Evaporation. â« molecules of a liquid move with a wide range of velocities. â« some of the more energetic molecules wi
Evaporation molecules of a liquid move with a wide range of velocities some of the more energetic molecules with more than average velocities break free and enter the space above the liquid surface even at a relatively low temperature some molecules are able to escape from the forces of attraction which confine the loss energetic molecules to the liquid.
Evaporation •the escape of water molecules from the liquid surface •can take place at any temperature
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Evaporation takes place more quickly
• the air above the liquid surface is blown • the surface area of the liquid is increased • the temperature of the liquid is raised.
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Saturation vapour pressure • If the container is closed • the evaporation process will reach equilibrium • When the number of molecules bouncing back into the surface is equal to the number leaving. • The vapour is then said to be saturated and the pressure exerted on the container walls is the saturation vapour pressure.
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Saturation vapour density
m V The mass of water vapour per unit volume under this conditon is the saturation vapour density. #saturation vapour density and pressure α temperature
Boiling The liquid will boil when its saturated vapour pressure equals the applied pressure acting on the liquid.
Difference between evaporation and boiling
Evaporation can takes place at any temperature. Boiling -----------------------at definite temperature. Evaporation can takes place at liquid surface Boiling -------------------------in a liquid
Boiling point
the temperature saturated vapour pressure of the liquid is equal to the applied pressure.
boiling point= 100C
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100 C The boiling point above
-increase in pressure raise above 100 C -the water to be superheated Example -in a pressure cooker - instrument sterilizer ( autoclave).
Cooling the body by evaporation of perspiration Cooling the body by evaporation of perspiration is an efficient process because during evaporation the large latent heat of vaporization, , is extracted from the skin.
L V 580 cal g 1 at 37C Even when no perspiration is evident, evaporation from the skin and exhaled water vapour are on the order of 600g per day
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Perspiration
Exhaled water vapour
The associated energy loss rate(P) is
Q m L v t t 600 580 4.186 24 3600 17 W Therefore perspiration heat loss is a significant fraction of the basic 90W energy loss rate.
