One of the first somewhat scientific observations and experiments I did was as a child washing dishes.
One thing I noticed was that dish soap foamed up more in hot water than in cold?
Why is this? I know many chemical reaction tend to occur faster at higher temps, but this seems more of a physical property than chemical…and doesn’t ring right to me as a reason. Yeah, yeah…molecules moving faster and all, but I don’t see the connection between faster molecular movement and macroscale influence on more bubbles.
I don’t think anybody will be able to give a specific answer. One factor in forming bubbles is surface tension. Surface tension varies with temperature. An intermolecular bond between molecules has a certain energy. As the temperature increases the average energy of the molecules changes. More molecules have enough energy to break these intermolecular bonds. The average time that a water molecules are bound to one another changes. The average time that the soap molecules stick to water changes. The average time that the soap molecule stays in a miscelle or other supermolecular structure changes. None of these changes take place proportionally. As a result the properties of the solution change.
A different soap will likely have a different ideal temperature for forming bubbles.
Well thanks, but I’ve never encountered a soap that became less foamy in hot water. But that could be because they all use essentially the same chemical.
The surface tension idea intrigues me a bit. But I would have thought surface tension decreases with increased temperature…and I would have thought reduced surface tension wouldn’t stabilize bubbles so well. Am I wrong on that aspect?
Soap actually lowers the surface tension. I’m not entirely familiar with the physics of bubbles. It looks relatively complicated. here is the wiki
You are probably right that for most substances, the surface tension decreases with increasing temperature. I wouldn’t want to make that bet about a mixture where intermolecular interactions are so prominant.
I love this little quote about interferrence and reflection:
Totally
Dielectric constant decreases with increasing water temperature too. That’s going to mess with micelle size and stability. ‘Complicated’ is a good watch word here.
Physics grad student here … this doesn’t seem all that complicated to me. When you first pour the dish soap out of the bottle, it’s a smoothly flowing liquid at room temperature. As a liquid, the soap molecules have a certain range of mobility - they will slide over and around each other very easily, but will not separate much. Agitation then produces bubbles in the liquid; this entails that some of the soap molecules escape the confines of the main volume of soap. However, since the energy added is insufficient to generate a phase change, the liberated molecules do not become a gas, but remain a liquid film anchored to other soap molecules, aka a bubble.
Adding extra heat in the form of hotter water means we have more soap molecules in just the right range of energy to, once the agitation happens, liberate themselves in this fashion. An analogy may be made to political protesters. Most of the population flows along as a smooth liquid, not protesting the issue. Agitation induces a certain number of people to march on the capital with signs and chants (forming bubbles). However, if you add more heat first, in the form of rousing speeches by advocates of change, then when the agitation happens you get more people marching on the capital.
so with that line of thinking, you are saying th molecules in a drop of soap in cold water is not fully solvated and diffused through the solution? Implying that in cold water there are “clumps” of soap molecules, but in hot water they are more evenly and thouroughly distributed?
Is that your thought?
If so, we can try an experiment, where we take soap in hot water, so that the molecules are totally distributed…cool half of it. (make the assumption the soap molecules don’t “reclump”) and see if the cool water and hot water foam at the same amounts.
My guess is that they will foam at different amounts (hot with more foam)
Do I have your hypothesis correct?
I think you are neglecting the complex dynamic in the solution. The soap solution is made up of at least two types of molecules that are ordering themselves in transient structures (lamellar layers and miscelles). The sizes and shapes of these structures are going to vary as the temperature increases. The ability of soap solutions to form bubbles is more dependent on the interactions between the solvent and the surfactant than your kinetic theory on liquids. Pure liquids do not form bubbles very well.