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-Hello everyone,

Yesterday I was preparing some tomato sauce and, in order to remove some water from the sauce, I put it in a pot on the burner and waited until it started to evaporate. When I sow a steady quantity of vapor coming out of the pot I started to blow on it in order to remove some vapor and let the new one create. Is blowing on the pot lowering the time the water needs to evaporate because the already created vapor 'get out of the way' faster or is it enlarging the time because the "cool" air is lowering the temperature of the water/sauce in the pot?

To summarize the question:

Is blowing constantly over a pot with some watery food in it making a difference in time the water needs to evaporate?

and

In the case of the sauce, blowing on it lower or enlarge the time the water needs to evaporate?

This question is more related to physics of liquids instead of how to properly cook a sauce.

Please feel free to update the title and/or the tags if necessary

Update:

The sauce was at (water) boiling temperature. On the top of the sauce bubbles created from boiling was visible.

Assuming that blowing on the pot lower the time the water takes to evaporate obviously there is a "limit" on how much you can blow on the pot before the quantity of the air blown starts to cool the sauce instead of only removing the vapor. How can I try to determine that amount of air?

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2 Answers 2

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Blowing on the pot will significantly increase the evaporation of vapor.

When the sauce boils, it means that vapor bubbles are forming throughout the pot, rising to the top. Blowing may slightly lower the temperature on the surface, but it will not influence the boiling throughout the pot.

On the other hand, the rate of evaporation is determined by the relative humidity of the air above the pot, to put it simply (more technical: the gradient of chemical potential). Blowing on the pot replaces the vapor-saturated (high-humidity) air with air that can take up water vapor (technical: you maintain a large gradient in chemical potential).

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If you do blow then this helps vapor to go away and make free space for the next portion, as you said. That's why blowing makes evaporation faster.

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    $\begingroup$ Have you done the experiment? A counter-argument could be that blowing cools the water, and therefore slows down the evaporation. $\endgroup$
    – Luboš Motl
    Commented May 25, 2016 at 6:12
  • $\begingroup$ I have no firm understanding about temperature of sauce but guess it is pretty close to water boilng temperature since vapor is visible. When the pot is on the boiler its temperature is maintained on the same level. Anywhere you cannot exceed boiling temperature even if you cover the pot, so at this time no much matter for the temperature if you blow or don't. Cooling of the sauce would even help it to pick up more heat from the flame beause colder sauce will be warmed up quicker than the hot. $\endgroup$
    – dmafa
    Commented May 25, 2016 at 6:21
  • $\begingroup$ UPDATE: Being not a good cooker sometimes I do, and can confirm with own experience that water evaporates from covered pot slower than from that is not covered, because opened pot gives vapor free way to leave while cover obstructs. The question is effectively about the same matter. $\endgroup$
    – dmafa
    Commented May 25, 2016 at 6:33
  • $\begingroup$ I find these comments implausible. Covering the pot physically prevents the vapor from getting out - it is not the same condition as "not being covered and not being blown at". So there's no reliable analogy here. Also, the temperature on and near the surface of the water is what matters and it's affected both by the fire beneath the pot as well as the air above it. I can't imagine in what sense the cooler air at the top could be irrelevant. With blowing, there's probably a greater gradient of the temperature (temperature difference between bottom and top of pot). Much of water is cooler. $\endgroup$
    – Luboš Motl
    Commented May 25, 2016 at 6:42
  • $\begingroup$ I will try the experiment in the next few days $\endgroup$
    – Rocco Mancin
    Commented May 25, 2016 at 7:11

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