Why Salt Makes an Ice Bath Colder
Written by Studio AM.
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A bowl containing only ice and liquid water tends toward a balance near water's normal freezing point. Add salt, and that balance changes. The salt dissolves into sodium and chloride ions, creating brine whose freezing point is lower than that of pure water. Liquid can therefore remain beside ice at a temperature below zero degrees Celsius.
Salt first needs a film of water in which to dissolve. As a result, some ice melts to make more brine. Melting requires energy to disrupt the ordered structure that holds water molecules in ice. That energy comes from the mixture and anything exchanging heat with it, so the bath becomes colder. Salt does not act as a packet of cold; it changes which phase is favored, and the resulting melting absorbs thermal energy.
Concentration matters. More dissolved particles can lower the freezing point across a useful range, but the effect is not unlimited. The mixture has a lowest freezing temperature at a particular concentration; adding salt beyond that does not keep lowering it. Incoming heat and melting ice also change the mixture. Hand-cranked freezers used this behavior by placing an ice-and-salt bath around a separate container. Heat moved from the inner mixture into the colder brine as the crank turned it. Dissolution, melting, and heat transfer allowed the bath to approach a colder equilibrium while cooling the food.
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Questions
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Question 1 of 4
What is the main idea of the passage?
The answer is A: Dissolved salt lowers water's freezing point, causing ice to melt and absorb energy so an ice bath can become colder.
The text connects freezing-point change, melting, energy absorption, and the colder brine; that full chain is the passage's central explanation.
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Question 2 of 4
Why does the temperature fall after some of the ice begins to melt?
The answer is B: Melting draws thermal energy from the mixture and nearby material.
The second paragraph says melting requires energy and that this energy comes from the mixture and objects exchanging heat with it.
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Question 3 of 4
In this passage, what is “brine”?
The answer is C: Water containing dissolved salt
The first paragraph says salt dissolves into ions and creates brine, so brine is the resulting saltwater solution.
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Question 4 of 4
How did historic hand-cranked freezers use the ice-and-salt mixture?
The answer is D: They placed colder brine around a separate inner container.
The final paragraph states that an ice-and-salt bath surrounded a separate container and received heat from the mixture inside.
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