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Home Glossary

# Glossary: Liquids

Glossary
Attraction between different substances on either side of a phase boundary.

Antoine equation Antoine's equation.
A simple 3-parameter fit to experimental vapor pressures measured over a restricted temperature range:

 log P = A - BT + C

where A, B, and C are "Antoine coefficients" that vary from substance to substance. Sublimations and vaporizations of the same substance have separate sets of Antoine coefficients, as do components in mixtures. The Antoine equation is accurate to a few percent for most volatile substances (with vapor pressures over 10 Torr).

boiling.
Conversion of liquid into gas as bubbles of gas that form within the liquid. Boiling begins at the temperature where the vapor pressure of a liquid would be equal to the external pressure on the liquid.

boiling point. (bp) standard boiling point; normal boiling point.
The temperature at which the vapor pressure of a liquid would be equal to the external pressure on the liquid. The standard boiling point is the temperature at which the vapor pressure of a liquid equals standard pressure.

Clausius-Clapeyron equation.
The Clausius-Clapeyron equation predicts the temperature dependence of vapor pressures of pure liquids or solids:

 ln (P/P°) = HR ( 1T° - 1T )

where P is the vapor pressure, P° is a vapor pressure at a known temperature T°, H is an enthalpy of vaporization if the substance is a liquid or an enthalpy of sublimation if it's a solid, R is the ideal gas law constant, and T is the temperature (in kelvins).

cohesion. Compare with adhesion.
Attraction between like molecules.

critical point. critical state.
State at which two phases of a substance first become indistinguishable. For example, at pressures higher than 217.6 atm andtemperatures above 374°C, the meniscus between steam and liquid water will vanish; the two phases become indistinguishable and are referred to as a supercritical fluid.

critical molar volume. (Vc)
The molar volume at the critical point.

critical pressure. (Pc)
The pressure at the critical point.

critical temperature. (Tc)
The temperature at the critical point. A gas above the critical temperature will never condense into a liquid, no matter how much pressure is applied. Most substances have a critical temperature that is about 1.5 to 1.7 times the standard boiling point, in kelvin.

dipole-dipole interaction. dipole-dipole force.
Electrostatic attraction between oppositely charged poles of two or more dipoles.

dynamic equilibrium. equilibrium. Compare with position of equilibrium.
Dynamic equilibrium is established when two opposing processes are occuring at precisely the same rate, so that there is no apparent change in the system over long periods of time.

enthalpy of fusion. (Hfus) heat of fusion; molar heat of fusion; molar enthalpy of fusion.
The change in enthalpy when one mole of solid melts to form one mole of liquid. Enthalpies of fusion are always positive because melting involves overcoming some of the intermolecular attractions in the solid.

enthalpy of vaporization. (Hvap) heat of vaporization.
The change in enthalpy when one mole of liquid evaporates to form one mole of gas. Enthalpies of vaporization are always positive because vaporization involves overcoming most of the intermolecular attractions in the liquid.

flash point. Compare with auto-ignition temperature.
The temperature when vapor pressure of a substance becomes high enough to allow the air/vapor layer over the substance to be ignited. Ether and acetone have flash points below room temperature, which makes them very dangerous.

immiscible. immiscibility. Compare with miscible and partial miscibility.
Two liquids are considered "immiscible" or unmixable if shaking equal volumes of the liquids together results in a meniscus visible between two layers of liquid. If the liquids are completely immiscible, the volumes of the liquid layers are the same as the volumes of liquids orginally added to the mixture.

intermolecular force.
An attraction or repulsion between molecules. Intermolecular forces are much weaker than chemical bonds. Hydrogen bonds, dipole-dipole interactions, and London forces are examples of intermolecular forces.

liquid.
A state of matter that has a high density and is incompressible compared to a gas. Liquids take the shape of their container but do not expand to fill the container as gases do. Liquids diffuse much more slowly than gases.

London force. dispersion force.
An intermolecular attractive force that arises from a cooperative oscillation of electron clouds on a collection of molecules at close range.

meniscus. meniscuses; menisci.
A phase boundary that is curved because of surface tension.

miscible. miscibility; liquid miscibility. Compare with immiscible and partial miscibility.
Two liquids are considered "miscible" or mixable if shaking them together results in a single liquid phase, with no meniscus visible between layers of liquid.

Newtonian fluid. Compare with non-Newtonian fluid.
A fluid whose viscosity doesn't depend on gradients in flow speed. Gases and low-molecular weight liquids are usually Newtonian fluids.

non-Newtonian fluid. Compare with Newtonian fluid.
A fluid whose viscosity changes when the gradient in flow speed changes. Colloidal suspensions and polymer solutions like ketchup and starch/water paste are non-Newtonian fluids.

nucleation.
The process of providing sites for 1) new bubbles to form in a liquid that is boiling or supersaturated with gas; 2) new droplets to condense from a supersaturated vapor, or 3) new crystals to form in a supersaturated solution. Nucleation sites can be scratches in a surface, dust particles, seed crystals, and so on.

partial miscibility. partially miscible. Compare with miscible and immiscible.
Two liquids are considered partially miscible if shaking equal volumes of the liquids together results in a meniscus visible between two layers of liquid, but the volumes of the layers are not identical to the volumes of the liquids originally added.

phase. in phase; out of phase; wave phase.
1.A phase is a part of a sample of matter that is in contact with other parts but is separate from them. Properties within a phase are homogeneous (uniform). For example, oil and vinegar salad dressing contains two phases: an oil-rich liquid, and a vinegar-rich liquid. Shaking the bottle breaks the phases up into tiny droplets, but there are still two distinct phases. 2. In wave motion, phase is the fraction of a complete cycle that has passed a fixed point since the current cycle began. The phase is often expressed as an angle, since a full cycle is 360^deg; (2 ). Two waves are "in phase" if the peaks of one wave align with the peaks of the other; they are "out of phase" if the peaks of one wave align with the troughs of the other.

phase diagram. phase map.
A map that shows which phases of a sample are most stable for a given set of conditions. Phases are depicted as regions on the map; the borderlines between regions correspond to conditions where the phases can coexist in equilibrium.

poise. (P)
A cgs unit of resistance to fluid flow (viscosity). If a force of 1 dyne is needed to force two fluid layers with 1 cm2 area that are 1 cm apart past each other at a speed of 1 cm/s, the liquid has a viscosity of 1 poise.

specific gravity. specific gravities. Compare with density.
The mass of a unit volume of a substance relative to the mass of a unit volume of water. Temperature must be specified when reporting specific gravities, since the density of the substance and of water change with temperature. Specific gravities are often reported relative to water at 4°C; at that temperature, water has a density of 1.00000 g/mL and the specific gravity of a substance is equal to its density in g/mL.

supercritical fluid.
A fluid state that occurs when the pressure and temperature exceed the substance's critical pressure and critical temperature. Supercritical fluids fill their containers like gases but dissolve substances like liquids, which makes them very useful as solvents. Their density and other properties are intermediate between gases and liquids.

supercooling. supercooled; supercool.
Liquids at temperatures below their normal freezing points are said to be "supercooled".

surface tension.
The work required to expand the surface of a liquid by unit area.

surfactant.
A material that spreads along a surface, changing the properties of the surface. For example, soap spreads over a water surface and lowers its surface tension.

thixotropic fluid. thixotropy.
A liquid that becomes less viscous when stirred. Paint and printing inks are thixotropic fluids; they are formulated so that they flow more freely when brushed or rolled.

van der Waals force.
A force acting between nonbonded atoms or molecules. Includes dipole-dipole, dipole-induced dipole, and London forces.

van der Waals radius. van der Waals radii.
One half the distance between two nonbonded atoms, when attractive and repulsive forces between the atoms are balanced.

viscosity. () coefficient of viscosity.
The resistance a liquid exhibits to flow. Experimentally, the frictional force between two liquid layers moving past each other is proportional to area of the layers and the difference in flow speed between them. The constant of proportionality is called "viscosity" or "coefficient of viscosity", and is given the symbol . The time required for a liquid to drain out of a capillary tube is directly proportional to its viscosity. The poise is a non-SI unit frequently used to express viscosities.

wetting. wet.
Covering with a surface with thin film of liquid. Liquid beads up on a surface if it cannot wet it.

General Chemistry Online! Liquids