The Watched Pot

Have you ever heard that expression, ‘a watched pot never boils’, which basically means that time feels longer when you’re waiting for something to happen?  Of course people are referring to the water inside of the pot boiling, and not the pot itself.  If you are concentrating on the water in the pot and staring at it constantly, it may seem like the very act of watching the pot could be preventing the water from boiling. Has that kettle boiled yet?  Well tell it to hurry up, because I’m dying for a cup of tea!  It may be a nuisance if you’re cooking or making drinks, but the length of time that it takes water to absorb heat is very useful to us in other ways.

In the movie, ‘My Cousin Vinny’ Joe Pesci who plays an un-licensed lawyer named Gambini asks a witness if he cooks Regular or Instant grits?  Then he speculates how it could only take him only five minutes to cook his grits, when it takes the entire grit-eating world at least 20 minutes.  He goes on to ask, “Are we to believe that boiling waters soaks into a grit faster in your kitchen than on any place on the face of the earth?”  He continues, “Well perhaps the laws of physics cease to exist on your stove!  Were these magic grits?  I mean, did you buy them from the same guy who sold Jack his beanstalk beans?”

The basic laws of physics fall into two categories, being classical physics that deals with the observable world (classical mechanics), and atomic physics that deals with the interactions between elementary and sub atomic particles (quantum mechanics).  The laws of physics govern action and reaction, the progression of time, and all interactions between the fundamental building blocks of the universe.  In a hypothetical way, the laws of physics don’t actually exist at all, but still the universe goes on.  It is important to realize that what we call natural laws aren’t prescriptive laws.  They are regularities that we have found in nature when we investigated and tried to express certain phenomena that we found.  The natural laws are trustworthy in the extreme, not because they are so very good, but because they describe what is already going on anyway.  Water has its own type of physics and I will try to educate my readers on some of its fantastic properties.

An amazing fact about water is that in some circumstances, it actually takes longer for cold water to drop down to the freezing temperature of 32 degrees Fahrenheit than it does for hot water and this is called the Mpemba effect.  No one knows why hot water actually freezes faster than cold water when the two bodies of water are exposed to the same subzero surroundings.  One possible solution could be that a heat circulation process called convection.  In a container of water, warmer water rises to the top, pushing the colder water beneath it and creating a ‘hot top’.  Scientists speculate that convection could somehow accelerate the cooling process, allowing hotter water to freeze faster than cooler water, despite how much more degrees of temperature it has to cover to get to the freezing point.

If you start off with a glass of water and cool it down, the molecules start to move closer and lock together. But at a temperature of about 4°C (39°F), the molecules are as close as they can possibly get.  In other words, the water has reached its maximum density.  If you keep on cooling it down, the molecules rearrange themselves into a slightly more open structure.  This means ice is a little bit less dense than freezing water and that’s why ice floats on water that’s the same temperature.  The anomalous expansion of water can cause water pipes which may be running under your home to freeze solid in winter, as the water inside of these pipes might turn to ice and then it would take up more volume, thus causing the pipes to burst open and then leak when the ice thaws out.

Although the solid form of almost every substance is denser than its liquid form, due to the fact that atoms in solids normally pack tightly together, this does not hold true for H2O.  When water freezes, its volume increases by about 8 percent.  This is the strange behavior that allows ice cubes, and even gargantuan icebergs, to float.  When water cools to its freezing point, there is a lot less energy which causes its molecules to slosh around, thus these molecules are able to form steadier hydrogen bonds with their neighbors.  Over time, the water molecules will gradually lock into position, this is the basic process that causes all liquids to solidify.  Just like in other solids, the bonds between molecules in ice are indeed shorter and tighter than the loose bonds in liquid water; the difference is that the hexagonal structure of ice crystals leaves a lot of empty space, which makes ice less dense than water overall.  Water is a relatively dense substance because it packs an awful lot of mass into a relatively small space. Water isn’t dense compared to metals such as gold, which is almost 20 times heavier  by volume.  But it is much heavier and denser than wood and plastic, which is why those things will float.  Anything less dense than water floats on it, and anything that is more dense will sink in it.

The amount of heat required per unit of mass is known as the specific heat of a substance, which is an intensive property characteristic (one that depends on the type of matter) of that substance.  Water has a high specific heat capacity and that means it can hold or carry more heat per kilogram (or pound) than virtually any other substance.  That’s why we use water in heating systems such as radiators, because each liter of water that trickles through the pipes carries and delivers more heat.  Of course the drawback is that the water takes some time to heat up in the first place.

One of the unique things about water is that it exists in three very different forms (or states of matter as they are known) which are solid, liquid, and gas.  Ordinary, liquid water is the most familiar to us because water is a liquid under everyday conditions, but we are also very familiar with solid water (ice) and gaseous water (steam and water vapor) as well.  Converting water between these three different states is remarkably easy.  All you have to do is change its temperature or pressure.  Take some ice and heat it up and you’ll soon have a pool of liquid water.  Heating it up will cause the water to evaporate and become steam.  It takes a terrific amount of energy to turn ice into water and water into steam, because you have to physically rearrange the structure of the substance in each case and push the molecules further apart.  This is why kettles take so long to boil.  An easier way to turn water from a solid or liquid into a gas would be to simply to leave it out in the open air, and gradually, the more energetic molecules in the water will escape and turn into a cool vapor up above it.

When you heat water to make steam, there comes a point where you keep heating the water but the temperature doesn’t increase.  The energy supplied seems to be vanishing into thin air, but it’s actually pushing apart the molecules in liquid water and turning them into a gas.  In the process, that energy is becoming locked inside the steam as something called latent heat (the word latent just means ‘hidden’). Latent heat is like an immense reserve of energy locked in steam that the inventors of yesteryear used to power factory machines and vehicles using their mighty steam engines.

Early scientists were unable to grasp the concept of latent heat, which seemed to disappear and then reappear later, somewhere else.  To make matters worse, in these times the distinction between heat and temperature was poorly understood, and the instruments to measure them were crude and unreliable. Finally in 1762, a Scottish scientist, Joseph Black made a brilliant leap of scientific intuition, where the bizarre behavior of latent heat was unmasked.  Man had at last discerned a profound truth hidden in poorly understood and seemingly unrelated observations.  Black’s attention was drawn to the latent heat puzzle by an observation on supercooled water, made by a German physicist and engineer Gabriel Daniel Fahrenheit.  Fahrenheit reported the now well-known fact that water can be supercooled, or chilled below the freezing point, without turning to ice.  When shaken, the supercooled water turns instantly to ice, and the temperature rises to the freezing point.  Black meditated on Fahrenheit’s experiment, and on his own observations of the slow melting of ice.  Taken together, the two suggested that a large quantity of heat was absorbed as ice melts, and a corresponding quantity released by the freezing of water.  Starting from this simple insight, he soon realized that a form of heat must exist that mysteriously disappears and reappears as water changes phases.  Black based his reasoning in part on the fact that something expected to happen did not.

In 1712, Thomas Newcomen a British inventor created the first practical steam engine for pumping water. Scottish inventor James Watt helped take humans away from the farm and into the factory and this opened up a new era known as the modern world.  His steam experiments revealed the revolutionary theory of latent heat (simplistically, the ‘hidden’ heat and energy in steam).  With it, he’ll eventually was able to increase the power of the steam engine fivefold.  Only then did he find out that another resident professor, Robert Black, has already discovered the theory and had been teaching it to his students for several years.  The two pair up.  Watt continues to experiment and realizes that the Newcomen steam engine wastes around three quarters of its heat.  To achieve the mechanical motion its parts (the piston and chamber) were constantly being cooled and heated.  More energy was being expended on this than on delivering mechanical force.  While he was out walking one Sunday afternoon in 1765, Watt saw the solution.  He envisioned a separate chamber in which the steam was allowed to condense.  This meant that there would be no more need for cooling and reheating, making the engine faster and more fuel efficient.  His insights converted a machine of limited use into one that powered the industrial revolution.

When there’s a huge temperature gradient between water and the outside air say, when a pot of boiling water measuring 212 degrees Fahrenheit (100 C) is splashed into air measuring minus 30 F (-34 C) a surprising effect occurs.  The boiling water will instantly turn to snow, and blow away.  This happens because extremely cold air is very dense, with its molecules spaced so closely that there isn’t much room left over for carrying water vapor.  Boiling water, on the other hand, emits vapor very readily.  When the water is thrown into the air, it breaks into droplets, which have even more surface area for vapor to rise off of.  When there is more vapor being emitted than the air can hold, the vapor will ‘precipitate out’ by clinging to microscopic particles in the air, such as sodium or calcium, and forming crystals.  This is how snowflakes are formed.

10 thoughts on “The Watched Pot

  1. My watched pots boil over before I notice – and I’m watching all the time. Maybe the trouble is what’s inside my head, not what’s inside the pot!
    Very interesting read, I have to say, and the pot has now boiled.

    Liked by 1 person

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