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<ul><li>Pupils should be able to </li></ul><ul><li>describe the rise in temperature of a body in terms of an increase in its internal energy (random thermal energy). </li></ul><ul><li>describe melting/solidification and boiling/condensation as processes of energy transfer without a change in temperature. </li></ul><ul><li>explain the difference between boiling and evaporation. </li></ul>Thermal Properties of Matter Lesson objectives
Internal energy is defined as the energy associated with the random, disordered motion of molecules . What is internal energy? Consider a beaker of water passively resting on a table. Thermal Properties of Matter At microscopic level, the kinetic energy of moving molecules is part of the internal energy of the system. The potential energy of the molecules arising from the intermolecular interaction forms the other part of the internal energy of the system.
When a kettle of water at room temperature (say, 28 C) is heated: Internal energy and Temperature Hence, temperature is a measure of the internal energy of a system. Thermal Properties of Matter <ul><li>the temperature increases. </li></ul><ul><li>the internal energy increases, since the kinetic energy of the </li></ul>molecules increase. <ul><li>the molecules move more vigorously. </li></ul>
When the same kettle of water is further heated, we will expect its temperature to continue rising . Change in states of matter However, the temperature of the heated water does not rise continuously. After some time, the temperature of water remains constant for a period of time. The water is now undergoing a change of state called boiling . It is changing from liquid to gaseous state. Thermal Properties of Matter temperature/ C time/min 28 100 boiling point
During change of states, the energy supplied to a body is no longer used to increase its molecular motion. Hence, its temperature does not rise any more. What happens during change of states temperature/ C time/min During change of states, energy absorbed is used to overcome the intermolecular forces of attraction. Thermal Properties of Matter 100 Molecules breaking away from one another, no further increase in molecular motion. Temperature remains constant. Increasing molecular motion. Temperature rises. 28
Melting and boiling Let's take a look at what happens to a piece of ice taken out of refrigerator at -4 C. Thermal Properties of Matter 100 temperature/ C time/min - 4 0 ice is melting, temperature does not change. water is boiling, temperature does not change. boiling - change state at 100 C melting - change state at 0 C ice is heating up, temperature rises from – 4 C to 0 C. water is heating up, temperature rises.
Freezing The reverse happens when a beaker of warm water (e.g. 45 C) is placed in the freezer compartment (set at -4 C) of a refrigerator. temperature/ C time/min 45 - 4 0 Thermal Properties of Matter warm water is cooling down, temperature drops. water is freezing to form ice, temperature remains unchanged at 0 C. ice is cooling down, temperature drops further until –4 C.
Evaporation Evaporation is a change in state from liquid to gas that takes place at the surface of a liquid. less energetic molecules remain in the liquid. more energetic molecules escape. Molecules in the liquid differ from one another in that; some are more energetic, while the others are less excited. As the more energetic liquid molecules escape into the air, they carry with them their higher internal energy. Thermal Properties of Matter Consequently: Average kinetic energy of the molecules remaining in the water is lower. Evaporation produces cooling effect.
Thermal Properties of Matter Factors affecting the rate of evaporation <ul><li>Air pressure above the fluid – a greater air pressure above the liquid implies that the air molecules collide more frequently with the liquid molecules. </li></ul>More resistance for liquid molecules to escape 2. Temperature of fluid - when temperature is high, the kinetic energy of the molecules will be large enough to escape from the surface hence rate of evaporation increases with temperature. 3. Surface area of liquid - with larger surface , more liquid molecules are exposed to the air which facilitate the rate of evaporation. The greater the air pressure, the lower the rate of evaporation Go to Exp3a.mpg
Differences between Boiling and Evaporation Thermal Properties of Matter Boiling is a vigorous process, with the formation of bubbles throughout the liquid. Evaporation is a gradual process. Liquid temperature remains unchanged during boiling. Liquid temperature drops during evaporation. Takes place throughout the liquid. Takes place only on the surface. Boiling Evaporation
Summary By the end of this lesson, pupils are able to: Describe a rise in temperature of a body in terms of an increase in its internal energy. Describe melting/solidification and boiling/condensation as processes of energy transfer without a change in temperature. Explain the difference between boiling and evaporation.