Question

when you boil water it evaporates but when you boil milk it overflows.why?don't copy from Google own answers only.

Answer 1

Milk overflows when boiled for thefirst time because it contains entrapped air. The air expands on heating and tends to escape lifting thesurface of milk. Once all the air is expelled, the boiling would be regularand smooth. A second boiling would not lead to overflow.

Answer 2

I will need to make some assumptions about grammar and syntax to answer this.

By “falls from the container” I will assume you mean that boiling milk “boils over” unless the heat is promptly reduced once it comes to a boil; that is, the milk becomes foamy, its volume within the stewpot or other container in which it is breing boiled increases, and, if allowed to continue long enough, the expanding, foamy mass will soon fill up the container and spill over the edge.

Milk does this because it is not a pure liquid but a mixture of water with various other solid particulates — relatively heavy molecules of proteins, fats, and carbohydrates — some of which react to heat by trapping rising bubbles of water vapor (steam) caused by the heat boiling the water it contains, and keeping those bubbles intact (with a membrane of denatured — “cooked” — milk solids) when they reach the surface of the milk in the pot. This creates a foam as the persistent bubbles pile on top of one another, increasing the volume of the mixture dramatically.

Basically, the steam in the bubble “cooks” the milk-sold molecules that immediately surround the bubble — not to any great depth (or else the whole pot of milk would turn solid, like a cooked egg white), but enough to form a membrane around the steam bubble. Those solid but microscopically thin membranes continue to exist — forming a “scum” on top of the boiled milk — even after the bubbles “pop” and release the steam.

Pure water does not do this when boiled, because the liquid water itself does not “cook” and turn more solid when exposed to steam. Yes, steam bubbles form, and rise to the water surface, and exist briefly (due, in this case, to water surface tension) before they pop and release the steam, then they blend imperceptibly back into the liquid mass that is being brought to a boil.

Know what else “foams” and “boils over” when it is heated to a boil? Just about any liquid food item containing solid particulates: soup, chocolate sauce, pasta in water, whatever. Also sea water — which is not pure water, either, but also contains solid particulates. That’s also why a foamy scum forms at the seashore when waves break.

Foaming can be caused by other methods than application of heat to “cook” solid molecules of organic matter floating in the mix. For instance, carbonated beverages (those containing dissolved carbon dioxide in the liquid) may foam up when poured into a container that has an irregularities — rough surfaces, dust particles, or whatever — which stimulate release of the dissolved gas, especially if the beverage is not just carbonated water, but also contains particulates, such as sugar, fruit juices, or food coloring. That’s why your soda or beer may fuzz up when you pour it into a glass, and even more so when you pour it into a styrofoam cup (with a very rough surface



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