(a) The concentration of a solution can be calculated using the equation concentration of solution = mass of solid volume of solution A student dissolved 9.25g of ammonium chloride in water and made up the solution to a volume of 200 cm³ - Edexcel - GCSE Chemistry: Combined Science - Question 1 - 2021 - Paper 1

Dissolving ammonium chloride in water is an endothermic process, meaning it absorbs energy from its surroundings.

1. The ammonium chloride solid requires energy to break the ionic bonds between its molecules, which is greater than the energy released when water molecules surround the dissolved ions.
2. As a result, heat is absorbed from the environment, causing a drop in temperature of the solution, which is characteristic of endothermic reactions.

In the reaction profile, the activation energy is illustrated as the peak of the energy curve. Label the highest point of the curve to indicate this energy barrier that the reactants must overcome for the reaction to proceed.

In the investigation, the results indicate that solid ammonium chloride did not light up the lamp, demonstrating that it does not conduct electricity. In contrast, the ammonium chloride solution lit up the lamp brightly, indicating that the ions in the solution can carry an electrical current. This supports the idea that ionic compounds conduct electricity when dissolved in water due to the mobility of ions, while solid ionic compounds cannot conduct electricity because their ions are fixed in place within the crystal lattice.