Month: October 1997

When I heat a cup of water in my microwave oven to 200 degrees, then put a spoonful of instant coffee in the hot water, it foams up. Hot water from a coffee maker does not do this. Why does water heated in a microwave oven do this? — WAH, Library, Pennsylvania

The microwave oven is superheating the water to a temperature slightly above its boiling temperature. It can do this because it doesn’t help water boil the way a normal coffee maker does. For water to boil, two things must occur. First, the water must reach or exceed its boiling temperature—the temperature at which a bubble of pure steam inside the water becomes sturdy enough to avoid being crushed by atmospheric pressure. Second, bubbles of pure steam must begin to nucleate inside the water. It’s the latter requirement that’s not being met in the water you’re heating with the microwave. Steam bubbles rarely form of their own accord unless the water is far above its boiling temperature. That’s because a pure nucleation event requires several water molecules to break free of their neighbors simultaneously to form a tiny steam bubble and that’s very unlikely at water’s boiling temperature. Instead, most steam bubbles form either at hot spots, or at impurities or imperfections—scratches in a metal pot, the edge of a sugar crystal, a piece of floating debris. When you heat clean water in a glass container using a microwave oven, there are no hot spots and almost no impurities or imperfections that would assist boiling. As a result, the water has trouble boiling. But as soon as you add a powder to the superheated water, you trigger the formation of steam bubbles and the liquid boils madly.

Is it possible to make ice with neutral buoyancy, so that if you placed it halfway down a glass of water and released it, it would remain there and not float to the top or sink? B, Kent, England

Not without using something other than pure, normal water for the ice. The density of ice is always less than that of water at the same pressure. While squeezing the ice will increase its density, it will also increase the density of the water so the ice will always float. Of course, you could add dense materials to the ice to weight it down to neutral buoyancy, but then it wouldn’t be pure ice any more.

I know it’s difficult to get drinking water from salt water, but why is it so expensive? — MP, Chicago, IL

The simple answer is entropy—the ever-increasing disorder of the universe. Salt water is far more disordered than the salt and water from which it’s formed, so separating those components doesn’t happen easily. The second law of thermodynamics observes that the entropy of an isolated system cannot decrease—you can’t reduce the disorder of the salty water without paying for it elsewhere. In effect, you have to export the salty water’s disorder somewhere else as you separate it into pure water and pure salt.

In most cases, this exported disorder winds up in the energy used to desalinating sea water. You start with nicely ordered energy—perhaps electricity or gasoline—and you end up with junk energy such as waste heat. While some desalination techniques such as reverse osmosis can operate near the efficiency limits imposed by thermodynamics, they can’t avoid those limits. If you want to desalinate water, you must consume ordered resources and those resources usually cost money (an exception is sunlight). The desalinating equipment is also expensive. Until water becomes scarce enough or energy cheap enough, desalinated water will remain uncommon in the United States.

I have found that turning on all the burners of my stove on a cold winter day makes the kitchen feel moderately warm but putting a pot of water on to boil as well makes it feel much warmer, even if I use fewer burners. Why is that? — PM, Little Rock, Arkansas

When you simply heat the cold air, you lower its relative humidity—the heated air is holding a smaller fraction of its maximum water molecule capacity and is effectively dry. Dry air always feels colder than humid air at the same temperature. That’s because water molecules are always evaporating from your skin. If the air is dry, these evaporating molecules aren’t replaced and they carry away significant amounts of heat. On a hot day, this evaporation provides pleasant cooling but on a cold day it’s much less welcome. If the air near your skin is humid, water molecules will return to your skin almost as frequently as they leave and will bring back most of the heat that you would have lost to evaporation. Thus humid air spoils evaporative cooling, making humid weather unpleasant in the summer but quite nice in the winter.