations and testing it

I think that experiment was done in Victorian times.

I imagine so, for school if you could do it with a reasonable amount of hassle

Cymek said:

Tamb said:

Cymek said:

That in itself would make for interesting experiments going to various elevations and testing it

I think that experiment was done in Victorian times.

I imagine so, for school if you could do it with a reasonable amount of hassle

You could simulate the conditions with a vacuum pump.

Tamb said:

Cymek said:

Tamb said:

I think that experiment was done in Victorian times.

I imagine so, for school if you could do it with a reasonable amount of hassle

You could simulate the conditions with a vacuum pump.

Or climb Mt Everest.

• You could simulate the conditions with a vacuum pump.

That. Is what she said…

Can you attach a photo of the digital thermometer?

Yes, the speed at which it is heated can also affect the measured temperature. That’s why distillation apparatus has a fractionating column above it. Unless you have a column like that, what he says is correct. The cold water above where it is boiling will reduce the measured temperature.

See how in the following diagram, the thermometer is placed in the gas not in the liquid.