Physical properties of alcohols
As with most compounds, the physical properties of the alcohols will depend very much on the intermolecular forces between molecules. Remember that there are three main forms of intermolecular forces:
- dipole-dipole forces
- hydrogen bonding
- dispersion forces.
Because alcohols contain the -OH functional group, hydrogen bonds are formed between molecules of alcohols, as shown by the following diagram.
Hydrogen bonds can also be formed between alcohol molecules and water molecules, as shown by the following diagram.
This ability to form hydrogen bonding results in alcohols being soluble in water, and also having boiling points higher than that of other organic molecules (with a similar molecular weight) that do not exhibit hydrogen bonding.
Alcohol in wine
The solubility of alcohol in water means that it can be used in alcoholic drinks. In order to measure the amount of alcohol in a sample of wine, the alcohol is separated by distillation. Watch the video of this process and answer the questions that follow.
Text alternative to the wine distillation video
A scientist demonstrates the process of determination of the alcohol content (Ethanol (CH3CH2OH)) of a sample of wine. It is a distillation process. The main parts of the process are as follows.
- Measure out 250 mL of wine and mix with water.
- Boil the wine to evaporate the alcohol which boils off at 80 degrees. (The alcohol will boil off first because water boils at 100 degrees.)
- The alcohol evaporates as a gas, is cooled and condensed to collect the alcohol.
- Place alcohol in measuring cylinder, and make up to 250 mL with distilled water.
- Use a hydrometer to measure the density of the water containing the distilled alcohol.
The hydrometer measures the density of the alcohol/water mix which indicates the percentage of alcohol in the wine, which in this case is 13.0%.
Click here to open the video in a new window
The answers can be accessed here 
.
Alcohol and other solvents
If you look at the structure of an alcohol, for example pentan-1-ol, there is also a part of the molecule which is non-polar.
This allows the alcohol to also dissolve (and be dissolved in) other non-polar solvents, such as petrol and oil. This is as a result of the formation of dispersion forces between the non-polar part of the alcohol molecule and the other molecule.
Click here for a diagram showing dispersion forces between alcohol and hydrocarbon molecules. 
Now go back to the examples of the primary, secondary and tertiary alcohols, either in the videos or on your summary sheet and answer the following questions.
1 |
Solubility in waterWhich of the three types of alcohol are generally likely to be least soluble in water? |
2 |
Solubility in petrol (a non-polar solvent)Which of the following alcohols is likely to be the most soluble in petrol? |
Alcohols as cleaning agents
Because alcohols are able to dissolve non-polar substances such as grease and oil-based paints, alcohol-based cleaning agents are used in a variety of situations.
Methylated spirits is an example. This is made up mainly of ethanol, but has up to 10% methanol added, along with a variety of other chemicals such as propanone, methylpropan-2-ol and butanone. In some countries a colouring agent is also added to the methylated spirits.
- Why do you think that other compounds are added to ethanol before it is sold as methylated spirits?
- Turpentine is another cleaning agent that contains mainly hydrocarbons. Suggest why methylated spirits and turpentine can be used in different situations.
