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Encyclopedia > Colligative properties

In chemistry, colligative properties are factors that determine how the properties of a bulk liquid solution change depending on the concentration of the solute in the bulk solution. It is important to note that this is not relative concentrations, as one sees in azeotropes for example, but bulk concentrations where the solvent volume is held constant. Chemistry (derived from the Arabic word kimia, alchemy, where al is Arabic for the) is the science that deals with the properties of organic and inorganic substances and their interactions with other organic and inorganic substances. ... Dissolving table salt in water In chemistry, a solution is a homogeneous mixture of one or more substances (the solutes) dissolved in another substance (the solvent). ... In chemistry, concentration is the measure of how much of a given substance there is mixed with another substance. ... A substance is soluble in a fluid if it dissolves in the fluid. ...


The identity of the solute(s) has no importance in the consideration of colligative properties, and the degree to which their manifestation can be observed and measured is affected only by the number of solute particles in the solution.


The four colligative properties are:

  • Vapor pressure: The change in vapor pressure where the solute is less volatile than the solvent is regulated by Raoult's law, which states that the pressure is equal to the mole fraction of the solvent times the vapor pressure of pure solvent: P=Xsolvent*P°. This holds truest for ideal solutions.
  • Freezing point depression: The presence of a solute decreases the freezing point as compared to a pure solvent. The exact change (ΔT) can be calculated as van't Hoff factor (i) of the solute multiplied by its molality (m) multiplied by the freezing point depression constant of the solvent (Kf): ΔT=iKfm. Alternatively, it can be calculated as the total molality of all solutes in solution times the depression constant: ΔT=Kf∑m.
  • Boiling point elevation: Because of the lowered vapor pressure, the boiling point of a solution is elevated as compared to the pure solvent. The change in boiling point (ΔT) can be calculated in the same way as the change in freezing point, except a different constant is used: the van't Hoff factor (i) of the solute multiplied by its molality multiplied by the boiling point elevation constant of the solvent (Kb): ΔT=iKbm. ΔT can also be calculated using the sum of all molalities: ΔT=Kf∑m.
  • Osmotic pressure: The presence of solute can cause pressure to be exerted across a permeable membrane according to an equation quite similar to the ideal gas law: π is the osmotic pressure, n is the number of moles of solute, R is the ideal gas constant, T is the absolute temperature in kelvins, and V is the volume: π = (nRTi) / V


In (1), the two columns of pure solvent (blue) under the same pressure. When solute (green) is added to the right column (2), osmotic pressure is exerted and solvent flows through the permeable membrane (red) to the right side. The vapor pressure is the pressure (if the vapor is mixed with other gases, the partial pressure) of a vapor (this vapour being formed from molecules/atoms escaping from a liquid/solid). ... The vapor pressure is the pressure (if the vapor is mixed with other gases, the partial pressure) of a vapor (this vapour being formed from molecules/atoms escaping from a liquid/solid). ... In chemistry, Raoults law states that the vapor pressure of mixed liquids is dependent on the vapor pressures of the individual liquids and the molar vulgar fraction of each present in solution. ... In chemistry, an ideal solution is a solution where the enthalpy of solution is zero. ... Freezing-point depression is the difference between the freezing points of a pure solvent and a solution of a nonelectrolyte in that solvent. ... In physical chemistry, the van t Hoff factor i is the number of moles of solute actually in a solution in water, per mole of solid solute added. ... This page refers to concentration in the chemical sense. ... Boiling-point elevation is a colligative property that states that a solution will have a higher boiling point than that of a pure solvent. ... In physical chemistry, the van t Hoff factor i is the number of moles of solute actually in a solution in water, per mole of solid solute added. ... This page refers to concentration in the chemical sense. ... Osmotic pressure or turgor (also called turgor pressure) is the pressure produced by a solution in a space that is enclosed by a differentially permeable membrane. ... The ideal gas law or equation is the equation of state of an ideal gas. ... Molar gas constant (also known as universal gas constant, usually denoted by symbol R) is the constant occurring in the universal gas equation, i. ... Absolute zero is the lowest temperature that can be obtained in any macroscopic system. ... The kelvin (symbol: K) is the SI unit of temperature, and is one of the seven SI base units. ... Simple diagram of osmotic pressure. ...


  Results from FactBites:
 
SparkNotes: Colligative Properties of Solutions: Colligative Properties (1802 words)
Colligative properties depend only on the number of dissolved particles in solution and not on their identity.
Non-colligative properties depend on the identity of the dissolved species and the solvent.
Note that we did not need to identify the nature of the solvent or the solute (except for its lack of volatility) to derive that the vapor pressure should be lower for a solution relative to the pure solvent.
  More results at FactBites »

 
 

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