Tag: water steam and ice

How does fog form? — KB

The interface between a droplet of water and the air around it is a busy place. Water molecules are constantly leaving the droplet to become water vapor in the air and water molecules in the air are constantly returning to the droplet as liquid water. What determines whether the droplet grows or shrinks is the difference between these two rates. If more water molecules return to the droplet than leave, the droplet will grow. If more water molecules leave the droplet than return, the droplet will shrink. How often water molecules leave the droplet depends on the droplet’s temperature. How often water molecules return to the droplet depends on the moisture content of the air.

This dynamic balance of growth and shrinkage occurs right in the middle of the air all the time. Tiny water droplets form by accident, even in reasonably dry air, but in most cases they quickly shrink back to nothing because the leaving rate is higher than the returning rate. However, when air that contains lots of moisture experiences a decrease in temperature, the returning rate can exceed the leaving rate. When that happens, the tiny droplets that appear by accident don’t immediately disappear. Instead, they grow larger and larger. Depending on the altitude, we call the white mist that results clouds or fog.

Our problem concerns temperature. At different temperatures, solubility of compounds varies. If we extract water from a pond at two degrees Celsius and then test it at room temperature, our reading isn’t going to be accurate. On the other hand, it isn’t practical for us to perform out tests outside. The substances we are testing are nitrites, nitrates, ammonia, pH, hardness, oxygen level, phosphates, temperature, and ORP. — J&E, Missouri

If you collect pond water at 2° C and then bring it into a room at 20° C, there will be a few subtle changes in the water’s contents. While the amounts of various dissolved materials can’t change unless atoms move in or out of the water, how they interact with one does change somewhat with temperature. I would be very surprised if anything that’s dissolved in that pond water comes out of solution when you warm it to room temperature, so if all you want to do is to determine the concentrations of various dissolved materials, go ahead and do it at room temperature. You might have to be careful with dissolved gases, because it’s relatively easy for gas molecules to enter or leave the pond water without your noticing that it’s happening, but the nitrites, nitrates, hardness, and phosphates aren’t going anywhere. Ammonia can leave as a gas, so you should be a little careful with it. I don’t know enough about ORP (oxidization reduction potential) to say anything about it. But you’ll have to be very careful with oxygen concentration because you can modify this just by pouring the water through air and making bubbles.

However, to be sure that the contents of the pond water are interacting with one another just as they were in the pond, you should cool the water back down to 2° C before making any measurements. This is particularly important for pH measurements, since water’s pH decreases slightly with increasing temperature.

What is pH and why is it so important to my garden pond and spa? — NW, California

pH is a measure of the concentration of dissolved hydrogen ions in water. When a hydrogen atom loses an electron and becomes a hydrogen ion—a proton—it can dissolve nicely in water. Actually, this proton sticks itself to the oxygen atom of a water molecule, producing a hydronium ion (H₃O⁺) that is then carried around by shells of water molecules. The higher the concentration of hydrogen (or hydronium) ions in water, the lower the water’s pH. More specifically, pH is negative the log (base 10) of the molar hydrogen ion concentration. That means that water with a pH of 6 has ten times as many hydrogen ions per liter as water with a pH of 7.

Pure water naturally contains some hydrogen ions, formed by water molecules that have spontaneously dissociated into hydrogen ions (H⁺) and hydroxide ions (OH^–). Pure water has enough of these hydrogen ions in it to give it a pH of 7. But if you dissolve acidic materials in the water, materials that tend to produce hydrogen ions, the pH of the water will drop. If you dissolve basic materials in the water, materials that tend to bind with hydrogen ions and reduce their concentration, the pH of the water will rise. Water with too many or too few hydrogen ions tends to be chemically aggressive and we do best in water that has a pH near 7.

Could you describe the process of an ice cube melting only from ambient (room) temperature? — JAS, Malta, NY

An ice cube is a crystal of water molecules. It is only stable up to a temperature of 32° F (0° C). When you place it in ambient temperature, it gradually warms until it reaches 32° F and then its surface begins to melt. As heat from the room flows into the ice cube, its molecules begin to separate briefly from one another and to exchange neighbors. These molecules lose their crystalline rigidity and structure and to become liquid. The liquid that forms is still at 32° F, but it has less order than the crystalline ice had.

As more heat flows into the mixture of ice and water, the ratio of solid ice to liquid water gradually changes and the fraction of liquid water increases. But only after all the ice has converted to water does the temperature of the water begin to rise significantly above 32° F.

What makes a soap bubble round? — MZ, Massachusetts

The molecules in a liquid are touching one another and this touching reduces the molecules’ potential energies. Separating the molecules and reducing the extent of their touching requires energy and is something that the liquid won’t normally do on its own. The molecules at the surface of a liquid have fewer neighbors than they would have if they were in the body of the liquid. Those molecules thus have higher potential energies than they would have in the body of the liquid. To minimize the overall potential energy of a liquid, it naturally tends to minimize its surface area.

When a soap solution has trapped some air to form a bubble, that solution can no longer shrink into a tiny droplet. The air keeps bubble large and it can’t avoid having lots of molecules on its surfaces, where they have higher than normal energies. But what the soap solution can do to minimize its total potential energy is to minimize the number of its molecules that are on the surface. The soap solution experiences what is called “surface tension”—an elastic tightening of its surface. This surface tension tends to minimize the surface area of the soap solution to minimize its potential energy. The soap solution minimizes its surface area around the trapped air by forming a spherical shape. A spherical shell makes the most efficient use of its surface area in enclosing a volume.

How can you speed up the process of dissolution? — T

Dissolution occurs when a solid material is disassembled into its constituent atoms, molecules, or ions and these particles are carried around in a solvent liquid. Since the disassembly is a statistical process, with thermal energy allowing particles to leave the solid and chance allowing them occasionally to return to the solid again, anything you can do to accelerate the leaving process and impede the returning process will speed dissolution. Heating the solid and liquid will speed dissolution by making it easier for particles to leave the solid and harder for them to stick when they try to return. Keeping fresh, pure solvent in contact with the solid will also prevent molecules from returning.

What is surface tension? — C

The molecules in a gas are independent and only collide with one another briefly before separating again. In contrast, the molecules in a liquid cling to one another so that they always remain in contact. While their mutual attachments aren’t as strong as normal chemical bonds, these molecules have reduced their overall potential energies by moving as close as possible to one another. However, the molecules at the surface of a liquid have no neighbors on one side and don’t benefit from the full energy-lowering effects of moving as close as possible to other molecules on all sides. Molecules at the surface of the liquid thus have higher potential energies than molecules within the liquid.

Because physical systems tend toward arrangements that minimize their overall potential energies, a liquid tends to minimize its surface area in order to minimize the number of high-energy molecules it has at its surface. This tendency to minimize surface area is the origin of surface tension in a liquid. The liquid behaves as though its surface were a taut elastic membrane. If you poke at a liquid, you can deform its surface but as soon as you stop pushing on it, it will spring back to its original flat or smoothly curved shape. That springiness is the result of surface tension.

What chemical properties of water cause it to be a medium of life? — WZ, Pacific Palisades, CA

Water is such a remarkable chemical that I hardly know where to begin. First, it is one of the lightest, simplest molecules and yet it remains a liquid even at temperatures approaching 100° C, making it well suited as a medium for chemistry of all sorts. Second, it is an extremely good solvent for a vast range of ionic and organic materials, so that it is an ideal medium for the complicated chemical mixtures of biology. Third, water has enormous latent heats of melting and vaporization that make it hard to freeze and its evaporation very effective at cooling a hot animal.

Which substance, calcium chloride or sodium chloride, melts ice faster and why? — MT, Fenton, MI

Without trying the experiment, I would expect sodium chloride to melt ice more quickly than calcium chloride simply because sodium chloride is more soluble in water. Anything that dissolves easily in water can melt ice, even sugar! A water-soluble material interferes with the crystalline structure of ice and, assisted by the tendency of everything to maximize randomness, converts the orderly arrangement of solid ice and soluble solid to the less orderly mixture of soluble material dissolved in liquid water. Both calcium chloride and sodium chloride are water soluble and thus melt ice, but sodium chloride is substantially more soluble than calcium chloride and ought to work faster.

However, molecule for molecule, calcium chloride will melt more ice than sodium chloride. That’s because a single calcium chloride molecule decomposes into three separate ions in solution (one calcium ion and two chlorine ions). In contrast, a sodium chloride molecule only forms two separate ions in solution (one sodium ion and one chlorine ion). Since each ion contributes to the ice melting process, calcium chloride molecules are about 50% more effective than sodium chloride molecules. But even this increased molecular efficiency has a price: calcium ions are heavier than sodium ions, so a kilogram of sodium chloride actually yields more ions and more ice melting than a kilogram of calcium chloride. Still, salt is messy and corrosive so calcium chloride is often a good alternative.

I know that adding salt to water will raise its boiling point, which would seem to imply that it would take longer to come to a boil. But does it take longer? As a cook I’ve always been told to add a little salt to the water to bring it to a boil faster. It seems to work or is that just the power of suggestion? If it does boil faster, why does it? — ND, Ashland, OR

I think that power of suggestion is at work here. Salt water boils at a higher temperature than pure water. Thus if you set two identical pots of water, one salty and one pure, on burners and heat them at equal rates, the pure water will reach its boiling temperature first.

However, water boils more vigorously when it contains impurities that can nucleate bubbles of water vapor. Just before the water in a pot reaches a full boil, its temperature is often nonuniform and there are some regions that are boiling while others aren’t. The edges and corners of crystals are particularly good at nucleating bubbles, so that tossing salt grains into such nearly boiling water will encourage its hot regions to boil more vigorously, at least until those salt grains dissolve away. The appearance of bubbles makes you think the water is at a full boil when it really isn’t.