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when mom fixes instant coffee, she boils water in her mug (in the microwave). When she adds the coffee, it "foams" up very rapidly. We guess the boiling point of water is lowered by the addition of the instant coffee granules, but this does not make sense in light of your explanations about a solute RAISING the boiling point of the solvent. So what gives?
Millard North High School, Omaha, NE
The water may be slightly super-heated, above its actual boiling
point. Adding the coffee would provide a lot of little sites to trigger
('nucleate') the boiling process. The water doesn't all boil away
because when a little bit boils, that lowers the temperature enough so
the rest doesn't boil.
It's also possible that the water isn't quite at its boiling
point, but that the coffee provides nucleation sites for excess
dissolved air to come out. (I think the solubility of oxygen and
nitrogen drop as the temperature is raised.) However, I suspect that
would be unlikely to be so routinely obvious.
Instant coffee is made so that it has a large surface area in the
granules so it mixes and dissolves as quickly as possible for impatient
coffee consumers. It is often made by freeze-drying a liquid, which
makes a light and powdery product. There are lots of air spaces tucked
inside the grains. As the coffee dissolves, the air gets let out. The
foam should be no bigger than the volume of the instant coffee stirred
in. I've noticed a similar effect when making chocolate milk from a
(republished on 07/24/06)
Follow-Up #1: why does instant coffee foam?
I've noticed that instant coffee only foams if (1) the coffee is added to water "boiled" in a microwave, and (2) does not foam if it is added to water brought to the boil (whistling point) in a tea kettle, or (3) if the water is poured into the cup after the instant coffee is in the cup.
- John Thomas (age 72)
That's very useful data. It rules out the possibility that the foam is from trapped air in the coffee, since that would be present in each case. The microwave method can easily superheat the water in the middle of the cup. In a tea kettle, the heat comes in from the very hot bottom surface, which triggers boiling rather than superheating. I guess that pouring allows a lot of surface evaporation and cooling, followed by mixing, also reducing the superheating.
(published on 09/26/12)
Follow-Up #2: measuring superheating
To Mike W.: This morning, I heated the water for my coffee in "my cup" -- a mug filled with exactly 1 and 1/4 cup of water -- in a microwave over for 90 seconds. Then I stirred the water and then I checked the temperature. It was 170 degrees F. Then I added the instant coffee. While there was some foam, there wasn't a violent foaming reaction. On several occasions, the reaction has been so violent, the foaming actually cause the water in the mug to overflow, which is why I posted what I posted in the first place. The next step is to try to measure the water in the cup without stirring it, to see if in fact there is a columjn of superheated water in the center, a column that would be, I assume, surrounded by much cooler water. Since I don't have a thermometer that is instantaneous, it will be interesting to figure out how to accomplish the measurement.
- John Thomas
This is an interesting challenge. One way is to test if you can trigger boiling. You'd want something with a good boiling-triggering surface but with low thermal conductivity, so it doesn't cool the water too much. I think a small teflon tube would be ideal. If water is superheated, it should trigger boiling. Be careful though, since you don't want your hand directly above some exploding hot water. The tube could be held with a long-nose pliers. If you can't find a teflon tube maybe you could just use some teflon pipe thread tape wrapped on a toothpick.
(published on 10/01/12)
Follow-up on this answer.