Tag: water steam and ice

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.

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.

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.

Why can ice, water and steam co-exist at “triple point”? — CL

Let’s start with three simpler problems: the coexistences of ice and water, of water and steam, and of ice and steam. Each pair of phases can coexist whenever the water molecules leaving one phase are replaced at an equal rate by water molecules leaving the second phase. This isn’t as hard as it sounds. In ice water, the water molecules leaving the ice cubes for the liquid are replaced at an equal rate by water molecules leaving the liquid for the ice cubes. In a sealed bottle of mineral water, the water molecules leaving the liquid for the water vapor above it are replaced at an equal rate by water molecules leaving the water vapor for the liquid. And in an old-fashioned non-frostfree freezer with a tray of ice cubes, the water molecules leaving the ice cubes for the water vapor around them are replaced at an equal rate by water molecules leaving the water vapor for the ice cubes.

In each case, there is some flexibility in temperature—these coexistence conditions can be reached over at least a small range of temperature by varying the pressure on the system. In fact, at 0.03&deg C and a pressure of 6.11 torr; pure water, pure ice, and pure steam can coexist as a threesome. At this triple point, water molecules will be moving back and forth between all three phases but without producing any net change in the amount of ice, water, or steam.

Why does cold water defrost things faster than hot water? — BS, Chicago, IL

I can’t think of any situation in which what you say would be true. Hot water should always defrost things faster than cold water. That’s because the rate of heat flow between two objects always increases as the temperature difference between them increases. When you put frozen food in hot water, heat flows into that food faster than it would from cold water because the temperature difference is larger.

Why does water sound loudest just before it reaches the boiling point, and then why does it get quieter once it actually boils? — KS

When you heat water on the stove, heat flows into the water from below and the water at the bottom of the pot becomes a little hotter than the water above it. As a result, the water at the bottom of the pot boils first and its steam bubbles begin to rise up through the cooler water above. As they rise, these steam bubbles cool and collapse—they are crushed back into liquid water by the ambient air pressure. These collapsing steam bubbles are noisy. When the water finally boils throughout, the steam bubbles no longer collapse as they rise and simply pop softly at the surface of the liquid.

Will ice cubes made out of heavy water (water that is rich in the heavy isotopes of hydrogen) sink to the bottom of a glass of water? — RN, Denmark

Yes. Heavy water ice is about 1% more dense than liquid water at its melting temperature of 3.82° C. I wouldn’t recommend drinking large amounts of heavy water, but you could make sinking ice cubes out of it.

What is reverse osmosis and how it is used in the process of purifying seawater for drinking water? — CS

In the form used for water desalination, reverse osmosis involves a special membrane that allows water molecules to pass through it while blocking the movement of salt ions. When water molecules are free to move between two volumes of water, they move in whichever direction reduces their chemical potential energy. The concept of a chemical potential is part of statistical physics—the area of physics that deals with vast collections of particles—and it depends partly on energy and partly on probability. Factors that contribute to a water molecule’s chemical potential are the purity of the water and the water’s pressure. Increasing the salt content of the water lowers a water molecule’s chemical potential while increasing the water’s pressure raises its chemical potential.

Because salty water has a lower chemical potential for water molecules than pure water, water molecules tend to move from purer water to saltier water. This type of flow is known as osmosis. To slow or stop osmosis, you must raise the chemical potential on the saltier side by applying pressure. The more you squeeze the saltier side, the higher the chemical potential there gets and the slower water molecules move from the purer side to the saltier side. If you squeeze hard enough, you can actually make the water molecules move backwards—toward the purer side! This flow of water molecules from the saltier water toward the purer water with the application of extreme pressure is known as reverse osmosis.

In commercial desalination, high-pressure seawater is pushed into jellyroll structures containing the semi-permeable membranes. The pressure of the salty water is so high that the water molecules flow through the membrane from the salty water side to the pure water side. This pure water is collected for drinking.

Why are there no bubbles in carbonated water until you open the sealed cap? Why are the bubbles inside the opened bottle then larger than the ones in a glass? — EP

When the bottle is sealed, its contents are in equilibrium. In this context, equilibrium means that while carbon dioxide gas molecules are continuously shifting from solution in the water to independence in the gas underneath the cap, there is no net movement of gas molecules between the two places. Since the company that bottled the water put a great many gas molecules in the bottle, the concentration of dissolved molecules in the water is high and so is the density of molecules in the gas under the cap. This high density of gaseous carbon dioxide molecules under the cap makes the pressure inside the bottle quite high, which is why the bottle’s surface is taut and hard.

While you can’t see it in this unopened bottle, there is activity both at the surface of the water and within the water. At the water’s surface, carbon dioxide molecules are constantly leaving the water for the gas under the cap and returning from the gas under the cap to the water. The rates of departure and return are equal, so that nothing happens overall. Within the water, tiny bubbles are also forming occasionally. But these tiny bubbles, which nucleate through random fluctuations within the liquid or more often at defects in the bottle’s walls, can’t grow. Even though these bubbles contain gaseous carbon dioxide molecules, the molecules aren’t dense enough to keep the bubbles from being crushed by the pressurized water. So these tiny bubbles form and collapse without ever becoming noticeable.

However, once you remove the top from the bottle, everything changes. The bottle’s contents are no longer in equilibrium. To begin with, carbon dioxide molecules that leave the surface of the water are no longer replaced by molecules returning to the liquid. That’s one reason why an opened bottle of carbonated water begins to lose its dissolved carbon dioxide and become “flat.” Secondly, without its trapped portion of dense carbon dioxide gas, the bottle is no longer pressurized and it stops being taut and hard (assuming that it’s made of plastic rather than gas). Thirdly, with the loss of pressure, the water in the bottle stops crushing the tiny gas bubbles that form within it. In fact, once one of those bubbles forms, carbon dioxide molecules can enter it from the liquid just as they enter the gas at the top of the bottle. As a result, each bubble that forms grows larger and larger. Since the gas in a bubble is less dense than water, the bubble begins to float upward until it reaches the top of the bottle. Because the bottle is taller than a typical water glass, a bubble has more time to grow before reaching the top in the bottle than it would have in the glass. That’s one reason why the bubbles in a bottle are taller than in a glass. Another reason is that the concentration of dissolved carbon dioxide molecules is higher while the water is in the bottle than it is by the time the water reaches the glass, so that bubbles grow faster in the bottle than in the glass.

Is there rain above the clouds? — JM, Arlington Heights, Illinois

No. If you are above the clouds, then the sky above you is free from droplets of condensed moisture. While that doesn’t mean that there is no water overhead, that water must be entirely in the form of gaseous water molecules. Since rain forms when droplets of condensed moisture grow large enough to descend rapidly through the air, the absence of any condensed droplets makes it impossible for full raindrops to form. In short, no clouds overhead, no rain.