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.
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).
In a solid the strong attractions between the particles hold them tightly packed together. Even though they are vibrating this is not enough to disrupt the structure. When a solid is heated the particles gain energy and start to vibrate faster and faster. Initially the structure is gradually weakened which has the effect of expanding the solid. Further heating provides more energy until the particles start to break free of the structure. Although the particles are still loosely connected they are able to move around. At this point the solid is melting to form a liquid. The particles in the liquid are the same as in the solid but they have more energy. To melt a solid energy is required to overcome the attractions between the particles and allow them to pull them apart. The energy is provided when the solid is heated up. The temperature at which something melts is called its "melting point" or melting temperature. At room temperature a material is a solid, liquid or gas depending on its melting temperature. Anything with a melting temperature higher than about 20oC is likely to be a solid under normal conditions. Materials have widely differing melting temperatures e.g. mercury -39oC, ice 0oC, salt 1081oC, aluminium 660oC and steel 1535oC. Everyday materials such as ice, butter and wax have different melting temperatures and can be used as examples with children.
Ice is probably the melting substance most children are familiar with. It can be problematic when trying to develop an understanding of melting as it does often appear to them to melt without any source of heat. Ice melts at room temperature because the surrounding air is warmer than the ice and at a temperature above the melting temperature. The heat energy required to melt the ice comes from the surrounding air which will consequently become a little cooler.
Not all solids melt when they are heated. Some may undergo chemical changes as a result of heating. For example paper burns rather than melts
Answers & Comments
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.
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).
Answer:
Melting - increases
Explanation:
In a solid the strong attractions between the particles hold them tightly packed together. Even though they are vibrating this is not enough to disrupt the structure. When a solid is heated the particles gain energy and start to vibrate faster and faster. Initially the structure is gradually weakened which has the effect of expanding the solid. Further heating provides more energy until the particles start to break free of the structure. Although the particles are still loosely connected they are able to move around. At this point the solid is melting to form a liquid. The particles in the liquid are the same as in the solid but they have more energy. To melt a solid energy is required to overcome the attractions between the particles and allow them to pull them apart. The energy is provided when the solid is heated up. The temperature at which something melts is called its "melting point" or melting temperature. At room temperature a material is a solid, liquid or gas depending on its melting temperature. Anything with a melting temperature higher than about 20oC is likely to be a solid under normal conditions. Materials have widely differing melting temperatures e.g. mercury -39oC, ice 0oC, salt 1081oC, aluminium 660oC and steel 1535oC. Everyday materials such as ice, butter and wax have different melting temperatures and can be used as examples with children.
Ice is probably the melting substance most children are familiar with. It can be problematic when trying to develop an understanding of melting as it does often appear to them to melt without any source of heat. Ice melts at room temperature because the surrounding air is warmer than the ice and at a temperature above the melting temperature. The heat energy required to melt the ice comes from the surrounding air which will consequently become a little cooler.
Not all solids melt when they are heated. Some may undergo chemical changes as a result of heating. For example paper burns rather than melts
INCREASES
DECREASES
10. What happens to the particles when gas becomes liquid?
SPREAD APART
COMPRESSED TOGETHER
11. All of the following is an ENDOTHERMIC process EXCEPT __________.
Melting
Vaporization
Deposition
Sublimation