8 S
M
8.1
T HE STRUCTURE OF SOLIDS , LIQUIDS AND GASES
The three basic states of matter are solid, liquid and gas (there is a fourth called plasma, but that is an extreme conditions, such as exists in the Sun, so we won’t get too worried about it).
PRACTICE QUESTION
1. Compare the three states of matter in terms of:
- shape
- volume
- rate of diffusion or movement
The next question is why the three states of matter differ? What makes ice, water and steam different? Figure 8.1 shows the difference at the molecular level.
Solid Liquid Gas
FIGURE 8.1
Differences between the three states of matter (dotted lines indicates intermolecular bonds)
The difference in the form of the three states is the closeness of the particles
(atoms/ions/molecules) to each other and the strength of forces of attraction between the particles . In covalent molecules, they are known as intermolecular forces . These forces increase in strength as the molecules get close to each other.
In a solid , the particles are close together , and are held that way because the forces are strong enough to keep the particles in place. More often than not, the particles arrange themselves into regular patterns - known as a crystal lattice. While a solid has a fixed shape and cannot move (without assistance) the particles are actually vibrating, and as heat is added, they vibrate more. This is shown by a increase in temperature.
Only at a temperature called absolute zero does atomic motion cease. This temperature has never been reached, but can be calculated as -273.15
 C, which is given the value 0 in the Kelvin scale.
In a liquid , some of the intermolecular forces have been broken, and there is a limited ability of the molecules to move around and to adopt the shape of the container. However, there are enough bonds left to maintain a constant volume for the liquid.
In a gas, virtually all of the intermolecular forces have been broken, and the molecules are now free to move randomly without any restrictions, other than the container walls.
8.2
T HE E FFECT OF H EAT
At normal pressures, the difference between the solid, liquid and gas forms of a particular substances is temperature. If a solid is heated enough, the vibrations become so energetic that the forces between the particles become weakened. Extra heat energy then goes into breaking some of these forces, so that some of the particles become free to move. This is the

8. States of Matter melting point of the substance, and the heat required to cause the solid to melt is called the latent heat of fusion (or melting). While the substance is melting, its temperature does not increase (see Figure 8.2).
The process continues in the liquid state as more heat is added to cause more vibration in the intermolecular forces. When all forces between particles (but not within) are so strained that they will break, the boiling point of the substance is reached. The heat required to cause the solid to boil is called the latent heat of evaporation (or boiling).
While the substance is boiling, its temperature does not increase (see Figure 8.2). melting point
Temp. boiling point
Time of heating
FIGURE 8.2
Temperature changes during heating from the solid state
Different substances have different latent values, because of the different strengths of the intermolecular forces. Some of these listed in Table 8.1.
TABLE 8.1
Heats of fusion and vaporisation for selected substances
Substance Heat of fusion
(kJ/mole)
Heat of vaporisation
(kJ/mole)
Water
Carbon dioxide
Sodium chloride
Methane
6.02
7.9
30.2
0.94
40.7
23.2
--
10.4
PRACTICE QUESTION
2. Calculate the amount of heat required to (a) melt and (b) boil 100 g of each of water and methane.
During the stages where the substance is increasing in temperature, the rate of increase depends on three factors:
 the rate of heating,
 the type of material it is made from,
 the mass of the object
Different substances are able to absorb heat energy without the object increasing significantly in temperature. This is known as the heat capacity . Water has a relatively high heat capacity, which means it takes relatively a large amount of energy to cause the temperature to rise. If you heat a 1 kg block of iron, and 1 kg of water, the iron will increase in temperature by a factor of almost ten times that of the water. Table 8.2 gives the heat capacity of a number of common substances.
The high heat capacity of water is critical in stabilising the earth’s surface temperature.
The oceans and other surface water that cover 70% of the surface absorb large amounts of the sun’s heat without significantly changing in temperature. Consider how little the ocean water changes in temperature from winter to summer (typically 16-22ºC).
Chemistry 2 8.2

8. States of Matter
TABLE 8.2
Heat capacities for selected substances
Compound Heat Capacity
(J/g)
Compound Heat Capacity
(J/g)
Liquid water
Steam
Ice
Alcohol
4.18
1.86
1.88
2.42
Iron
Aluminium
Graphite
Ammonia gas
0.45
0.90
0.71
2.06
The heat of capacity of a substance can be used to calculate the relationship between temperature change and energy input/output for a process. Equation 8.1 gives this relationship. p
 T Eqn 8.1 heat = mC where m is the mass (in g) of the substance and Cp is the heat capacity in J/g/ºC -1 . This is used to measure heats of reaction - a field known as calorimetry – where the heat of a process is transferred with minimal loss to a substance (usually water) of exactly known mass and heat capacity.
EXAMPLE
What is the temperature rise in 500 g of water if it is heated for 1 minute at a rate of 1.5 kJ/second?
Total heat = 60 x 1500 = 90, 000 J

T = 90, 000 ÷ (500 x 4.18)
= 43.1ºC.
PRACTICE QUESTIONS
3. How much heat was released by a reaction if 125.3 g of water rose in temperature from
21ºC to 32ºC?
4. Which would heat up fastest: 10 g of iron or 20 g of water?
5. 250 mL of alcohol density 0.8 g/mL) at 18ºC is heated by the combustion of 28 g of methane. Would this bring the ethanol to the boil?
6. How long would you need to heat 1 L of water from 20ºC to (a) 100ºC (b) complete vaporisation, using a heating element capable of producing 2.4 kJ/s?
Note that these type of heat capacity calculations do not extend across change of state boundaries. For example, if the predicted rise in water at 25ºC was more than 75ºC, the calculation becomes invalid.
8.3
T HE GASEOUS STATE
Gases behave remarkably consistently, regardless of their chemical composition. At 0ºC and normal atmospheric pressure (known as standard temperature and pressure, STP), 1 mole of any gas occupies 22.4 L. At 25ºC, it occupies 24.5L. It doesn’t matter whether the gas is hydrogen, oxygen or even a mixture, such as air.
Gases, in theory, obey the following four rules:
 the volume of the gas is negligible
 the molecules exert no attraction upon each other
 all collisions are totally elastic (ie. no KE is lost during the collision)
 the gas molecules are in a state of constant rapid motion
Chemistry 2 8.3

8. States of Matter
Under most circumstances, these assumptions hold almost perfectly true, regardless of the chemical composition of the gas, even though they are not really true. At very low temperatures, assumptions 2-4 begin to fail, and at very high pressures, none of them are valid.
Gases are more “interesting” to study because their pressure and volume are variable, unlike those for solids and liquids. Table 8.3 lists some of the common units for pressure and the interconversion factors.
TABLE 8.3
Common units of pressure
Unit 1 atm = ? atmospheres (atm) 1 mm Hg
Pascals (Pa)
760 mm Hg
1.013 x 10 6 Pa
8.4
G AS L AWS
The variables in a given amount of gas – temperature, pressure and volume – are related by a simple equation (Eqn 8.2).
PV
 cons tan t Eqn 8.2
T
Written another to represent the situation where a change in one or more of these variables occurs, we get equation 8.3, which is known as the Combined Gas Equation .
P
1
V
1 
P
2
V
2 Eqn 8.3
T
1
T
2 where sides 1 and 2 represent the conditions before and after the change, and the temperatures are always in degree Kelvin. The pressure and volume units can be anything, but are the same on each side of the equation.
This equation was developed from the observations of a number of scientists, who concentrated on the relationships between two of the variables, as described in Table 8.4.
TABLE 8.4 Studies of two-variable gas relationships
Scientist
Boyle
Charles
Gay-Lussac
Relationshi p
P  V
V  T
T  P
Constant
T
P
V
PRACTICE QUESTIONS
7. Can you think of any everyday example to illustrate each of these relationships?
8. Modify equation 8.3 to express these three relationships.
Chemistry 2 8.4

8. States of Matter
EXAMPLES
1. Freon 12 is a widely used refrigerant gas. Consider a 1.53L sample of gaseous Freon at a pressure of 5.6kPa. If the pressure is changed to 15kPa at constant temperature, what will the new volume of gas be?
With calculation questions like this, always write down all the data you have and don’t have.
Condition 1 Condition 2
Pressure
Volume
5.6 kPa
1.53 L
15 kPa
? L
Temperature constant constant
Rearranging equation 8.3 to make V
2
the subject, we get
V
2

P
1
V
1
T
2
T
1
P
2
Now substituting into this and remembering that T1 = T2,
V
2

5 .
6 x 1 .
53

0 .
57 L
15
2. A sample of gas at 15°C and 1 atm. has a volume of 2.58L. What pressure will the gas exert at 38°C in a container of volume 890 mL?
Pressure
Condition 1
1 atm
Condition 2
? atm
Volume
Temperature
2.58 L
15ºC = 288K
890 mL = 0.89 L
38ºC = 311K
Rearranging equation 8.3 to make P
2
the subject, we get
P
2

P
1
V
1
T
2
T
1
V
2
Now substituting into this,
P
2

1 x 2 .
58 x 311

3 .
13 atm
288 x 0 .
89
PRACTICE QUESTIONS
9. A gas is collected at 24°C and 735mm Hg pressure in a bulb of volume 0.763L. What would the volume be at 0°C and 760mm Hg pressure?
10. Nitrogen gas is stored in a steel cylinder with a volume of 0.5 m3 at a pressure of
1500kPa and at a temperature of 27°C. Calculate the increase in pressure if the temperature rises to 30°C.
11. A volume of 3.0L of air is warmed from 50oc to 100oc. What is the new volume if the pressure remains constant?
12. A gas sample occupies 200 mL at 760 mm Hg. What volume does the gas occupy at 400 mm Hg if the temperature remains unchanged?
13. Nitrogen gas in a steel cylinder is under a pressure of 106 Pa at 290K. What will be the pressure in the tank if it is left in the sun and the internal temperature rises to 320K?
Chemistry 2 8.5

8. States of Matter
14. A sample of gas occupies a volume of 0.08 L at a pressure of 0.50 atm and a temperature of 0ºC. What will be its volume at a pressure of 1.50 atm and a temperature of 50ºC?
15. If 0.5 m 3 of a gas at STP is allowed to expand to 1.2 m 3 by heating it to 400K, what will be the new pressure?
16. A flask of volume 1.25 L is filled with gas at a pressure of 65.7 kPa. The flask is then connected up to another (which has been emptied of all gas). The tap between the two is opened, and the gas from the first flask fills both flasks. The pressure within the two flasks is measured at 41.2 kPa. What is the volume of the second flask?
17. A sample of gas evolved from a chemical reaction is collected at 100ºC and 1 atm pressure and found to occupy a volume of 1.592 L in a gas trap. It is cooled to STP conditions, the volume being allowed to change as necessary. What is the new volume occupied by the gas, and how many moles must have been collected?
18. A balloon designed to monitor weather in the upper atmosphere is filled with gas at sea level (1 atm pressure, 20ºC) and occupied a volume of 5 m 3 . It rises to a high altitude, where the surrounding pressure is 0.12 atm and the temperature is -25ºC. The balloon changes its volume so that the pressure inside is the same as that outside. What is the new volume of the balloon?
19. A gas container of 150 m 3 volume is designed to release a safety valve if the pressure inside reaches 10 atm. At what temperature will the valve open if the vessel was initially filled at 30ºC with 8 atm pressure of gas?
20. A sample of gas collected at 55ºC occupied 12.6 L at atmospheric pressure. It is then compressed to 2.45 L and cools to 32ºC. What pressure must have been applied?
21. Enough carbon dioxide is placed in a 250 mL bottle of soft drink to produce a pressure of 200 kPa at 20ºC when the bottle contains no liquid. The bottle is filled to 90% capacity with the drink, and the only thing stopping an excess pressure buildup because of the small gas space is the solubility of the carbon dioxide. If the drink were frozen, the carbon dioxide is forced out, being insoluble in ice. What pressure would form inside the gas space if the bottle were chilled to -5ºC? (Assume no change in volume due to expansion of ice)
22. Use the gas laws to explain why a stoppered flask containing hot water is difficult to open once the water has cooled.
The combined gas law is valid only for a given constant sample of gas. Put simply this means that if we add or remove any gas from the system it does not work. If we need to work with changing amounts of gas we must use another equation – the ideal gas equation (equation
8.4).
PV = nRT Eqn 8.4 where n is the number of moles of gas and R is the universal gas constant. R has a number of different values, depending on the units of pressure and volume being used. Table 8.5 gives these values.
TABLE 8.5
Values of the universal gas constant
Pressure
Unit hPa =(10 5 Pa) mm Hg mm Hg atm atm
Volume Unit Value m
L
L
3 mL mL
8.314
62.4
62, 400
0.082
82.1
Chemistry 2 8.6

8. States of Matter
PRACTICE QUESTIONS
23. Calculate the pressure exerted in a 1, 000 L vessel by 2 moles of gas at 750K.
24. 25 g of oxygen gas is heated to 100ºC in an expandable container. Given an atmospheric pressure of 102 hPa, calculate the volume of container.
25. What mass of helium would be required in a 2.5 m 3 cylinder at 200 atm pressure.
8.7
C RITICAL TEMPERATURES AND PRESSURES
Gases can be condensed by application of increased pressure. However, there is a temperature above which a gas cannot be condensed. This is known as the critical temperature and the pressure required at this temperature is the critical pressure .
These values for a range of gases are compared in Table 8.6.
TABLE 8.6
Critical properties of some common gases
Gas Boiling Point
(  C)
Critical Temperature
(  C)
Critical Pressure
(atm)
Ammonia -78 133 113
CCl
2
F
2
(CFC-12)
Helium
Nitrogen
-30
-272
-210
112
-268
-147
40
2.3
34
Oxygen -218 -118 50
Water 100 374 218
This is important for engineers designing chemical process and refrigeration equipment. A refrigerant gas must be able to be condensed at relatively high temperatures, since this is how the cooling process occurs.
The refrigerant compound as a liquid is pumped through coils to the chamber where the cooling is required. The heat from this chamber is conducted through the metallic coils into the refrigerant which heats sufficiently to boil. It leaves the chamber as a gas and returns to the compressor where it is subject to high pressure and is condensed. This releases heat which is radiated from the vanes on the outside. The process is shown in Figure 8.3. refrigerant as liquid heat released to atmosphere
COMPRESSOR
(gas to liquid) heat taken from chamber
COOLED CHAMBER
(liquid to gas) refrigerant as gas
FIGURE 8.3
The refrigeration process
Chemistry 2 8.7

8. States of Matter
PRACTICE QUESTIONS
32. Why does condensation of a gas to a liquid release heat?
33. Why are ammonia and CFC-12 the superior refrigerants?
W HAT Y OU N EED T O B E A BLE T O D O
 describe the difference at the atomic level between the three states of matter
 describe the temperature changes for a substance as it is heated
 perform calculations associated with changes of state
 explain the significance of differing heat capacities
 perform gas law calculations
 describe assumptions behind these equations
 perform heat capacity calculations calculate partial pressures
 perform gas diffusion calculations
 explain the meaning critical temperature and pressure of a gas
 explain how a refrigerator works
Chemistry 2 8.8