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Most Common GCSE Chemistry Questions

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In GCSE Chemistry, students frequently encounter questions that test foundational concepts. Understanding these questions is crucial for a strong grasp of the subject – this guide provides detailed explanations for some of the most common questions.

Question: Magnesium oxide is a compound formed from the metal magnesium and the non-metal oxygen. Describe what happens when a magnesium atom reacts with an oxygen atom. You should refer to electrons in your answer.

Answer: When a magnesium atom reacts with an oxygen atom, the magnesium atom loses two electrons to become a Mg^2+ ion, and the oxygen atom gains those two electrons to become an O^2- ion.

Explanation: Magnesium, a metal, tends to lose electrons to achieve a stable electron configuration, forming cations (Mg^2+). Oxygen, a non-metal, tends to gain electrons to fill its valence shell, forming anions (O^2-). This transfer of electrons from magnesium to oxygen results in the formation of ionic bonds, creating magnesium oxide (MgO), a compound where these ions are held together by electrostatic forces of attraction.

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Question: Sodium carbonate reacts with hydrochloric acid in an exothermic reaction. Plan a method to investigate the effect of changing the mass of sodium carbonate powder on the highest temperature reached by the reaction mixture.

Answer: To investigate this, measure equal volumes of hydrochloric acid in a polystyrene cup and add different masses of sodium carbonate to separate trials. Use a thermometer to measure the highest temperature reached in each reaction.

Explanation: Using a polystyrene cup as an insulator minimizes heat loss. Starting with the same volume and concentration of hydrochloric acid ensures that the only variable affecting the temperature change is the mass of sodium carbonate. This method allows for observing how the reaction’s heat changes with different amounts of sodium carbonate, indicating the relationship between reactant mass and the energy released during the reaction.

Question: A student produces pure crystals of zinc chloride by reacting zinc oxide with hydrochloric acid. The equation for the reaction is: ZnO(s) + 2 HCl(aq) → ZnCl2(aq) + H2O(l). The student adds zinc oxide to hydrochloric acid until the zinc oxide is in excess. Give one observation that the student could make to show that the zinc oxide is in excess.

Answer: The student would observe undissolved zinc oxide at the bottom of the reaction container, indicating it is in excess.

Explanation: When zinc oxide is added to hydrochloric acid, it reacts to form zinc chloride and water. Once all the hydrochloric acid has reacted and no more can react with additional zinc oxide, any further addition of zinc oxide will not dissolve. This excess of undissolved solid indicates that the reactant (zinc oxide) is in excess.

Question: The elements in Group 1 become more reactive going down the group. Rubidium is below potassium in Group 1. Predict one observation you would see that shows that rubidium is more reactive than potassium when both are added to water.

Answer: When added to water, rubidium would react more vigorously than potassium, potentially producing a more violent reaction or explosion and a faster rate of hydrogen gas release.

Explanation: The reactivity of Group 1 elements increases down the group due to the outer electron being further from the nucleus and more easily lost. Rubidium, being below potassium, has its valence electron further from the nucleus and less strongly attracted, making it more easily lost in reactions, such as with water. This results in rubidium reacting more vigorously, showcasing its higher reactivity compared to potassium.

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Question: During the electrolysis of molten sodium chloride, what products are formed at the cathode and anode?

Answer: At the cathode, sodium metal is formed, and at the anode, chlorine gas is produced.

Explanation: In the electrolysis of molten sodium chloride (NaCl), sodium ions (Na^+) move towards the cathode (negative electrode), where they gain electrons (are reduced) to form sodium metal (Na). Meanwhile, chloride ions (Cl^-) move towards the anode (positive electrode), where they lose electrons (are oxidized) to form chlorine gas (Cl2). This process separates the sodium chloride compound into its elemental components.

Explain why carbon dioxide is considered a greenhouse gas and its effect on Earth's climate.

Answer: Carbon dioxide is considered a greenhouse gas because it traps heat in Earth’s atmosphere, contributing to the greenhouse effect and global warming.

Explanation: Carbon dioxide (CO2) molecules absorb infrared radiation (heat) emitted from Earth’s surface and re-emit it in all directions, including back towards the surface. This trapped heat increases the global temperature, a phenomenon known as the greenhouse effect. An enhanced greenhouse effect, driven by increased levels of CO2 from human activities, leads to global warming and climate change, affecting weather patterns, sea levels, and biodiversity.

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