Discover PHYSICS for GCE ‘O’ Level Science Unit 9: Thermal Properties of Matter

9.1 Temperature and Internal Energy Learning Outcomes In this section, you’ll be able to: describe a rise in temperature of a body in terms of an increase in its internal energy

9.1 Temperature and Internal Energy Recall Heat is the amount of thermal energy that flows from a hotter region to a cooler region. Unit for thermal energy is the joule (J). Supplying thermal energy to an object leads to a gain in the internal energy of the object.

9.1 Temperature and Internal Energy What is internal energy? Particles in a solid, held together by strong interatomic or intermolecular bonds, vibrate about fixed positions. The total energy of the particles is called internal energy. Internal energy comprises two components: Kinetic energy Potential energy

9.1 Temperature and Internal Energy What is internal energy? Kinetic component of internal energy is due to the vibration of the particles. It is directly related to temperature i.e. the higher the temperature, the more vigorous the vibration. In liquids and gases, the kinetic energy is due to their movement instead of vibrations.

9.1 Temperature and Internal Energy What is internal energy? Potential component of internal energy is due to the stretching and compressing of the interatomic or intermolecular bonds as particles vibrate. The amount of energy stored is dependent on the force between the particles and the distance between the particles.

9.1 Temperature and Internal Energy What is internal energy? Hence, when the temperature of a substance rises, it is due to an increase in the average kinetic energy of its particles only.

9.1 Temperature and Internal Energy Key Ideas Internal energy is made up of kinetic energy and potential energy. An increase in temperature leads to an increase in kinetic energy component of the internal energy.

9.1 Temperature and Internal Energy Test yourself 1. Based on the kinetic model of matter, explain the nature of the internal energy of an object in the solid state. Answer: The internal energy of a solid object comprises of kinetic energy and potential energy. The kinetic component is related directly to the temperature and is due to the vibrations of the particles. The potential component is due to the molecular bonds between the particles.

9.2 Melting and Solidification Learning Outcomes In this section, you’ll be able to: Describe the process of melting and solidification as transfer of energy without a change in temperature. Sketch and interpret a cooling curve.

9.2 Melting and Solidification The change of state from solid to liquid is called melting. Melting occurs at a definite or constant temperature. This temperature is called the melting point.

9.2 Melting and Solidification Table 9.1 Melting points of some substances

9.2 Melting and Solidification Possible Errors?

9.2 Melting and Solidification

9.2 Melting and Solidification Melting occurs at a constant temperature. During the change of state, there is no change in temperature even though thermal energy is being absorbed. Figure 11.10 Melting in action

9.2 Melting and Solidification How does a solid melt? Molecules in a solid are held by strong intermolecular bonds. Thermal energy supplied to solid is used to break these bonds during melting, hence there is no increase in temperature (Section QR of the curve in Figure 11.8) The thermal energy that is absorbed without a change in temperature is called the latent heat of fusion of a substance.

9.2 Melting and Solidification Solidification and freezing point The reverse process of melting is called solidification i.e. changing from liquid to solid state. A pure substance will solidify or freeze at a temperature equal to its melting point.

9.2 Melting and Solidification

9.2 Melting and Solidification

9.2 Melting and Solidification Solidification and freezing point Freezing occurs at a constant temperature. During the change of state, there is no change in temperature even though thermal energy is lost to the surroundings. When liquids molecules come together and solidify, intermolecular bonds are formed. Thermal energy is released during the formation of bonds, hence there is no change in temperature.

9.2 Melting and Solidification Key Ideas Melting is the change of state from solid to liquid, without a change in temperature. During melting, the temperature remains constant at the melting point. Solidification is the change of state from liquid to solid, without a change in temperature. During solidification, the temperature remains constant at the freezing point. Thermal energy is released by the substance.

9.2 Melting and Solidification Test yourself 1. In melting, there is no change in temperature even though thermal energy is being absorbed. Where has the thermal energy gone to? Answer: The thermal energy absorbed is used to break the intermolecular bonds in order for the solid to melt.

9.2 Melting and Solidification Test yourself 2. In freezing, there is also no change in temperature even though thermal energy is being released. Where does the thermal energy come from? Answer: When liquid molecules come together and solidify, intermolecular bonds are formed. In the process, thermal energy is released.

9.3 Boiling and Condensation Learning Outcomes In this section, you’ll be able to: Describe the process of boiling and condensation as transfer of energy without a change in temperature

9.3 Boiling and Condensation When pure liquid is heated and changes to vapour at fixed or constant temperature, this change of state is called boiling. The temperature at which this occurs is called the boiling point of the substance.

9.3 Boiling and Condensation Table 9.2 Boiling points of some substances

9.3 Boiling and Condensation The reverse of boiling is condensation. It is a change of state from vapour to liquid state at the same constant temperature as in boiling. Thermal energy is given out during condensation.

9.3 Boiling and Condensation Figure 9.15 Thermal energy is still being absorbed in portion YZ of the graph but there is no change in temperature. Why is this so?

9.3 Boiling and Condensation How does a liquid boil? Thermal energy supplied to liquid but no rise in temperature (YZ line in Figure 9.15). Thermal energy is used to do work to separate the molecules and also to push back on the surrounding atmosphere. Molecules become further apart with negligible intermolecular forces of attraction. Change of state (boiling) from liquid to gas has taken place. The thermal energy that is absorbed without a change in temperature is called latent heat of vaporisation of the substance.

9.3 Boiling and Condensation Key Ideas Boiling is the change of state from a liquid into vapour, occurring at a constant temperature called the boiling point. 2. Condensation is the process whereby vapour changes into liquid at the same constant temperature. Thermal energy is given out during condensation.

9.3 Boiling and Condensation Key Ideas During boiling, the temperature remains constant at its boiling point. Thermal energy is being absorbed by the substance. 4. During condensation, the temperature remains constant at the condensation point. Thermal energy is released by the substance.

9.3 Boiling and Condensation Test yourself 1. In boiling, there is no change in temperature even though a large amount of heat is being absorbed. Where has the thermal energy gone to? Answer: During boiling, thermal energy is needed to completely break the intermolecular bonds and separate the molecules, as well as to push back the atmosphere.

9.3 Boiling and Condensation Test yourself 2. In condensation, there is also no change in temperature even though heat is being released. Where has the thermal energy gone to? Answer: During condensation, molecules come together and intermolecular bonds are formed. In the process, thermal energy are released.

9.4 Evaporation Learning Outcomes In this section, you’ll be able to: Explain the differences between boiling and evaporation. Explain the effects of evaporation. State the factors affecting the rate of evaporation.

9.4 Evaporation What is evaporation? Evaporation, like boiling, is the change of state from a liquid to vapour. Unlike boiling, evaporation can occur at any temperature.

9.4 Evaporation What is evaporation? Table 9.3 Differences between boiling and evaporation.

9.4 Evaporation Evaporation causes cooling Evaporation requires thermal energy from the surroundings. If you step out of the swimming pool on a dry and sunny day, your body will feel cold (especially if a wind is blowing). Why is this so? This is because the water on your body is evaporating.

9.4 Evaporation

9.4 Evaporation How does evaporation occur? Evaporation causes cooling. This can be explained using the kinetic model of matter. Figure 9.19 Evaporation of ether takes place even at room temperature. Energetic molecules are able to escape from the liquid surface.

9.4 Evaporation How does evaporation occur? Molecules of liquids are always moving randomly at different speeds. These molecules can absorb thermal energy from the surroundings. At the liquid surface, molecules are more energetic and can overcome the downward attractive forces of other molecules and escape into atmosphere. The slower-moving molecules left behind are cooler as temperature is proportional to the average kinetic energy of the molecules.

9.4 Evaporation Applications of evaporation Cooling caused by spraying of perfume or when perspiring. Drying of clothes or puddles of water. Sponging person with fever. Refrigeration.

9.4 Evaporation Factors affecting the rate of evaporation Temperature Raising the temperature of liquid will increase the rate of evaporation. A warmer liquid means a greater number of energetic molecules at the surface layer Hence more molecules are energetic enough to escape from the surface.

9.4 Evaporation Factors affecting the rate of evaporation Humidity of the surrounding air Humidity is the measure of how much water vapour there is in the air. The higher the amount of water vapour, the higher the humidity. Evaporation decreases with increasing humidity.

9.4 Evaporation Factors affecting the rate of evaporation Surface area of the liquid Rate of evaporation increases when there is more exposed surface area of the liquid. This is because evaporation only takes place at the exposed surface of a liquid. Larger exposed surface allows more molecules to escape.

9.4 Evaporation Factors affecting the rate of evaporation Movement of air Moving air removes the molecules of liquid as soon as they escape from the surface. Increasing air movement will increase the rate of evaporation.

9.4 Evaporation Factors affecting the rate of evaporation Pressure Reducing atmospheric pressure increases the rate of evaporation. For example, wet objects dry faster at mountaintops or at high altitudes where atmospheric pressure is lower than at sea level.

9.4 Evaporation Factors affecting the rate of evaporation Boiling point of the liquid Liquids with lower boiling points will evaporate faster. For example, ether will evaporate much more rapidly than water under the same physical condition. On the other hand, mercury hardly evaporates at room temperature because it has high boiling point.

9.4 Evaporation Key Ideas Evaporation is a cooling process. Evaporation of liquid is due to molecules at the surface with energy greater than the average kinetic energy escaping from the rest of the liquid.

9.4 Evaporation Test yourself 1. State the differences between boiling and evaporation. Answer:

9.4 Evaporation Test yourself 2. Explain why putting a layer of perfume on the skin produces a cooling effect. Answer: Perfume is very volatile and evaporate rapidly. To evaporate, the perfume absorbed the thermal energy from the skin. Hence the skin now feels a cooling effect.

9.4 Evaporation Test yourself 3. Explain why the rate of evaporation of a liquid increases with temperature. Answer: Raising the temperature of liquid will increase the rate of evaporation. A warmer liquid means that a greater number of energetic molecules at the surface layer. Hence more molecules are energetic enough to escape from the surface.