Change of State
(A) Latent heat
Matter exists in three states: solid, liquid and gas.
(1) Temperature during change of state
A substance may change from one state toanother if it is heated or cooled.
At boiling point, water can change to steam(boiling), and steam can change to water(condensation).
At freezing point, water changes to ice(solidification); at melting point, ice can changeto water (fusion).
A cooling curve is a graph that plots thetemperature of a substance against time as thesubstance cools. From the cooling curve, it shows that temperature of a substance remains constant as it changes from one state (e.g. liquid) to another state (e.g. solid).
(2) Latent heat and specific heat capacity
When a substance is changing its state, latentheat is absorbed or given out without a change inits temperature.
Latent heat is related to the change of PE of thesubstance’s particles during a change of state.
The specific latent heat of a substance is the energy transferred byheating to change the state of 1 kg of the substance without a change intemperature.
The symbol for specific latent heat island the unit is J kg–1.
If energy E is required to change mass m of a substance from one state to another. E, m and l are related by
The specific latent heat of fusion of ice lfis theenergy needed to change 1 kg of ice to waterwithout a change in temperature. Its value is3.34 × 105 J kg–1.
The specific latent heat of vaporization ofwater lvis the energy needed to change 1 kg ofwater to steam without a change intemperature. Its value is 2.26 × 106 J kg–1.
The following figure shows the energy involved in heating 1 kg of water (from ice to steam).
(3)Measuring specific heat capacity
The specific heat capacity can be measured using the equation.
Measuring specific latent heat of fusion of water / Measuring specific latent heat of vaporization of waterExperimental set-up / /
Calculation / /
Precaution / Before switching onthe heater, pack thecrushed ice in the twofunnels such that thedrip rates are steadyand about the same.
After switching offthe heater, do notremove the cups; waituntil the drip rateshave become steadyand about the same.
A small piece of wiregauze or steel wool atthe neck of thefunnels can preventthe crushed ice fromdropping into thebeakers directly. / Do not switch on the heater unless the heating part of it istotally immersed in water. Overheating in air may damage the heater.
Possible source(s) of error / Difficulty in keeping the water dripping down the two funnels at the same rate.
Energy lost to the surroundings. / Steam condensing on the heater drips back into the cup.
Energy is lost to the surroundings.
(B) Evaporation
Evaporation is the changing of a liquid to avapour which occurs at temperatures lowerthan the boiling point. Evaporation takes placeat the surface of the liquid.
Evaporation is related to the fast-movingparticles escaping from the surface of a liquid. As fast-moving particles fly away, the average KE of the remaining particlesis lowered; so the liquid becomes colder. This produces a cooling effect. Latent heat is required as the liquid evaporates.
Water vapour in warm humid air condenseswhen the air is cooled. Latent heat is releasedas water vapour condenses (during condensation).
Fast-moving particles in liquid escape and become vapour while slowmovingparticles in vapour stick back to the liquid. The rate of evaporation(drying up) is the difference between the rate of escape and the rate ofreturn.
The rate of evaporation increases with
• increasing temperature and surface area ofthe liquid,
• decreasing density of vapour/humidity ofair, and
• increasing movement of vapour.
The following table shows the comparison between evaporation and boiling.
Evaporation / BoilingOccurs at any temperature. / Occurs at a definite temperature —the boiling point.
Occurs at surface. / Occurs within liquid.
No bubbles formed. / Bubbles appear.
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Change of State