Category: water steam and ice

How does steam work? — SS, Nairobi, Kenya

Steam is the gaseous form of water. When the water molecules in liquid or solid water have enough thermal energy, they can break free of one another and become independent particles. Even at room temperature, the air you are breathing is several percent water molecules. But at higher temperatures, the rate at which water molecules leave the surface of solid or liquid water increases so much that these water molecules can form a dense, high-pressure gas. This gas is called steam.

How fast can glaciers move? — SF, Burton, OH

Glaciers move at a variety of rates, ranging from inches per year to many feet per day. Currently there are very few “galloping” glaciers (those that move many feet per day) and most are either stagnant or retreating.

What happens when salt is added to water? If I mix 1 cup of salt with 1 cup of water, will I end up with 2 cups of solution? – RT

As a crystalline solid, salt consists of a beautiful cubic lattice of sodium atoms that have lost one electron to become sodium positive ions and chlorine atoms that have gained one electron to become chlorine negative ions. The crystal is held together by the attractive forces between these oppositely charged atomic ions. When a salt crystal dissolves in water, it decomposes into individual sodium positive ions and chlorine negative ions that are then carried about by shells of water molecules. Water molecules are electrically polar, meaning that they have positively charged ends and negatively charged ends. The water molecules line up around a positively charged sodium ion with their negatively charged ends inward and carry that ion about. Similarly, water molecules line up around a negatively charged chlorine ion with their positively charged ends inward and carry that ion about. Whether you will end up with 2 cups of solution after mixing 1 cup of salt and 1 cup of water depends on how tightly the atoms and molecules pack together in each case. Remember that your 1-cup of salt contains a fair amount of air between the salt grains. You’ll have to try it to find out the answer—I’m not sure what the answer will be.

Why does popcorn pop? – AB

Inside the hard, dry hull of a popcorn kernel is a portion of moist starch. When you heat the kernel well above the boiling temperature of water, the water in the starch converts to hot, high-pressure steam. The hotter this steam gets, the higher its pressure rises and the stronger the outward forces it exerts on the hull. Eventually, the hull rips open under the stress and exposes the starch to the low-pressure air around it. The pressurized steam then pushes the starch outward, expanding it to many times its original size. The kernel “pops.”

How does desiccant absorb and hold water? — JP, Houston, TX

Water molecules from the air are continuously colliding with surfaces and sometimes one of those water molecules will stay attached to a surface for some amount of time. That water molecule forms a weak chemical bond with the surface and remains there until thermal energy knocks it back into the air. As a result of this occasional sticking, most surfaces have a thin layer of water molecules on them. Desiccants are materials that tend to keep those water molecules for a relatively long time and that have lots of surface area on which those water molecules can stick. However, the strongest desiccants react chemically with water molecules so that those water molecules essentially never leave.

What effect does ice have on potholes? – AH

Water and ice are major contributors to potholes. When water flows into cracks in the road and then freezes, it tears the roadway apart. That’s because ice takes up more room than the water from which it’s formed—ice is less dense than water. Since the water expands as it freezes, it enlarges the cracks that contain it and gradually breaks up the roadway.

Can I soften small quantities of tap water by merely adding table salt to it? Any idea how much salt to add for tape water that is medium to very hard? I want enough to use in a steam iron regularly? — HD, Kintnersville, PA

There are two issues here. First, hard water is water that contains dissolved calcium, magnesium, and iron salts. The metal ions in these salts interfere with soaps and detergents, causing soaps to form soap scum and preventing detergents from effectively carrying away fats and oils. The standard way to soften water is to exchange sodium ions for the calcium, magnesium, and iron ions because sodium ions don’t have such bad effects on soaps and detergents. Adding salt to hard water, as you propose to do, won’t exchange sodium ions for the other ions. It will only add more metal ions to the water and the water will remain hard.

Second, a steam iron shouldn’t use hard water because when hard water boils away as steam, it leaves behind all the calcium, magnesium, and iron salts as unsightly scale. Again, adding salt to your hard water will simply leave more scale on the insides of your iron or on your clothes. You need demineralized water, not soft water, for your iron. The best way to demineralize water is to distill it.

At times a very thin invisible layer of ice forms on road surfaces. The road surface appears dry and does not have the telltale reflections of ice. Many people refer to this as “black ice.” How is this ice formed? What are the crystal properties that make it invisible? – BK

Black ice is a layer of ice that is almost free of internal defects or air bubbles and that does not have a smooth surface. The absence of internal defects or air bubbles is what makes it transparent rather than white. Snow and crushed ice appear white because they contain countless tiny surfaces. Whenever light changes speed, as it does in going from ice to air or air to ice, some of that light reflects. Since snow and crushed ice contain many ice/air interfaces, they reflect light extensively and appear white. In contrast, black ice contains no internal ice/air interfaces and doesn’t reflect any light from inside. Any light that makes it into the black ice goes all the way to the roadway. If the roadway reflects any of this light, it again passes unscathed through the black ice. The only evidence that the black ice exists at all comes from its surface, but here again the ice offers little that you can see. Since true black ice is microscopically rough, the small amount of light that reflects as it enters the ice from the air is reflected randomly in all directions. So little of that reflected light travels in any one direction that you can barely see it at all. Overall, black ice reflects so little light that you see only the roadway itself. While I am not sure, I think that it forms when moisture in the air condenses to dew on the roadway and then freezes into ice. Whatever process forms it must leave it almost without holes and therefore invisible.

How do stalactites and stalagmites form in caves? — GS, Conroe, TX

They form when various minerals come out of solution in water and crystallize on the surfaces of a cave. To understand how this process occurs, we must look at the interface between the water and the cave surface. Whenever water is in contact with a mineral surface, there is a chance that an atom of the surface will suddenly leave the surface and dissolve in the water. If there are atoms already dissolved in the water, there is also a chance that one of them will suddenly come out of solution in the water and attach to the surface. Atoms leave and return to cave surfaces all the time as water drips from the ceiling of a cave to its floor.

What is important for the growth of stalactites and stalagmites is that more atoms stick to the cave surfaces than leave those surfaces. That is exactly what happens and it does so because the water has already picked up more than enough dissolved atoms before it reaches the stalactite. Either because of temperature changes or because of evaporation, the water that runs across the cave roof and down the sides of a stalactite deposits more atoms on the stalactite’s surface than it removes. The same goes for the stalagmite after the water drips down to the cave floor. As the atoms build up on the cave surfaces, the stalactites grow down and the stalagmites grow up.

How does reverse osmosis work? – MC

Normal osmosis in water is a process in which pure water flows through a semi-permeable membrane to dilute a concentrated solution on the other side. It is driven by statistics—it’s much more likely for a water molecule on the fresh water side to pass through the membrane than it is for a water molecule on the concentrated solution side to pass through the membrane. There are simply more water molecules trying to cross the membrane from the fresh water side! In fact, water molecules will continue to flow from the fresh water side to the concentrated solution side until the solution has been highly diluted or an accumulation of pressure on the solution side slows the passage of water and brings it to a halt.

Reverse osmosis occurs when the pressure on the solution side is raised so high that the movement of water reverses directions. If you squeeze the concentrated solution hard enough, you can drive additional water molecules from that solution through the semi-permeable membrane and into the fresh water on the other side. The raised pressure on the solution changes the statistics, making it more likely for water molecules to go from the solution side to the fresh water side. This technique is used to purify water in homes and to desalinate water in desert countries.