Warm-up Are these examples of evaporation? Wet floor is left to dry up. Feeling cool under air-conditioner. Ironing wet clothes to dry them.
Warm-up Do you think the leaves of a plant get dry quickly in these cases? Put it inside a room with all windows closed. It is muggy and is going to rain. Strong sunlight shines on it. The plant is under a sea breeze.
Warm-up How does the formation of snow or rain affect the temperature of the air? A bit warmer than it would be. A bit cooler than it would be. No change. Others: _________
Liquid can change to vapour at temperatures below the boiling point.
A liquid changing into a vapour is said to be evaporating. The process is called evaporation .
Differences between evaporation and boiling
Evaporation and boiling require latent heat of vaporization .
1 Cooling effect of evaporation
Apply perfume/alcohol to your body...
What do you feel? COOL ! Perfume/alcohol evaporates easily and takes the energy from your body.
1 Cooling effect of evaporation Sweat comes out of the pores, evaporates thus taking energy away from the skin. So we feel cool!
1 Cooling effect of evaporation No cooling effect occurs. So, we feel hot. On a humid day, the air is full of water vapour . This slows down the evaporation of sweat. S weat tends to stay on the skin.
You can easily catch a cold if you are wet.
1 Cooling effect of evaporation If it is windy, you cool down even more . As you dry out, latent heat is taken from your body.
2 Evaporation and particle motion Evaporation is the escape of fast-moving particles from the surface of a liquid. Thus, some have higher K.E.! Different particles have different velocity.
2 Evaporation and particle motion average KE of the remaining molecules decreases temperature of liquid cooling effect slow molecules left behind E fast molecules (high KE) near the surface may escape
3 Factors affecting evaporation 1 Fast-moving particles in liquid escape and become vapour ; Points to consider:
3 Factors affecting evaporation 3 The rate of evaporation (drying up) is a balance between the rate of escape and the rate of return. 2 While slow-moving particles in vapour stick back to the liquid.
The rate of evaporation increases with the temperature of the liquid
Temperature of water Molecules move faster on average. Molecules have more KE More molecules can escape.
Surface area of water larger the surface area , more molecules can escape at the same time. G reater is the rate of evaporation. When surface area is increased…
If the air is humid, it is full of water vapour.
Humidity of air
Water particles in vapour have greater chance to return to the liquid.
more humid the air ,
the lower the rate of evaporation. Some return evaporation
Movement of air If there is a breeze, the particles that escape from the surface of water get blown away. fewer particles in the vapour return to the liquid. a breeze increases the rate of evaporation.
Condensation the opposite of evaporation
Warm air can hold more water .
If warm humid air suddenly cools,
some of the vapour has to condense.
This is how clouds and mist are formed from millions of tiny water droplets.
Condensation the opposite of evaporation
Latent heat of vaporization is released when water vapour condenses.
A E = ml v applies to boiling but not to evaporation. B Boiling always occurs at a definite temperature but evaporation does not. C Boiling takes place at the surface, but evaporation occurs within the liquid. D Bubbles are formed violently in boiling, but slowly in evaporation. Which is the difference between boiling and evaporation?
Soup covered with oil… … because the oil layer __________ (slows/enhances) the evaporation of water ( soup) longer slows Soup covered with oil takes ________ (longer/shorter) to cool down...
True or false: When... True or false: When water vapour condenses, the surrounding air is warmed. (T/F)
____________________________________________________________________________________________________________________________________________ Increasing water temperature, Increasing surface area of water, Decreasing humidity, Increasing air movement Name any TWO factors that increase the rate of evaporation.
Example 6 A person (mass 60 kg) doing exercise is covered with sweat . (a) If 1 litre (1 kg) of sweat is evaporated in 1 hour... … how much energy is required to evaporate this amount of water?
Energy required to evaporate 1 kg of water
Example 6 = ml v = 2.26 10 6 = 2.26 MJ = 1 2.26 10 6
(b) If this amount of energy were not removed from the body by sweating ...
Example 6 … by how much would the body temperature of the person ? The average specific heat capacity of the human body is 3500 J kg -1 o C -1
Example 6 Apply E = mc T , T = E / mc = 2.26 10 6 / (60 3500) If the energy were not removed from the body by sweating,... ... the body temperature would increase by 10.8 °C. T = 10.8 °C
Example 7 The experimental set-up is used to find the specific latent heat of vaporization of water . the electronic balance measures the mass of water boiled away
Switch on the heater only when it is immersed in water; otherwise it may be damaged.
Example 7 (a) State a precaution in this experiment.
A lid should not be added although it can reduce heat loss.
Example 7 Steam will condense on the lid and drip back into the cup. error in the mass of water boiled away. (b) A student suggests that a lid should be added to the polystyrene cup to reduce heat loss . Comment on his suggestion.
The error may be caused by
Example 7 1 Steam condensing on the top part of the heater dripping back into the cup. 2 Energy loss to the surroundings. (c) The result obtained in the experiment is larger than the standard value. Explain.
Example 7 Some water in the polystyrene cup evaporates ... … and thus the balance reading drops. (d) After the heater has been switched off for a period of time, the reading shown on the balance drops slightly. Why?