Thread by @BucknellThermo

Charlie and Laney now taking over with our explainer on the concept of boiling point elevation and freezing point depression! 1/

How much salt do YOU add to YOUR pasta water?
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Why do you add the salt?
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Today, we’re going to look at raising the boiling point (BP) of things, namely water. We’re going to explore the question: Does the salt ACTUALLY change the boiling point and make your pasta boil faster?
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The pop science explanation of salting pasta water is that salt makes the pasta cook faster by raising the BP of the water. This is true, but how much does it really change?
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Some chemical reactions happen quickly, some happen more slowly. What’s the difference? “Activity” is the ‘force’ that makes reactions happen. Low activity means a slow reaction and high activity means a fast reaction.
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When you add salt to one side of a cup of water with a semipermeable membrane (water can cross but salt can’t), what will the water do? 🌊🌊
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The water on the pure side flows towards the salty side. This is because the water wants equal activity on both sides of the membrane.
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The amount of water that flows from one side to the other depends on the activity, the more salt (lower activity), the more water flows. When talking about activity, the amount of salt is measured using molality (more on molality later).
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Molality is called a colligative property. A colligative property is one that doesn’t depend on what you are measuring, but only on how much.
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For example, you could have a dozen eggs or you could have a dozen frogs. They are completely different things, but the amount is the same.
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If you take four eggs away and replace them with four frogs, you still only have 8 eggs to scramble. 13/

Molality is the measure of how much salt is dissolved. A solution of water with NaCl (table salt) might have a different molality than CaCl2 (a popular road salt). 14/

When you dissolve salt in water, it splits into its respective pieces. NaCl splits into 1xNa, and 1xCl. CaCl2 would split into 1xCa, and 2xCl. So the molality of a NaCl solution might be 2, but the molality of a CaCl2 solution might be 3.
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Molality is related to the unit mol. A mol is a measure of how many molecules there are. Different chemicals have different amounts of mols per amount of grams. For example, 1 mol of NaCl is 58.44 grams, but 1 mol of CaCl2 is 110.98g.
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Molality relates to mols in that molality is the count of how many mols there are of each individual component. So 1 mol of NaCl, has a molality of 2 because there is 1 mol of Na, and 1 mol of Cl.
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The reason molality changes activity is because the solution isn’t pure water anymore. Now, it’s water diluted with salt. The higher the molality, the bigger the change in activity.
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Another thing that changes the rate of a reaction is the temperature, because temperature also changes activity. This graph of activity versus temperature shows how the activity of water changes with temperature.
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A gas behaves differently than a liquid, which is why the graph also shows that each of the phase lines has a different slope.
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If we move one of the lines, the point where that line intersects the others changes. So if we move the liquid line a little lower, we can see the freezing point and boiling point have changed.
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These lines are just the activity of the water, so when we add salt to water, we are moving the liquid line down. And that's why salt changes boiling point and freezing point!
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Every liquid is different, so each liquid changes its boiling point differently. We calculate the change in boiling point using the ebullioscopic constant K_b.
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(‼️Fun side note: boiling point is measured just by boiling a pot of liquid. There is actually a special tool that you use called an ebullioscope!😱)
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Once you have the ebullioscopic constant, it’s super easy to calculate how much the boiling point changes. Just use the formula: ΔT=K_b*b, where b is the molality and K_b is the ebullioscopic constant.
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As we saw when looking at the graph of temperature and activity, adding salt changes the freezing point too, but we can’t calculate that with the ebullioscopic constant.
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Luckily, the freezing point depression is just as easy to calculate. You just use the formula: ΔT=K_f*b, where b is the molality and K_f is the cryoscopic constant.
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K_f is just like K_b, each liquid has a unique constant, which is measured by doing a bunch of boring experiments in a lab.
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One example of where this property is useful is the northern parts of the world where it snows for half the year. Workers put salt on the roads to help melt the ice. But how well does that work? 29/40

The most popular salt to use on roads is NaCl. This is the same as table salt, but for roads they use the mineral version, which is coarser and a little less pure. It’s also much cheaper. 30/40

To calculate how much the freezing point changes for a certain amount of salt, we need to convert the amount of salt from mass into moles, then into molality. 31/40

Using the molar mass (number of mols in 1g) of salt, we find that 5g of salt has 0.0856mol. Because there is 1mol of Na and 1mol of Cl, this would be a molality of 0.171 in 1L of water.
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Using the equation we talked about earlier and a cryoscopic constant for water of 1.853, we find that 5g of salt in 1L of water changes the freezing point of water by 0.32C. This is a change of about half a degree Fahrenheit. 33/40

The change in freezing point isn’t all that much, but we know that salting roads actually works. So what is going on? ❓⁉️ 34/40

When you put down salt on a road with snow on it, the salt is only interacting with the very tiny amount of water around it, so the molality is actually huge because there is so much salt and so little water. 35/40

Because there is so little water and so much salt, salting a road can actually pull the freezing point down to about -6F. That’s pretty cold! 36/40

What about the boiling change? Unlike our example with salting the road, we have a lot of water when we are trying to do something like make pasta. Let’s take a look. 37/40

The ebullioscopic constant for water is 0.512, so using the same 5g of salt and 1L of water, that comes out to a change of boiling point of 0.0876C. That’s a tiny change in boiling point. 38/40

If you do the calculation backwards to see how much salt you need to change the boiling point of water by 1 degree C, you would need 116g of salt. That’s about half a cup of salt. 39/40

To summarize: a little salt goes a long way in freezing point depression, but the amount of salt ~normally~ used to cook doesn't actually change the boiling point of water much at all. 40/40