Water can exist in the solid, liquid, or gas phase. Most substances can exist in different phases. Changes in phase are also physical changes. For example, the physical properties of ice and steam are quite different but they are both water. There is no change in the chemical nature of the two substances.
Solid gold and liquid gold are exactly the same chemically even though the phases solid and liquid are different. Examples of phase changes include melting, freezing, condensation, evaporation, and sublimation.
Melting occurs when a solid changes to a liquid. Freezing occurs when a liquid becomes a solid. Condensation involves a gas becoming a liquid. Evaporation involves a liquid becoming a gas and sublimation is the change of a solid directly to a gas.
Phase changes require either the addition of heat energy melting, evaporation, and sublimation or subtraction of heat energy condensation and freezing. Drag your mouse over the link to open a new browser window - then watch a demonstration of the differences between the different phases of water.
Close the window to return to this page. Changing the amount of heat energy usually causes a temperature change. If the vapor pressure of the drop is greater than the partial pressure of vapor in the gas phase, the drop will evaporate. Interactive: Boiling Point : Non-polar molecules gray evaporate or boil more quickly than polar molecules blue and red. Attractions between molecules are shown with dotted lines. Run the model, then heat the liquids. What does boiling look like at the molecular level?
A bubble is a hole in a liquid; molecules at the liquid boundary are curved inward, so they experience stronger nearest-neighbor attractions. As a consequence, the vapor pressure P w of the liquid facing into a bubble is always less than that of the bulk liquid P w at the same temperature. When the bulk liquid is at its normal boiling point that is, when its vapor pressure is 1 atm , the pressure of the vapor within the bubble will be less than 1 atm, so the bubble will tend to collapse.
For both of these reasons, a liquid will not boil until the temperature is raised slightly above the boiling point, a phenomenon known as superheating. How does a liquid become a gas? A supercritical fluid is a substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist.
A supercritical fluid is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist. This can be rationalized by thinking that at high enough temperatures above the critical temperature the kinetic energy of the molecules is high enough to overcome any intermolecular forces that would condense the sample into the liquid phase.
On the other hand, high enough pressures above the critical pressure would not allow a sample to stay in the pure gaseous state. Therefore, a balance between these two tendencies is achieved and the substance exists in a state between a gas and a liquid. Phase Diagram for a Substance : The figure highlights the critical point, above which in either temperature or pressure the substance does not exist in either the liquid or gas phase. It can effuse through solids like a gas , and dissolve materials like a liquid.
Carbon dioxide and water are the most commonly used supercritical fluids, as they are used for decaffeination and power generation, respectively. In general terms, supercritical fluids have properties between those of a gas and a liquid. The critical properties of some substances used as solvents and as supercritical fluids are shown in Table 1. Table 2 shows density, diffusivity, and viscosity for typical liquids, gases, and supercritical fluids.
Critical Properties of Various Solvents : Supercritical fluids have properties between those of a gas and a liquid. In addition, there is no surface tension in a supercritical fluid, as there is no liquid to gas phase boundary. One of the most important properties of supercritical fluids is their ability to act as solvents.
Solubility in a supercritical fluid tends to increase with the density of the fluid at constant temperature. Since density increases with pressure, solubility tends to increase with pressure. The relationship with temperature is a little more complicated. At constant density, solubility will increase with temperature. However, close to the critical point, the density can drop sharply with a slight increase in temperature. Therefore, close to the critical temperature, solubility often drops with increasing temperature, then rises again.
All supercritical fluids are completely miscible with each other; therefore a single phase for a mixture can be guaranteed if the critical point is exceeded. The critical point of a binary mixture can be estimated as the arithmetic mean of the critical temperatures and pressures of the two components,. For greater accuracy, the critical point can be calculated using equations of state, such as the Peng Robinson or group contribution methods. Other properties, such as density, can also be calculated using equations of state.
In the pressure-temperature phase diagram of CO 2 , the boiling separates the gas and liquid region and ends in the critical point, where the liquid and gas phases disappear to become a single supercritical phase.
At well below the critical temperature, e. The system consists of 2 phases in equilibrium, a dense liquid and a low density gas.
As the critical temperature is approached K , the density of the gas at equilibrium becomes denser, and that of the liquid becomes lower. At the critical point, Thus, above the critical temperature a gas cannot be liquified by pressure.
At slightly above the critical temperature K , in the vicinity of the critical pressure, the line is almost vertical. A small increase in pressure causes a large increase in the density of the supercritical phase. Many other physical properties also show large gradients with pressure near the critical point, such as viscosity, the relative permittivity, and the solvent strength, which are all closely related to the density. A close look at supercritical carbon dioxide : A pressure vessel made of aluminum and acrylic is filled with pieces of dry ice.
The dry ice melts under high pressure, and forms a liquid and gas phase. When the vessel is heated, the CO2 becomes supercritical — meaning the liquid and gas phases merge together into a new phase that has properties of a gas, but the density of a liquid.
Supercritical CO2 is a good solvent, and is used for decaffeinating coffee, dry cleaning clothes, and other situations where avoiding a hydrocarbon solvent is desirable for environmental or health reasons. Freezing is a phase transition in which a liquid turns into a solid when its temperature is lowered to its freezing point.
Freezing, or solidification, is a phase transition in which a liquid turns into a solid when its temperature is lowered to or below its freezing point. All known liquids, except helium, freeze when the temperature is low enough. Liquid helium remains a liquid at atmospheric pressure even at absolute zero, and can be solidified only under higher pressure.
For most substances, the melting and freezing points are the same temperature; however, certain substances possess different solid-liquid transition temperatures. Most liquids freeze by crystallization, the formation of a crystalline solid from the uniform liquid. Crystalline Solid : Model of closely packed atoms within a crystalline solid.
This is a first-order thermodynamic phase transition, which means that as long as solid and liquid coexist, the equilibrium temperature of the system remains constant and equal to the melting point. 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. 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 H 2 O, which is Because the substance is melting, the process is endothermic, so the energy change will have a positive sign.
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. 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 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.
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