If the substance has enough energy, it can overcome the bonding forces holding the particles together and, in doing so, undergo a change in state. Cooling, on the other hand, removes energy thus making the particles less active and allowing the bonding forces to take hold within the substance. Melting - solid to liquid Melting occurs when a solid is heated and turns to liquid. The particles in a solid gain enough energy to overcome the bonding forces holding them firmly in place.
Typically, during melting, the particles start to move about, staying close to their neighbouring particles, then move more freely. For pure substances, the temperature at which this change occurs is quite precise and is called the melting point of the substance. Freezing - liquid to solid Freezing occurs when a liquid is cooled and turns to a solid.
Eventually the particles in a liquid stop moving about and settle into a stable arrangement, forming a solid. This is called freezing and occurs at the same temperature as melting.
Hence, the melting point and freezing point of a substance are the same temperature. The melting and freezing point of a substance are defined as the temperature above which, the substance is liquid and below which, it is solid. Boiling - liquid to gas Boiling occurs when the particles in a liquid gain enough energy to overcome the bonding forces holding them loosely in place in the liquid and they become free, fast moving, individual particles in a gas.
For pure substances, the temperature at which this change occurs is quite precise and is called the boiling point of the substance.
A gas will also liquefy turn into a liquid if its pressure is increased enough. This is because the particles are moved close enough for bonds to form between the particles.
Gas cylinders used for camping stoves and barbecues contain liquefied petroleum gas LPG under high pressure. As soon as the pressure is released, the liquid turns back to a gas. State changes Substances can exist as a solid, liquid or gas. 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 The phase change between a liquid and a gas has some similarities to the phase change between a solid and a liquid.
At a certain temperature, the particles in a liquid have enough energy to become a gas. The process of a liquid becoming a gas is called boiling or vapourization , while the process of a gas becoming a liquid is called condensation. This means that the temperature at which a liquid becomes a gas, the boiling point , can change with surrounding pressure.
Therefore, we define the normal boiling point as the temperature at which a liquid changes to a gas when the surrounding pressure is exactly 1 atm, or torr.
Unless otherwise specified, it is assumed that a boiling point is for 1 atm of pressure. To determine the magnitude of the energy change, we must first convert the amount of Br 2 to moles. As with melting, the energy in boiling goes exclusively to changing the phase of a substance; it does not go into changing the temperature of a substance.
So boiling is also an isothermal process. Only when all of a substance has boiled does any additional energy go to changing its temperature.
What happens when a liquid becomes a gas? We have already established that a liquid is composed of particles in contact with each other. When a liquid becomes a gas, the particles separate from each other, with each particle going its own way in space. This is how gases tend to fill their containers. Indeed, in the gas phase most of the volume is empty space; only about one one-thousandth of the volume is actually taken up by matter see Figure Under some circumstances, the solid phase can transition directly to the gas phase without going through a liquid phase, and a gas can directly become a solid.
The solid-to-gas change is called sublimation , while the reverse process is called deposition. Sublimation is isothermal, like the other phase changes. There are several common examples of sublimation. A well-known product — dry ice — is actually solid CO 2.
Dry ice is dry because it sublimes, with the solid bypassing the liquid phase and going straight to the gas phase. If you have ever noticed that ice cubes in a freezer tend to get smaller over time, it is because the solid water is very slowly subliming. The food is still good but looks unappetizing.
Reducing the temperature of a freezer will slow the sublimation of solid water. Chemical equations can be used to represent a phase change. In such cases, it is crucial to use phase labels on the substances.
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