Frequent Questions Following are some questions that are repeatedly asked in P4 (5070) and P6 (0620) direcly or indirectly. Topic : Collection of gases Question 1: What is the purpose of ice or cold water? Answer: To cool down the gas so that it condenses and turns into a liquid. Question 2: Why is the gas collected in a jar that is upside down? Answer: Because the gas is less dense than air. Question 3: State the purpose of the mineral wool Answer: To hold/ absorb/ soak the water Question 4: Why is this experiment done in a fume cupboard? Answer: It releases harmful gases that are poisonous. It is toxic. Topic: Chromatography Question 1: Why is a pencil used in drawing the baseline in chromatography? Answer: Because the pencil will not dissolve to interfere with dissolved dyes. Question 2: What precautions must be taken while using ethanol as a solvent? Answer: Cover the apparatus with a lid, because ethanol is volatile. Question 3: How does the chromatogram show that a substance is pure? / Not Pure? Answer: There is only one spot / There is more than one spot. Question 4: Why is the baseline always drawn above the solvent level? Answer: The dyes will dissolve and get washed away with solvent. Topic: Electrolysis Question 1: Explain why inert electrodes are used. Answer: They are unreactive/ inactive Question 2: Explain why Magnesium/Zinc/Iron etc. is not a suitable material to be used as the electrode. Answer: It is a reactive metal. Question 3: Suggest suitable material to be used as an electrode in this experiment Answer: Graphite/ platinum Question 4: Suggest what can be done with solid NaCl to allow it to conduct electricity Answer: Heat to melt/ Dissolve to make a solution. Question 5: Suggest what can be done with solid NaCl to collect sodium at the cathode. Answer: Heat to melt. Topic: Rate of reaction Question 1: Name another item of apparatus that can be used to collect and measure the volume of gas. Answer: Gas Syringe Question 2: Why is the delivery tube removed from water? Answer: To prevent the suck-back. Question 3: The volume of acid was measured using a measuring cylinder. State one advantage of using a pipette instead of a measuring cylinder. Answer: Pipette is more accurate. Question 4: State one disadvantage of using a pipette instead of a measuring cylinder. Answer: Pipette is slower. Question 5: State Why the bung must be placed immediately after the reactants are added to the flask. Answer: To prevent gas from escaping. Question 6: How could the accuracy of the result be checked? Answer: Repeat and compare the results. Question 7: What measurements should be taken to follow the rate of reaction? Answer: Measure the volume of gas per unit time. / Measure change in mass per unit time. Question 8: Explain why the rate of reaction is zero after -------minutes. / Explain why no more volume of gas produces after ____ minutes. Answer: Limiting reagent has finished. (Student will have to find from 1st line of the question that clearly tells which reagent was used in excess, the other reagent must be limiting then) Question 9: Which result appears to be inaccurate and why? Answer: (Read anomalous point from graph) ………… appears to be inaccurate as it doesn’t fit the smooth curve Question 10: What is used to crush a substance? Answer: when you crush, you use a pestle and mortar. Question 11: Why should the solid be crushed? Answer: It increases surface area. More particles are exposed hence a faster rate. Topic: Identification/ Distinguishing Question 1: What conclusion can you make about solid H from the results of test 2? Answer: It is a hydrated salt. Question 2: How can you distinguish between alkanes and alkenes? Answer: Use bromine water, in which the alkene will decolorise it to colorless, but nothing, happens with an alkane, Question 3: How can you distinguish between chlorine and sodium chloride? Answer: Add litmus paper, it will bleach with chlorine, but nothing happens to it with sodium chloride. Question 4: How can you distinguish between copper sulfate and copper carbonate? Answer: Acidify with hydrochloric acid, and add barium chloride, there will be white ppt with sulfate, but no white ppt. with carbonate. OR Add hydrochloric acid, in which nothing happens with sulfate but a fizz or effervescence of carbon dioxide will occur with carbonate. Question 5: State a chemical test for water Answer: Test: Add water to anhydrous copper II sulfate Observation: Anhydrous copper II sulfate will turn from white to blue OR Test: Add water to anhydrous cobalt II chloride Observation: Anhydrous cobalt II chloride will turn from blue to pink Question 6: State a Physical test to check if the water is pure Answer: check its boiling point, if it is exactly 100oC, it is pure. Question 7: State a Physical test to check if the substance is pure Answer: check its boiling point. Topic: Acid Bases and Salts Question 1: How will you make pure dry crystals? Answer: 1) heat till the point of crystallization. 2) Leave to cool gradually. 3) Filter, dry and collect the crystals! Question 2: How can you detect the point of crystallization? Answer: Place a stirring rod in the solution and see the formation of the first crystals on it. Question 3: Give two observations when acid reacts with Mg/ Zinc Answer: i. Effervescence of a gas/ Bubbles of a gas ii. Metal dissolves Question 4: Give two observations to show that acid that reacts with metal is a stronger acid. Answer: i. The effervescence of gas produces faster/ Bubbles of gas produce faster ii. Metal dissolves faster Question 5: State three conditions that should be kept constant to ensure a fair test Answer: Temperature, Surface area of solid, Volume of liquid used, Concentration of liquid used, mass of sold used. Question 6: How could you show that the acid is strong/ weak? Answer: Use pH paper to check the pH, if pH is less than 3 it is a strong acid if it is above 3 it is a weak acid. Planning question: Here is a general outline or a checklist students must follow while writing the answer to this question 1. 2. 3. 4. 5. 6. 7. Apparatus that is required What to measure? How to measure? Factors to keep constant. When will you stop? Calculation of results. Conclusion Example: 1. Catalysts are substances that increase the rate of reaction but are unchanged at the end of the reaction. Aqueous Hydrogen peroxide decomposes slowly to form water and oxygen. H2O2 (aq) Copper II oxide is an insoluble solid. 2H2O(l) + O2(g) Plan an investigation to find out if copper II oxide is a catalyst for the decomposition of hydrogen peroxide. You must include how your results tell you that copper II oxide is a catalyst. You have access to copper II oxide, Hydrogen peroxide, and all normal laboratory apparatus. 1. Apparatus that is required Top pan balance, Conical flask, Bung, gas syringe, Spatula, Filter funnel, Filter paper, 2. What to measure? a. To check if speed is increased after adding copper II oxide, the Volume of the gas collected will be measured b. To check if it remains the same at the end of the reaction, the mass of copper oxide before and after the reaction will be measured 3. How to measure? a. Either draw a diagram b. or describe the setup Connect the flask to the gas syringe, place a bung and measure the volume of oxygen gas collected after in 30 seconds. 4. Factors to keep constant. The same volume of Hydrogen peroxide with and without a catalyst 5. Factors to change Two experiments will be performed Exp-1 with catalyst and Exp-2 without catalyst 6. When will you stop? 30 seconds. (Students can choose any duration depending on the type of reaction.) 7. Calculation of results. a. Volume of gas collected per 30 seconds for Experiment 1 and Experiment 2 b. Mass of copper oxide in Experiment 2 8. Conclusion a. Volume of Oxygen collected in Experiment 2 should be more b. Mass of copper II oxide should before and after the reaction should be same. Answer: Using a top pan balance weigh about 5g Copper II oxide, Record the mass. In a conical flask put 50cm3 of Hydrogen peroxide, connect it with a gas syringe and place the bung. Measure the volume of oxygen collected in 30 seconds. Record the volume. Repeat the same experiment using the same volume of Hydrogen peroxide, but add 5g copper II oxide and place the bung immediately. Record the volume of gas collected in 30 seconds. Disconnect the apparatus, and filter the mixture in the flask. Copper II oxide will be received as residue, dry, and weigh again. Compare the volume of oxygen collected with and without a catalyst. The volume collected with the catalyst should be more. The mass of copper II oxide measured after the reaction should be 5g. Tip: Drawing a labeled diagram of the apparatus will save students from writing a description and will get them easy marks.
