In the previous section we saw liquefaction of gases. In this section, we will see liquid state
• The study about liquefaction in the previous section, gives us some interesting information
• They can be written in 4 steps:
1. When a gas is liquefied, the molecules remain the same
• For example:
♦ When gaseous NH3 is liquefied, the newly formed liquid ammonia has the same NH3 molecules
♦ When gaseous CO2 is liquefied, the newly formed liquid carbon dioxide has the same CO2 molecules
2. If the molecules are the same, what is the difference between the two states?
• The answer is that:
When the gas becomes a liquid, the inter molecular attractions are increased. So the molecules of the liquid remain close together. That means, the liquid can be considered as a dense gas
3. So there is a continuity between gaseous state and liquid state
• The word fluid is used to recognize this continuity
4. In order to turn the gas into liquid, two conditions must be satisfied:
(i) Molecules must be close together
♦ We must apply pressure and thus bring the molecules of the gas closer together
(ii) Temperature must be low
♦ We must reduce the temperature (to a value below Tc)
♦ Then, the molecules will no longer have the kinetic energy to break away from the inter molecular attractions
Now we will learn about vapor pressure. It can be written in 10 steps:
1. We have seen that, the kinetic energy of the molecules need to be brought down to achieve liquefaction
• Conversely, we can say:
When the molecules are in the liquid state, their kinetic energy will be low
2. But in a liquid sample, even when it is at normal temperature, there will be some molecules that have a ‘high value kinetic energy’
• Those molecules can 'break away' from the other liquid molecules
3. The consequence of such ‘break away’ can be demonstrated using an example. It can be written in 5 steps:
(i) Consider a sample of liquid water placed in an open vessel
• Let it be at room temperature
(ii) Some of the molecules in that sample will be having enough kinetic energy to break away
• Those molecules will escape from the liquid water body and reach a level above the water surface
This is shown in fig.5.29(a) below
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| Fig.5.29 |
(iii) Those molecules are no longer attracted to any other molecules. Because, they are in the gaseous state
• So even a small current of air will carry them away
(iv) If the climate is hot, some more molecules will gain the required kinetic energy
• They will also break away from the liquid body and will be carried away by air currents
(v) This process continues and soon all the liquid water will be turned into the gaseous state
• This process is called evaporation
■ So the consequence of the 'break away' is that, there will always be some molecules above the liquid surface
4. Now consider fig.5.29(b) above
• The vessel is closed with a lid
• This time, the ‘molecules which break away’ are confined in a 'definite space'
5. As time passes, more and more molecules will break away from the liquid body
• All those free molecules will be collected and confined in the definite space
♦ The lid is the upper boundary of this definite space
♦ The surface of the liquid is the lower boundary of this definite space
♦ The vessel is the side boundary of this definite space
6. We see that, as time passes, more and more gaseous molecules are added into this definite space
• If this continues, there will not be any liquid left. All liquid will turn into gas
• But such a situation does not occur. The reason can be explained in 8 steps:
(i) The molecules in the definite space will be moving in random directions
♦ They will hit each other
♦ They will hit the boundaries mentioned in (5)
(ii) As a result, some of those molecules will lose energy and will fall back into the liquid state
(iii) But remember that:
Even when some molecules are falling back to the liquid state, some molecules (as mentioned in step 2), are breaking away from the liquid state and are passing into the gaseous state
(iii) So we have motion in two directions:
♦ Some molecules are breaking away from the liquid state into the gaseous state
♦ Some molecules are falling back from the gaseous state into the liquid state
(iv) If the ‘number of molecules breaking away’ is greater than the ‘number of molecules falling back’, all the liquid will soon become gas
(v) If the ‘number of molecules falling back’ is greater than the ‘number of molecules breaking away’, all the gas will soon become liquid
(vi) Such situations mentioned in (iv) and (v) do not arise because, an equilibrium will be reached
• That is:
♦ The ‘number of molecules breaking away’
♦ will soon become equal to
♦ The ‘number of molecules falling back’
(vii) It is important to understand the nature of this equilibrium. This can be explained in steps:
♦ Even at equilibrium,the molecules are travelling in both directions
♦ That is., even at equilibrium,
✰ some molecules are breaking away
✰ some molecules are falling back
♦ We call it an 'equilibrium' because, the numbers are the same
(viii) Once equilibrium is attained, there will be no change in the number of gaseous molecules in the definite space
■ That is the reason why, in fig.5.29(b), all the liquid is not turned into gas
7. So we learnt that:
• At equilibrium:
♦ The ‘number of molecules breaking away’
♦ will be equal to
♦ The ‘number of molecules falling back’
• In other words, at equilibrium, the ‘number of gaseous molecules’ will be a constant
8. A ‘definite space’ is same as ‘constant volume’
• So at equilibrium, we have:
♦ A constant volume
♦ A constant number of gaseous molecules in that volume
♦ A constant temperature
• When those three items are constant, the 'pressure exerted by the gaseous molecules' will be a constant
■ This pressure at equilibrium is called equilibrium vapor pressure or saturated vapor pressure
9. We attached the words ‘equilibrium/saturated’ to vapour pressure
• So, is there an ‘ordinary vapour pressure’ also ?
• The answer is: Yes
• This can be explained in 4 steps:
(i) In step (6), we saw that:
♦ As time passes, more and more gaseous molecules are being added
✰ This is a situation which continues till equilibrium
(ii) So before reaching equilibrium we can measure the vapour pressure at any instant
♦ Let the vapour pressure measured at time t1 be pvapour(1)
♦ Let the vapour pressure measured at time t2 be pvapour(2)
(iii) pvapour(1) Will be different from pvapour(2)
• This is because:
♦ The number of gaseous molecules at time t1
♦ will be different from
♦ The number of gaseous molecules at time t2
(iv) So we can write:
• The vapour pressures measured before attaining equilibrium are reported simply as: Vapour pressure
• The vapour pressure measured at equilibrium is reported as: Equilibrium vapour pressure or Saturated vapour pressure
10. Whenever we report a vapour pressure (equilibrium or ordinary), we must mention the temperature also
• This is because:
♦ Vapour pressure is temperature dependent
♦ When the temperature is high, there will be a greater number of gaseous molecules
✰ This will create a greater vapour pressure
♦ When the temperature is low, there will be a lesser number of gaseous molecules
✰ This will create only a lower vapour pressure
• Based on the above discussion, we can now write some basics about boiling
• The basics can be written in 5 steps:
1. Consider the vessel in fig.5.29(b) above
• We see that, the ‘molecules which break away from the liquid body’ are not free to escape because, there is a lid
2. Consider the vessel in fig.5.29(a) above
• We see that, the ‘molecules which break away from the liquid body’ are free to escape because, there is no lid
3. But in fact, in fig.a, there is an invisible lid. This can be explained in 5 steps:
(i) We know that, atmospheric pressure acts every where
(ii) Pressure is: Force per unit area
• So atmospheric pressure is: Force exerted by atmosphere on unit area
(iii) The atmosphere can be considered as a mixture of the molecules of N2, O2, CO2 etc.,
• These molecules are pulled down by gravity
• So these molecules apply a down ward weight on substances below them
• When we divide this weight by area, we get: Weight per unit area
♦ This 'weight per unit area' is the atmospheric pressure
(iv) Consider the molecules (both gaseous and liquid) in the vessel in fig.5.29(a)
• Each of those molecules will experience the atmospheric pressure
(v) That means, all the molecules in the vessel in fig.a are pushed down by the atmospheric pressure
• This, in effect, is a lid
4. If the gaseous molecules in fig.5.29(a), can over come the ‘pushing down by the atmosphere’, they can escape
• So let us consider the conditions by which the molecules can over come the ‘pushing down by the atmosphere’
• It can be written in 5 steps:
(i) Let us heat the liquid from the bottom of the vessel
• The temperature of the liquid will begin to increase
(ii) More and more liquid molecules will attain the required energy to break free from the liquid body
• So more and more molecules will turn into gaseous state
• So more and more molecules will get collected between the lid (here, the lid is atmospheric pressure) and the surface of the liquid
(iii) That means, number of gaseous molecules in that region increases continuously
• When the number of gaseous molecules increases, the ‘vapor pressure’ that we saw earlier increases
• Due to the continuous heating, the vapor soon becomes equal to the atmospheric pressure
(iv) With a little more heating, the vapor pressure becomes greater than atmospheric pressure
• The ‘vapor pressure is greater’ means:
♦ The force per unit area exerted by the vapor
♦ is greater than
♦ The force per unit area exerted by the atmosphere
• So the vapor pushes away the atmosphere and escapes from the vessel
(v) As the heating is continued, the vapor pressure also increases
• More and more liquid molecules first break away from the liquid body
• After that, they push away the atmosphere and escape from the vessel
5. At this stage, we see bubbles coming from the bottom of the vessel
• Bubbles originate at the bottom of the vessel because, heating is done at the bottom
• The liquid molecules at the bottom attains the required energy to break away
• They push the other molecules in all directions
• So a bubble is made up of ‘high energy liquid molecules’
• Since these ‘high energy molecules’ push the other molecules in all directions, bubbles have a near spherical shape
• When bubbles begin to appear, we say that: Boiling has begun
• Next we will see the influence of atmospheric pressure on boiling. It can be written in steps:
1. In our earlier classes, we have learnt that:
Boiling point is the temperature at which a liquid starts to boil at standard atmospheric pressure
2. From the above definition, it is clear that:
The temperature should be measured 'when the atmospheric pressure is at a standard value'
• In other words:
If we measure the temperature 'when the atmospheric pressure is not at a standard value', we cannot report it as an acceptable boiling point
3. For example, at mountain tops, the atmospheric pressure will be low
• The air molecules of the atmosphere will not be ‘pushing down’ with the same force as at sea level
• The vapour pressure will quickly become equal to the surrounding atmospheric pressure
• A lower temperature will be sufficient to build up the required vapour pressure
♦ So we will find that, the liquid boils at a lower temperature
♦ We cannot report this low temperature as the boiling point
4. In order to avoid any doubts, we can report the pressure value also. Thus we have:
The temperature at which the vapor pressure of a liquid is equal to the external pressure is called boiling temperature at that pressure
5. If the external pressure is 1 atm, we do not write the pressure
• Instead, we use the word ‘normal’
• This can be explained in 3 steps:
(i) Suppose that, the ‘temperature at which a liquid boils’ is T1 K
(ii) Also suppose that, when this T1 K is measured, the external pressure is 1 atm
(iii) We can report the result in two ways:
♦ The boiling point of the liquid is T1 K at 1 atm pressure
♦ The normal boiling point of the liquid is T1 K
✰ When the word ‘normal’ is used, a ‘pressure of 1 atm’ is implied
6. Similarly, if the external pressure is 1 bar, we do not write the pressure
• Instead, we use the word ‘standard’
• This can be explained in steps:
(i) Suppose that, the ‘temperature at which a liquid boils’ is T2 K
(ii) Also suppose that, when this T2 K is measured, the external pressure is 1 bar
(iii) We can report the result in two ways:
♦ The boiling point of the liquid is T2 K at 1 bar pressure
♦ The standard boiling point of the liquid is T2 K
✰ When the word ‘standard’ is used, a ‘pressure of 1 bar’ is implied
7. Let us see if there is any difference between the two methods mentioned in (5) and(6). We will take water as an example
• The normal boiling point of water is 100 oC
• The standard boiling point of water is 99.6 oC
• So indeed there is a difference
• The explanation can be given in 4 steps:
(i) 99.6 is less than 100
(ii) This implies that:
♦ If the external pressure is 1 bar,
♦ the molecules can escape a bit more easier than
♦ If the external pressure is 1 atm
(iii) This implies that:
♦ A pressure of 1 bar
♦ is less than
♦ A pressure of 1 atm
(iv) This is indeed true. Converting both units into M pa, we get:
♦ 1 bar = 0.1 M pa
♦ 1 atm =0.101 M pa
• It is clear that:
♦ If we take 1 atm, we get a pressure of 0.101 M pa
♦ If we take 1 bar, we get a pressure of only 0.1 M pa
• So 1 bar is slightly less than 1 atm
• Based on the above discussion, we can now explain why it is difficult to cook food on mountain tops
• The explanation can be written in 3 steps:
1. At mountain tops, water boils at low temperatures
• We know the reason:
A low temperature is sufficient to give the water molecules the required energy to break off
2. So compare the two items:
(i) Molecules of some boiling water at the mountain top
(ii) Molecules of some boiling water at the sea level
• obviously, (i) will be having lesser energy
3. Now compare the two items:
(i) Raw food material put into the water mentioned in 2(i)
(ii) Raw food material put into the water mentioned in 2(ii)
• The raw food mentioned in (i) has only a 'lesser energy available to absorb' when compared to (ii)
• So we will need to cook it for a longer time in order to make it edible
• That is why we say that, it is difficult to cook food at mountain tops
• Similarly, we can explain why it is easier to cook food in a pressure cooker
• The explanation can be written in 4 steps:
1. Inside the pressure cooker, the pressure is high
• That means, a high pressure is pressing down on the water molecules
2. Water in a pressure cooker will boil only at a high temperature
• We know the reason:
A high temperature is necessary to give the water molecules the required energy to break off
3. So compare the two items:
(i) Molecules of some boiling water in a pressure cooker
(ii) Molecules of some boiling water in an ordinary vessel
• obviously, (i) will be having greater energy
4. Now compare the two items:
(i) Raw food material put into the water mentioned in 3(i)
(ii) Raw food material put into the water mentioned in 3(ii)
• The raw food mentioned in (i) has a 'greater energy available to absorb' when compared to (ii)
• So we will need to cook it only for a shorter time in order to make it edible
• That is why we say that, it is easier to cook food in a pressure cooker
• Next we will see how the ‘relation between boiling point and external pressure’ can be shown graphically
• We will use water as an example. It can be written in 7 steps:
1. Apply a pressure of p1 on a sample of water
• Heat that water until it boils
• Note down the temperature T1 at which it boils
2. Apply a pressure of p2 on that sample
• Heat it until it boils
• Note down the temperature T2 at which it boils
3. Repeat the steps several times with different pressure values. We get the points: (p1,T1), (p2,T2), (p3, T3) . . . so on . . .
4. Plot temperature along the x-axis
• Plot pressure along the y-axis
• We will get the green curve shown in fig.5.30(a) below:
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| Fig.5.30 |
5. Now, 1 atm pressure is equivalent to 760 mm mercury
• So draw a horizontal white dashed line through 760 mm mercury
• This horizontal dashed line will intersect the green curve at a point
6. Through this point of intersection, draw a vertical white dashed line
• This vertical dashed line will meet the x-axis at the normal boiling point
• For water, it will be 373 k
7. Graphs of other liquids can be drawn in this way
• Fig.b shows the graphs of diethyl ether, carbon tetrachloride etc.,
• Once their graphs are plotted, we can easily find the normal boiling points by drawing vertical dashed lines
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