Changes of State

Thermal Energy and Heat

Shards of ice fly from the sculptor’s chisel. As the crowd looks on, a swan slowly emerges from a massive block of ice. As the day wears on, however, drops of water begin to fall from the sculpture. Drip by drip, the sculpture is transformed into a pud- dle of liquid water.What makes matter change from one state to another? To answer this question, you need to think about the particles that make up matter.


Simply stated, energy is the ability to do work or cause change. The energy of motion is called kinetic energy. Particles within matter are in constant motion. The amount of motion of these particles depends on the kinetic energy they possess. Particles with more kinetic energy move faster and farther apart. Particles with less energy move more slowly and stay closer together. The total kinetic and potential energy of all the particles in a sample of matter is called thermal energy. Thermal energy, an extensive property, depends on the number of particles in a substance as well as the amount of energy each particle has. If either the number of particles or the amount of energy in each particle changes, the thermal energy of the sample changes. With identically sized samples, the warmer substance has the greater thermal energy. In Figure 2, the particles of hot water from the hot spring have more thermal energy than the particles of snow on the surrounding ground.


Figure 1


Not all of the particles in a sample of matter have the same amount of energy. Some have more energy than others. The average kinetic energy of the individual particles is the temperature, an intensive property, of the substance.You can find an average by adding up a group of numbers and dividing the total by the number of items in the group. For example, the aver- age of the numbers 2, 4, 8, and 10 is (2  4  8  10)  4  6. Temperature is different from thermal energy because thermal energy is a total and temperature is an average. You know that the iced tea is colder than the hot tea, as shown in Figure 2. Stated differently, the temperature of iced tea is lower than the temperature of hot tea. You also could say that the average kinetic energy of the particles in the iced tea is less than the average kinetic energy of the particles in the hot tea.


Figure 2


When a warm object is brought near a cooler object, thermal energy will be transferred from the warmer object to the cooler one. The movement of thermal energy from a substance at a higher temperature to one at a lower temperature is called heat. When a substance is heated, it gains thermal energy. Therefore, its particles move faster and its temperature rises. When a substance is cooled, it loses thermal energy, which causes its particles to move more slowly and its temperature to drop.

Specific Heat

As you study more science, you will discover that water has many unique properties. One of those is the amount of heat required to increase the temperature of water as compared to most other substances. The specific heat of a substance is the amount of heat required to raise the temperature of 1 g of a substance 1°C. Substances that have a low specific heat, such as most metals and the sand in Figure 3, heat up and cool down quickly because they require only small amounts of heat to cause their temperatures to rise. A substance with a high specific heat, such as the water in  Figure 3, heats up and cools down slowly because a much larger quantity of heat is required to cause its temperature to rise or fall by the same amount.



Figure 3




Source: Glencoe Science-The Nature of Matter-SE_0078617650




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