Apparent molar property

The excess volume VE is the difference between the real volume of a mixture of chemical substances and the ideal volume which corresponds to the sum of the volumes of the components prior to mixing ( pure component volumes):

Is the volume of the mixture is greater than the real of the ideal, the excess volume is positive, negative in the opposite case. As the difference between real and ideal behavior of a mixture, the excess volume is an excess size.

Molar excess volume

Based on the amount of substance n of the mixture is called the molar excess volume:

The molar volume of the mixture is equal to the sum of the partial molar volumes of the components:

The partial molar volume of the substance A, the volume which contributes to the total volume of this material as a component of a mixture of several substances A and B. It is both of the other substance B as well as the mixing ratio dependent and not always identical to the molar volume occupied by the substance A as a pure substance:

With

• Molar mass M and
• Density.

Magnitude and examples

The volume effect of mixing of pure substances is relatively small. Usually, the difference is only about one to two percent.

• Mixtures of nonpolar and polar substances generally have a significantly positive excess volume, ie the volume of the mixture is greater than that of the ideal mixture ( volume dilatation ). Examples are: Di -n- propyl ether / heptane Methylcyclohexane and 2- pentanol (maximum = 0.50 at cm3/mol = 117.98 cm3/mol, 298 K = 25 ° C)
• Dichloromethane and 2-butanone (maximum = 0.06 at cm3/mol = 72.99 cm3/mol, 298 K = 25 ° C)

• Mixtures of small and large molecules polar component having a polar group, often have a negative excess volume, ie the volume is smaller than that of the ideal mixture (volume contraction). Examples are: Tetrahydrofuran / water N-methyl -2-oxazolidinone and water ( minimum = 0.54 cm3/mol, 298, K = 25 ° C)
• Pyridine and methanol ( minimal cm3/mol = -0.48 at cm3/mol = 57.53, K = 298 25 ° C)
• Carbon monoxide and methane (minimum = -0.35 cm3/mol at = 36.29 cm3/mol, 90 K = -183 ° C)