Question

In making candy or sugar syrups, sucrose is boiled in water with a little acid, such as lemon juice. Why does the product mixture taste sweeter than the starting sucrose solution?

Short answer

Answer: The product mixture tastes sweeter because the hydrolysis reaction breaks down sucrose molecules into glucose and fructose molecules, which have a higher relative sweetness compared to sucrose. The acid acts as a catalyst to speed up this process.

Step-by-step solution

Understand the chemical reaction taking place when boiling sucrose in water with a little acid

Sucrose (C12H22O11), commonly known as table sugar, undergoes hydrolysis when boiled in water with a little acid. Hydrolysis is a chemical reaction that breaks down a compound by reacting it with water. In our case, sucrose is broken down into glucose (C6H12O6) and fructose (C6H12O6). The acid, such as lemon juice, acts as a catalyst to speed up the reaction.

Compare the sweetness of sucrose, glucose, and fructose

The three sugars, sucrose, glucose, and fructose, have different sweetness levels. Fructose is the sweetest among the three, followed by sucrose and glucose. On a relative sweetness scale where sucrose has a value of 1, glucose is 0.75, and fructose is 1.7.

Analyze the taste change after the hydrolysis of sucrose

As the sucrose solution is boiled in water with a little acid, it breaks down into glucose and fructose. The mixture now has individual glucose and fructose molecules, which are sweeter than the original sucrose molecule. This is because fructose is almost twice as sweet as sucrose, and glucose is slightly sweeter than sucrose by default. As a result, the product mixture tastes sweeter than the starting sucrose solution. In conclusion, the product mixture tastes sweeter after boiling sucrose in water with a little acid due to the hydrolysis reaction that converts sucrose molecules into glucose and fructose molecules, which have a higher relative sweetness compared to sucrose.