How would you differentiate between salt and sugar without tasting them?

Though salt and sugar look similar and you may have accidentally substituted one in for another, there are ways to differentiate them during cooking or making tea. The most obvious difference appears to be their taste we could differentiate them using chemical characteristics. I think it is safe to assume that ‘salt’ refers to table salt or NaCl whereas sugar is table sugar which is usually ‘sucrose or C11H22O11 (which you may or may not know)’. Sucrose is an organic compound formed when fructose and glucose bond using a glycosidic linkage - a form of covalent bond. In contrast, NaCl is when there is an ionic bond between Na+ and Cl-. The differences in bonding provide an insight into how to answer the above question.

Electrical conduction in aqueous solutions

The first difference is that ionic compounds including salt can conduct electricity in an aqueous state because the mobile ions can carry a charge. Therefore, the first way to differentiate them is to see whether one substance has a voltage when an anode and cathode connected to a power source are placed inside the aqueous solution containing the substance. If a voltage is produced, the current is being conducted and the unknown substance is salt.

Comparing melting points

Additionally, most covalent substances such as carbon dioxide have low melting points. This is not due to their covalent structure but rather the weak London forces between the molecules that break during melting and require little energy to overcome. However, we know that this is not always the case since diamond is a covalent structure yet has a higher melting point than strong ionic compounds. One may think that since sugar is solid at room temperature, like diamond, it also melts at higher temperatures than salts. However, this is not the case because whilst diamond is linked via covalent bonds throughout its giant structure, sucrose molecules are bonded to one another with hydrogen bonds which are weaker than ionic bonds. On the other hand, since salt has a giant ionic lattice structure the ions are bonded to each other with ionic bonds. Therefore, less energy is needed to overcome the intermolecular forces in sucrose or sugar than salt hence when melted in a controlled environment, the substances which melt earlier - assuming the salt and sugar are pure - is the sugar.

Dissolving in alcohol

Moreover, as an ionic compound, salt is polar which means that whilst it can dissolve easily in other polar substances such as water, it dissolves less well in alcohol. Alcohol is more polar than hydrocarbons such as alkenes and alkanes due to the presence of the hydroxyl group however since the hydroxyl group is usually the only functional group contributing to polarity, unlike water and salt which has OH- and H+ or Na+ and Cl- respectively hence is less polar. Hence it is harder for alcohol to break up larger salt crystals into smaller ones. Therefore, the reduced polarity in alcohol means salt dissolves less readily within alcohol. Sucrose on the other hand is less polar than salt and has hydrogen bonds between the molecules which alcohol can more easily break meaning it more readily dissolves in alcohol. Therefore, when comparing the two unknown substances, the one which dissolves more readily in alcohol is sugar.

These are just three possible points and is not exhaustive. Don’t worry if you felt you did not know parts because a lot of the information involved in answers depends on what and in how much detail topics in detail at A-level. However, I think the main point to take away from this is that whilst salt is a giant ionic compound, sucrose is a covalently bonded molecule joined to other sucrose molecules with hydrogen bonds. From this, using your knowledge of the properties of the different types of attractions, you may be able to think of points to answer the questions.