Chemistry of water-ethanol mixtures

Ethanol is best known as alcohol since it contains hydroxyl group (-OH) attached to alkyl carbon. Ethyl alcohol is a straight chain alcohol with molecular formula C₂H₅OH. It is mainly found in alcoholic beverages, as a pure organic solvent and as a alcohol fuel (power alcohol). Water (H₂O) and ethanol molecules mix or dissoves each other, since both the solvents are polar in nature. The Hydrogen atom of the Hydroxyl (-OH) group on ethanol and water molecules are polar. The hydrogen bond is formed between the hydrogen of -OH group of ethanol and oxygen of water molecule. Water-ethanol(EtOH) mixtures at different temperatures exhibit a wide range of dielectric constant , viscosity, density and a high degree of hydrogen bonding effects. Interaction between water and ethanol can be studied at different quantities by the measurement of conductivity over that composition range. Appropriate measurements provide useful indications of solvent-solvent interaction and solvent structure.

Mixed solvents of water-ethanol can be prepared by adding known amount of ethanol to water in volume ratio or mass ratio. Differenet percentage compositions of ethanol in water, like 10%, 20 %, 30%, 40 % etc can be prepared by mixing known volume of ethanol in water (10 % ethanol = 10 cm³ ethanol + 90 cm³ water).

Volume fractions of ethanol in water is calculated as, Volume fraction = volume of ethanol / (volume of ethanol + water)

For example 10 cm³ of ethanol is mixed with 90 cm³ of water, volume fraction of ethanol = (10 / 100) = 0.1.

The various ways in which the concentration of a mixture of ethanol and water can be specified.

Mass %

This is calculated as

Mass % = (mass ethanol x 100) / (mass ethanol + mass water)

Note that the Mass % is not dependent on the temperature at which it is measured and no temperature has to be specified when giving a strength in Mass %.

Molar %

This is calculated as

Molar % = (moles ethanol x 100) / (moles ethanol + moles water)

Note that the Molar % is not dependent on the temperature at which it is measured and no temperature has to be specified when giving a strength in Molar %.

Volume %

The Volume % is bit strange in that the Volume % of ethanol and the Volume % of water add up to more than 100. The way the Volume % of ethanol is defined is that it is the parts of ethanol to which water has been added to bring the volume to a total of 100 parts.

For example, when working at 20°C, if you took 48.00 liters of pure ethanol you would find that you would have to add 55.61 liters of water to bring the total volume to 100.00 liters. The Volume % of ethanol in this case is 48.00% and the Volume % of water is 55.61%. The total of the separate components is 103.61 liters, but has contracted to 100.00 liters.

The degree of contraction is affected by the relative quantities ot ethanol and water, and also by the temperature. For example, when working at 50°F you would find that if you again started with 48.00 liters of ethanol you would need 55.82 liters of water to bring the total to 100.00 liters. This means that when a strength is specified in Volume % the temperature has to be specified as well.

Ethanol has a significantly higher coefficient of expansion than water. This means that as the temperature varies, the volume of the ethanol portion of the mixture changes faster than that of the water portion.