Hardest GCSE Chemistry Questions

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GCSE Chemistry challenges students with its mix of theoretical concepts and practical applications. From atomic structure to chemical reactions, the subject spans a wide array of topics. Certain questions, known for their difficulty, test the depth of understanding and application skills of students. These questions not only probe the fundamental principles of chemistry but also encourage critical thinking and problem-solving. In this article, we look at some of the hardest GCSE Chemistry questions, providing detailed explanations and worked answers to shed light on these challenging concepts.

Question: A student investigates the reaction rates of marble chips (calcium carbonate) with hydrochloric acid at different concentrations. Describe how the student could measure the rate of carbon dioxide production.

Answer: The student could measure the rate of carbon dioxide production by collecting the gas in a gas syringe and recording the volume of gas produced over time.

Explanation: To investigate the reaction rate, the student sets up a reaction flask containing hydrochloric acid and marble chips, connected to a gas syringe. As the acid reacts with the marble chips, carbon dioxide is produced and collected in the syringe. By measuring the amount of gas produced at regular intervals, the student can calculate the rate of reaction. The experiment can be repeated with hydrochloric acid at different concentrations to see how concentration affects the reaction rate.

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Explain the process of dynamic equilibrium in a closed system using the Haber process as an example.

Answer: Dynamic equilibrium in the Haber process occurs when the rate of the forward reaction, producing ammonia from nitrogen and hydrogen, equals the rate of the reverse reaction, where ammonia decomposes back into nitrogen and hydrogen, in a closed system.

Explanation: In the Haber process, nitrogen gas from the air reacts with hydrogen gas to form ammonia in a reversible reaction. At dynamic equilibrium, both the forward and reverse reactions occur at the same rate, so the concentrations of reactants and products remain constant. This state can be achieved under certain conditions of temperature, pressure, and the presence of a catalyst in a closed system, demonstrating how industrial processes optimize conditions to maximize product yield.

Predict the observation when lithium is added to water, and explain why lithium reacts in this manner.

Answer: When lithium is added to water, it reacts gently to form lithium hydroxide and hydrogen gas, often moving around the surface and fizzing.

Explanation: Lithium reacts with water to produce lithium hydroxide and hydrogen gas. This reaction occurs because lithium, as an alkali metal, readily loses its single outer electron to form a positive ion, reacting with water in a single displacement reaction. The reaction’s gentle nature compared to other alkali metals is due to lithium’s relatively higher ionization energy, requiring less energy to remove its outer electron.

Question: Plan an investigation to determine the Rf value for the dye in this food colouring using a beaker, a solvent, and chromatography paper.


  • – Draw a pencil line near the bottom of a piece of chromatography paper.
  • – Place a small spot of the food coloring on the line.
  • – Pour a small amount of solvent into the beaker, ensuring the level is below the pencil line when the paper is placed in the beaker.
  • – Hang the paper in the beaker so the solvent touches the paper but not the spot directly.
  • – Allow the solvent to travel up the paper, carrying the dye with it.
  • – Remove the paper when the solvent has nearly reached the top, and mark the solvent front.
  • – Measure the distance the solvent and the dye traveled from the baseline to calculate the Rf value:
  • Rf=

distance moved by solvent

distance moved by substance.


Explanation: This method allows the separation of different components in the dye based on their solubility and affinity for the chromatography paper. The Rf value, a ratio of distances moved by the substance and the solvent, helps identify specific compounds based on their characteristic movement through the paper.

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Question: Name the soluble salts produced when calcium phosphate reacts with nitric acid and phosphoric acid.


  • Nitric acid: Calcium nitrate
  • Phosphoric acid: Calcium hydrogen phosphate

Explanation: When calcium phosphate reacts with nitric acid, it forms calcium nitrate, a soluble salt, along with phosphoric acid as a byproduct. The reaction with phosphoric acid produces calcium hydrogen phosphate, another soluble salt, and further phosphoric acid, due to the acid’s ability to react with the calcium component of calcium phosphate.

Question: Evaluate the two methods for producing a large mass of ammonium sulfate.

Answer: The industrial process is more efficient for producing a large mass of ammonium sulfate due to continuous operation and the use of a warm column to evaporate water, allowing for the ongoing collection of dry crystals. The laboratory method is more labor-intensive and time-consuming, involving manual steps such as evaporation, cooling, filtration, and drying with filter paper. While the laboratory method might be suitable for small-scale production or educational purposes, its scalability is limited compared to the industrial process, which is designed for mass production with less manual intervention and potentially higher purity and yield of ammonium sulfate.

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