Four Bottles from the Chemistry Storeroom
A laboratory receives a 500 mL bottle labelled ethanol from a local supplier. The liquid is miscible with water in all proportions, gives brisk effervescence with sodium metal, and does not become turbid with Lucas reagent at room temperature even after 10 minutes. However, a few drops give an intense violet colour with neutral FeCl3 solution, and when 10 mL of the sample is diluted with 40 mL water a faint oily layer separates. The label says density 0.79 g mL⁻¹ and water content less than 1%. A second bottle in the storeroom contains diethyl ether. The teacher asks the student to decide whether the first bottle is pure ethanol, to identify the likely contaminant, and to propose a purification step that would be suitable before the ethanol is used as a solvent.
1. Interpret the tests and identify the likely contaminant. Explain why the sample is not diethyl ether and why it is not pure ethanol.
Read the worked answer
The sample is miscible with water, reacts with sodium to give hydrogen, and gives no immediate turbidity with Lucas reagent. These observations fit a primary alcohol such as ethanol. Diethyl ether would not react briskly with sodium and is not fully miscible with water in all proportions; it is also much less polar and would not behave like ethanol in the Lucas test. So the bottle is not ether.
Pure ethanol should not give a violet colour with neutral FeCl3. That colour is characteristic of phenols, which form coloured complexes with Fe³⁺. The oily layer produced on dilution also fits phenol, which is only moderately soluble in water and can separate when the ethanol content is reduced. The likely contaminant is therefore phenol, possibly together with a little water. The sample is ethanol containing phenol, not pure ethanol.
2. Why might simple distillation alone fail to remove the phenol completely? Propose a chemical purification step and explain how it works.
Read the worked answer
Phenol is steam-volatile and can be carried over with ethanol during simple distillation, especially if the mixture is acidic or if distillation is stopped late. Simple distillation therefore may not remove all the phenol, even though phenol has a much higher boiling point than ethanol.
A better step is to treat the sample with dilute sodium hydroxide solution. Phenol is weakly acidic and is converted into sodium phenoxide, an ionic, non-volatile salt:
C6H5OH + NaOH → C6H5ONa + H2O.
The mixture can then be distilled. Ethanol distils, while sodium phenoxide remains in the alkaline residue. The distillate is ethanol with much less phenol. If necessary, the ethanol can be dried and redistilled. This chemical conversion is more reliable than relying on a small boiling-point difference alone.
3. The label claims water content below 1%. How could water affect the sodium test and the density, and what simple test would check for water?
Read the worked answer
Water also reacts with sodium to produce hydrogen gas, so the brisk effervescence with sodium does not prove that the liquid is anhydrous ethanol. A small amount of water would not change the observation very much, because both ethanol and water react.
Density can help but is not decisive. Pure ethanol has density about 0.789 g mL⁻¹ at room temperature; water is denser, so adding water tends to raise the density slightly. A value of 0.79 g mL⁻¹ is consistent with ethanol containing little water, but density alone cannot quantify the water content.
A simple check is to add a little anhydrous copper(II) sulphate: it turns blue in the presence of water. A more precise method would be Karl Fischer titration, but the copper sulphate test is suitable for a quick practical check.