The Thermometer That Stopped Climbing

The Thermometer That Stopped Climbing

[1]Priya's class set a beaker of crushed ice from the freezer on a hot plate. A digital thermometer probe sat in the ice, and Priya stirred gently. The reading started at −18 °C and climbed quickly. Then, at 0 °C, it stopped. For several minutes the hot plate stayed on, the ice shrank into slush, and the number did not budge. Only when the last sliver of ice was gone did the reading begin to rise again, all the way to 100 °C. There it stopped a second time while the water bubbled hard. Where was the hot plate's energy going while the thermometer was stuck?

[2]The answer depends on three separate ideas. Temperature tracks how energetic a substance's particles are on average: it reflects their average kinetic energy, the energy they have because they move. Thermal energy adds up the energy of every particle in a sample, so it depends on how much matter there is as well as on how energetically the particles move. A teacup at 80 °C has a higher temperature than a swimming pool at 26 °C, because its particles are more energetic on average. Yet the pool holds far more thermal energy, because it contains hundreds of thousands of times more water.

[3]The third idea is heat: thermal energy on its way from something warmer to something cooler. Where two objects touch, their particles collide constantly. In these collisions, energy tends to pass from more energetic particles to less energetic ones, so energy flows from the warmer object into the cooler one. The flow continues until both reach the same temperature. After that, the particles still trade energy, but equal amounts go each way, so neither temperature changes. Cold never flows anywhere. An ice cube feels cold in your hand because energy is leaving your skin.

[4]In ice, each water molecule is held in a fixed spot by attractions to its neighbors, and it can only vibrate. Below 0 °C, the incoming energy makes the molecules vibrate harder, so the temperature rises. At 0 °C (at normal air pressure), the added energy goes into pulling molecules free of their fixed spots instead of making them move faster on average. Since the average kinetic energy stays the same, so does the temperature. The sample's thermal energy is still growing, though: liquid water at 0 °C has more thermal energy than the same mass of ice at 0 °C. On a graph of temperature against time, this level stretch is called a plateau.

[5]The same thing happens at the boiling point, 100 °C at sea-level air pressure. There the energy pulls molecules completely away from one another, and bubbles of water vapor form deep inside the liquid. Turning the hot plate higher makes the water boil away faster, but it does not make the boiling water hotter. Evaporation is different: it happens only at the surface, at almost any temperature, whenever a fast-moving molecule near the top escapes. That is why a puddle dries on a cool day. Water vapor itself is invisible. The white cloud above a boiling pot is made of tiny drops of liquid water that formed when the vapor cooled in the air.

[6]Every change can also run in reverse, and then energy leaves the substance. When water vapor touches a cold can or a cold window, it passes energy to the colder surface. The molecules slow down until their attractions hold them together in drops, which is condensation. A freezing puddle's temperature holds at 0 °C until it is frozen solid, because each molecule that locks into place gives up energy to the colder air around it.

[7]Each substance changes state at its own temperatures because the attractions between its particles differ in strength. Helium atoms attract one another so weakly that helium boils at about −269 °C and cannot be frozen at all at normal air pressure. Water molecules attract one another strongly for their small size, so water must reach 100 °C to boil. The atoms in iron hold on so tightly that iron does not melt until about 1,538 °C. Melting and boiling points are a measuring stick for how tightly particles hold on.