Why add alcohol to eau de Cologne

Eau de Cologne perfume is made from about 80-85 per cent water and 12-15 per cent ethanol. Volatile esters make up the remainder, and provide both the smell and colour. 

The vapour pressure of alcohol is higher than that of water, so adding alcohol to an aqueous perfume increases the pressure of the gases above the liquid. In this way, the activity a of the organic components imparting the smell will increase and thereby increase the perceived concentration of the esters. And increasing a(ester) 

The word perfume comes from the Latin per fumem, meaning through smoke.  

These esters are stable in the dark, but degrade in strong sunlight, which explains why so many perfumes are sold in bottles of darkened or frosted glass. 

Every day, electrochemists perform measurements that require a knowledge of the activity a. Measurements can be made in terms of straightforward concentrations if solutions are very dilute, but very dilute in this context implies c 10-4 mol dm-3, or less. Since most solutions are far more concentrated than millimoles per litre, from now on we will write all equations in terms of activities a instead of concentration c. 

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