Volume contraction

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Volume contraction with a mixture of tetrahydrofuran / water;
x is the mole fraction

The term volume contraction is used in chemistry in particular for a reduction in volume when two substances are mixed.

A volume contraction (decrease in volume) occurs when, when mixing several liquids to form a solution, the total volume is smaller than the sum of the volumes of the individual components. The volume difference is called the excess volume  and is negative when the volume contracts. In many solutions it is very low or practically nonexistent.

However, this effect is noticeable with a mixture of alcohol and water . For example, if you mix 48 ml of water with 52 ml of ethanol, the total volume is 96.3 instead of 100 ml. The reduction in volume depends non-linearly on the mixing ratio (see figure), which is why the alcohol content of the mixture cannot be measured by measuring the volume, but can only be determined from density or boiling point .

The cause of the volume contraction is the formation of additional binding forces (formation of hydrogen bonds between the molecules ), which means that they take up less space. In oceanography there is a similar, but smaller effect because of the salt content in seawater .

Influence on salary information

If the content of a component in a solution is given as a volume concentration , the volume contraction is taken into account.

On the other hand, if the volume contraction is to be disregarded, the content is preferably given as a volume fraction .

The article alcohol content demonstrates the volume contraction through sample values ​​of volume concentrations and volume fractions ( conclusion ).

Volume dilation

There are also liquids which, when mixed, lead to volume dilation , i.e. an increase in volume. In this case the excess volume is positive. Examples are mixtures of methylcyclohexane and 2-pentanol or of dichloromethane and 2-butanone .

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