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These curves show a visualization of how phase changes occur (via adding or removing heat). The three states of matter (i.e. Solid, liquid and gas) are interconvertible, depends on the energy of the molecules. As the energy increases the molecules move around faster, less influenced by their attractive forces results in phase change. In this section, we are going to discuss about the role of energy is related with the phase change.
The matter changes the state between solid, liquid and gas as summarized in the figure below. Freezing (liquid turns to solid) and melting (solid turns to liquid) are opposite to each other and both represent the equilibrium between the two states. Evaporation occurs when a liquid turns to a gas while Condensation is the opposite of vaporization and both represent the equilibrium between the liquid and gas states. Sublimation (solid turns directly to gas) is the opposite of deposition (gas turns directly to solid), represents the equilibrium between the solid and gas states.
Those transformations (or process) that absorb energy are called Endothermic process. For example, Solids absorbs energy to convert into liquid and liquid absorbs energy to transform into gas. This means, Solid to liquid and liquid to gas requires energy or absorbs energy so these are endothermic process.
Those transformations (or process) that release energy are called Exothermic process. For example, Gas molecules release energy to transform into liquid state and liquid molecules release energy to turn into solid state. This means, gas to liquid and liquid to solid releases energy so these are exothermic process.
Suppose you have an ice cube in a closed container, that is at a temperature of -20°C (position A in the figure below), well below its melting point. Now, if you provide heat to the ice cube, the water molecules absorb energy and will begin to vibrate faster and faster. Eventually, when the ice has warmed to 0°C (position B in the figure below), the added energy will start to break apart the hydrogen bonding (attractive force between water molecules) that keeps the water molecules in the solid state, resulting in melting (horizontal line BC in the figure below).
At the time of melting Ice, its temperature does not rise (horizonal line BC). All of the provided energy goes into the melting process and temperature becomes constant. During the melting process, the two states – solid and liquid – are in equilibrium with one another. The ice-water co-exists at 0°C.
Continued heating even after converting whole of ice to liquid water will now increase the kinetic energy of the liquid molecules and the temperature will rise (slope CD). The temperature will rise steadily until it reaches 100°C (position D). At this point, the kinetic energy of the molecule is too high that it overcomes the attractive forces that holds the water molecules together and the added energy from the heat will cause the liquid to begin to vaporize. The temperature will remain at 100°C at the time of vaporisation (Horizontal line DE). During the vaporization, the two states – liquid and gas – are in equilibrium with one another. Once all the liquid has completely boiled away (position E), continued heating of the steam will increase its temperature above 100°C (slope EF).
Cooling curves are just opposite to what we discussed in heating curves. Let say you have water vapours at 120 deg. C. in a closed container (position A in the figure below). Now, if you cool the water vapours, the water vapours release energy and decreases kinetic energy of the molecules. Eventually, when the water vapour has reached to 100 deg C (position B in the figure below), the attractive forces will overcome the kinetic energy of the water vapours, resulting in condensation (horizontal line BC in the figure below). During the condensation, the two states – gas and liquid – are in equilibrium with one another.
At the time of condensation, its temperature does not change (horizonal line BC).
On continued cooling after converting whole of water vapours to liquid water will now decrease the kinetic energy of the liquid molecules and the temperature will drop again (slope CD). The temperature will drop steadily until it reaches 0 deg. C (position D). At this point, the kinetic energy of the molecule is too low that water molecules will begin to freeze. The temperature will remain at 0 deg. C at the time of freezing (Horizontal line DE). During the freezing process, the two states – solid and liquid – are in equilibrium with one another. Once all the liquid has completely freeze (position E), continued cooling will decrease its temperature below 0 deg. C (slope EF).
Temperature is a measure of "Average Kinetic Energy". Any change in temperature leads to the change in Kinetic Energy. All of the slope lines on a heating or cooling curve show a temperature change and therefore a change in kinetic energy. During these regions, a single state of matter exists and the sample is either getting hotter or cooler. This makes potential energy to be constant. During the horizontal line segments, there is no change in temperature, so kinetic energy remains constant. However, all the energy that is absorbed or released is related to changes in potential energy.
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