According to an ancient folk tale, there was a wife who noticed that when she added salt to a pot of water on the stove, the water boiled faster than usual. So, does salt really make water boil faster?
“This story is true, but the difference between them is negligible” – a specialist responded to Live Science.
If it happens, the difference in boiling time is merely a few seconds.
“If you add a tablespoon of salt (less than 3 grams) to 1 liter of water, it won’t differ much from boiling water normally” – Lesley-Ann Giddings, an assistant professor of chemistry and biochemistry at Middlebury College in Vermont, stated.
“To put it simply: If it occurs, the difference in boiling time is merely a few seconds” – Giddings added.
It takes a considerable amount of energy to bring water to a boil. In fact, you need 1 calorie of energy to heat 1g of water by 1 degree Celsius (1.8 degrees Fahrenheit).
“To boil water, its vapor pressure must equal atmospheric pressure. This is part of the explanation for why water boils at a lower temperature on Mount Everest than it does at sea level. This is due to the lower air pressure, which affects the water at a height of 8,800 meters (29,000 ft)” – Giddings explained.
“However, we can imagine a kettle of water placed over a fire at sea level. When salt is added, it makes it harder for water molecules to escape from the kettle and enter the gas phase, which is what causes the water to boil. Therefore, saltwater boils at a higher temperature” – Giddings added.
“There are many explanations for this” – Giddings noted.
Saltwater temperature rises faster than pure water.
She explained that: “Specific heat – the amount of heat required to increase the temperature of a substance by 1 degree Celsius – is lower for saltwater compared to freshwater. This means that saltwater does not resist temperature changes like freshwater does; in other words, it releases less heat, which allows the temperature of saltwater to rise 1 degree Celsius more than freshwater.”
“Saltwater temperature rises faster than pure water. However, it still has a higher boiling point and a larger mass when salt is added to the same amount of water. Thus, this does not mean that saltwater boils faster” – Giddings affirmed.
You would need to add a lot of salt to the water to see a significant difference in boiling time for saltwater.
But the story changes if you consider two different amounts of water – meaning if you put less water in a kettle and add much more salt. Imagine there are two kettles: kettle A and kettle B. Kettle A has 100g of water while kettle B contains only 80g of water and 20g of salt.
100g of water in kettle A will have a higher specific heat, meaning it requires a significant amount of energy for the water in the kettle to boil. In contrast, the salt in kettle B dissolves, and dissolved salt has a lower specific heat compared to pure water, according to a post by Mike Dammann, manager of Inorganic Processes at the Southwest Research Institute in San Antonio, Texas.
“Moreover, kettle B has only 80g of water, which is less water to heat than kettle A. The 20% saltwater will heat faster than approximately 25% of pure water and will boil more quickly” – Dammann explained in an online statement.
“Therefore, kettle B will boil faster than kettle A because it contains less water and more salt” Dammann asserted.
“But a solution of water with 20% salt is quite salty; seawater only contains about 3.5% salt. Most people cannot use seawater for cooking because it is too salty” Giddings noted.
“In reality, you would need to add a lot of salt to the water to see a significant difference in boiling time for saltwater” – Giddings stated.
To better understand, let’s look at an experiment comparing boiling times between pure water and saltwater:
