Solid into Liquid
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Melting
to
al
ing
Conversion of solid state
of matter into liquid state
by application of heat
• Increase in temperature
increases the kinetic
energy of particles, they
start moving freely and
at one point change to
liquid state
• Temperature at which a
solid melts to become
liquid at atmospheric
pressure is melting point
Answers
Answer:
Substances can change phase—often because of a temperature change. At low temperatures, most substances are solid; as the temperature increases, they become liquid; at higher temperatures still, they become gaseous.
The process of a solid becoming a liquid is called melting. (an older term that you may see sometimes is fusion). The opposite process, a liquid becoming a solid, is called solidification. For any pure substance, the temperature at which melting occurs—known as the melting point—is a characteristic of that substance. It requires energy for a solid to melt into a liquid. Every pure substance has a certain amount of energy it needs to change from a solid to a liquid. This amount is called the enthalpy of fusion (or heat of fusion) of the substance, represented as ΔHfus. Some ΔHfus values are listed in Table 10.2 “Enthalpies of Fusion for Various Substances”; it is assumed that these values are for the melting point of the substance. Note that the unit of ΔHfus is kilojoules per mole, so we need to know the quantity of material to know how much energy is involved. The ΔHfus is always tabulated as a positive number. However, it can be used for both the melting and the solidification processes as long as you keep in mind that melting is always endothermic (so ΔH will be positive), while solidification is always exothermic (so ΔH will be negative).
Table 10.2 Enthalpies of Fusion for Various Substances
Substance (Melting Point) ΔHfus (kJ/mol)
Water (0°C) 6.01
Aluminum (660°C) 10.7
Benzene (5.5°C) 9.95
Ethanol (−114.3°C) 5.02
Mercury (−38.8°C) 2.29
Example 2
What is the energy change when 45.7 g of H2O melt at 0°C?
Solution
The ΔHfus of H2O is 6.01 kJ/mol. However, our quantity is given in units of grams, not moles, so the first step is to convert grams to moles using the molar mass of H2O, which is 18.0 g/mol. Then we can use ΔHfus as a conversion factor. Because the substance is melting, the process is endothermic, so the energy change will have a positive sign.
45.7 g H2O × (1 mol H2O/18.0 g H2O) × (6.01 kJ/ 1 mol H2O) = 15.3 kJ
Without a sign, the number is assumed to be positive.
Test Yourself
What is the energy change when 108 g of C6H6 freeze at 5.5°C?
Answer
−13.8 kJ
During melting, energy goes exclusively to changing the phase of a substance; it does not go into changing the temperature of a substance. Hence melting is an isothermal process because a substance stays at the same temperature. Only when all of a substance is melted does any additional energy go to changing its temperature.
What happens when a solid becomes a liquid? In a solid, individual particles are stuck in place because the intermolecular forces cannot be overcome by the energy of the particles. When more energy is supplied (e.g., by raising the temperature), there comes a point at which the particles have enough energy to move around but not enough energy to separate. This is the liquid phase: particles are still in contact but are able to move around each other. This explains why liquids can assume the shape of their containers: the particles move around and, under the influence of gravity, fill the lowest volume possible (unless the liquid is in a zero-gravity environment—see Figure 10.3 “Liquids and Gravity”).
Figure 10.3 Liquids and Gravity