Heat is the motion of molecules. The faster an object's molecules are moving, the hotter the object is. If the molecules slow down, the object gets colder. The same is true whether we are talking about water, cooking, weather, home heating, or refrigeration.

In liquid water, the molecules are moving in a range of speeds around xxx k/s. Some near the surface are at the upper range of the speed and simply break away into the air. This is evaporation. The water left behind is a tiny fraction cooler because the average speed of its molecules will have gone down. ("Evporation cools"). If we add heat to the water, some molecules within the water will move fast enough to become gaseous. When their neighbors get to that same speed, a bubble of water gas (steam) forms inside the water. It forms fast enough to be an explosion, which is why there is noise in water hear the boil. As most of the water reaches the point of evaporation, most steam is formed and escapes as steam at the water's surface. Then the explosions die down; boiling water is quieter than almost-boiling water.

When we put an ice tray filled with liquid water into the freezer, the temperature of the freezer slows the molecules of the tray. There in turn slow the molecules of the water. When they get around xxx kps in average speed, the water freezes--it turns to ice.

The temperature needed for freezing, melting, condensing and boiling depend on pressure. If the air pressure is lower, a water molecule is more easily released from the water surface. Being a mile high, Denver has lower air pressure, so water boils at a lower pressure. And thus hard-boiling an egg takes longer.

We can draw a phase diagram relating temperature and pressure to the solidity of a substance. See the diagram for water the the right. The left edge is low pressure and pressure rises from left to right. So the substance progresses from gaseous at the left to solid at the right. The bottom edge is low temperature and the temperature rises from bottom to top. So the substance will be solid at the bottom and gaseous at the top, We can draw a line on the diagram where the substance changes from solid to liquid, another for the change from liguid to gseous, and a third line across which the substance goes directly beween solid and gaseous. Somewhere in the middle the three lines will meet. This is called the "triple point". At this point a blob of the substance will have molecules in all three states. A small change in temperature of pressure will have a dramatic effect.

Sometimes a mountain valley will experience a waarm dry wind as water freezes on the mountainside. Why? Because of the freezing. Just before frreezing a water molecule will have rapid speed. As it freezes, that energy must go somewhere; energy does not disappear, it jsut goes somewhere else. On the mountainside, the energy is transferred to the surrounding air molecules. They warm up and the wind ensues.

How cold can it get? Since molecules cannot go slower than dead stop, there is a lowest temperature. That corresponds to -217.36° (????) celsius. At this temperature, even hydrogen turns into a solid. However, considerable skill and effort are requried to approach tis temprature; molecules are eluctant tobe still. One interesting phenomenon at such cold temperatures is that some conductors of electric lose all reisistance. Theelectiricty flows unimpeded around and around a loop until the temperature rises. This current produces a magnet. Its not free magnetism, however, energy must bepoured into keeping the circuit cold.

speed
velocity

direct transfer of heat by molecular contact
indirect transfer thru radiant heat

examples

hot air home heat
radiant home heat

heat arrives from sun as radiant heat

refrigreration

Your hand is typically around 37°, which corresponds to xxxxxxxxxx hz. If you touch something with slower vibration, it feels cool or cold; if faster, it feels warm or hot. And that's what heat is.

(Here I am talking about heat transfer by contact. Heat can also transfer by "radiation," that is through photons. This may well be easier to talk about after you learn something about photons.)

There is no upper limit to vibration, so things can always get hotter. But there is no vibration amount below zero. Hence there is a temperature callled "absolute zero". It is -2xx.xx° celcius. (Or zero degrees kelvin. Degrfees in the kelvin scale are the same size as celcius degrees, but kelvin's zero is absolute zero.)

It is not possible to eliminate all vibration, So we cannot cool objects to absolute zero. But researcher/tinkerers have come close. The latest in a long series of techniques is to zap a molecule with a laser beam at just the right time to counteract its wiggle. A tempertaure of .00xxxx degrees kelvin was attained. (One experiment reported a temperature below absoulte zero, but that effort relied on a rather different definition of temperature.)

To cook an egg we have to heat it; that is to motivate its molecules to vibrate faster. On a stove we light the gas, it burns from solar energy gathered by plants millions of years ago. The flame's vibrating molecules bounce off air molecules, speeding them up. They vibrate their neighbors and so on to warm the pan and it heats the egg.

If we heat water, its molecules move faster. At the surface, some move so fast they attain the degree of vibration that converted water to stream. Those molecules move away from the pot. Some molecules within the water also get that hot; they and nearby molecules form a steam buble within the water. The speed of the edge of the buble expanding is faster than the speed of sound, which is which water near boiling gives off sound.

Another way to affect the temperature of a gas is to compress it; squeeze it into a smaller space.

Cooling a room or a refrigerator relies on the principle that "evaporation cools". You can feel this effect by wetting your hand from the a faucet. The molecules of water on your hand will have an average speed, but some will vibrate more than others. Some on the surface will bounce right on off, becoming water vapor. As each molecule leaves, the average speed of those remaining goes down. Your hand feels--and is--cooler.

In an airconditioner or refrigerator, a hot liquid is squirted through a tiny mozzle into an empty space. It expands. A lot.