CGSS 2023 Chemistry Prelim Paper 1 Answers

2023 Chemistry Prelims Paper 1 Answers 1 A 11 C 21 B 31 C 2 B 12 D 22 A 32 C 3 D 13 D 23 C 33 B 4 C 14 A 24 A 34 C 5 C 15 D 25 A 35 A 6 A 16 D 26 D 36 B 7 A 17 B 27 B 37 B 8 C 18 A 28 B 38 C 9 C 19 B 29 A 39 D 10 C 20 A 30 D 40 B

1 Answer: A X consists of two substances R and Y. Therefore it is a mixture. Since Y cools at a fixed temperature, it is a pure substance. 2 Answer: B Bromine and molecules in the air are in constant random motion. 3 Answer: D When silver nitrate is added to barium iodide, a yellow precipitate of silver iodide is formed. The filtrate is aqueous barium nitrate. When dilute sulfuric acid is added to the filtrate, a white precipitate of barium sulfate is formed. The filtrate conta ins nitric acid, which is made up of hydrogen ions and nitrate ions. 4 Answer: C Helium, having a proton number of 2, has an electronic configuration of 2, and has 2 electrons in the outer shell. Lithium, having a proton number of 3, has an electronic c onfiguration of 2,1, has 1 electron in the outer shell. Neon, having a proton number of 10, has an electronic configuration of 2,8, has 8 electrons in the outer shell. 5 Answer: C since X forms cation of X2+, the ionic compound formed with fluorine is XF2. 6 Answer: A Ammonium chloride consists of ammonium ion and chloride ion – an ionic bond exists between them. Ammonium ion has covalent bonds between nitrogen and hydrogen atom 7 Answer: A The intermolecular forces of attraction between covalent molecules are overcome when the covalent liquid boils. 8 Answer: C No. of moles of chlorine gas = 71 71 = 1 mol Since volume ratio = mole ratio for gases, No. of moles of phosphorus vapour = 1 mol Molar mass of phosphorus molecule = mass no. of moles = 124 1 = 124 g/mol No. of moles of phosphorus atoms in a molecule of phosphorus = mass atomic mass = 124 31 = 4 mol Therefore, the formula is P4.

9 Answer: C Molar mass of metal chloride = 74.5 Mass of metal = 74.5 – 35.5 = 39 Therefore, the metal is in group I and the formula of the chloride formed is YCl. 10 Answer: C H+(aq) + OH−(aq) → H2O(l) Since 0.1 mol of H+ and 0.1 mol of OH− are used, both reactants are limiting. 0.1 mol of water would be formed. The energy given out would be 0.1 x 54 = 5.40 kJ 11 Answer: C Relative atomic mass = 1 5 × 10 + 4 5 × 11 = 10.8 12 Answer: D Ammonium salt reacts with alkali to produce ammonia, which turns damp red litmus paper blue. Based on the given information, ammonium nitrate, NH4NO3, is present in the fertiliser. The empirical formula is N2H4O3. 13 Answer: D The spoon should be the negative electrode so that copper(II) ions can be reduced to plate the spoon with copper. This is a electrolytic cell. 14 Answer: A Chemical equation for the electrolysis of concentrated sodium chloride 2H+(aq) + 2Cl− (aq) → H2(g) + Cl2(g) No. of moles of sodium chloride = 58500 23+35.5 = 1000 mol No. of moles of Cl− discharged = 1000 mol No. of moles of H+ ions discharged = 1000 mol 2Cl− (aq) → Cl2(g) + 2e− Mass of Cl2 formed = 1000 2 × (35.5 × 2) = 35500 g = 35.5 kg 2H+(aq) + 2e−→ H2(g) Mass of H2 formed = 1000 2 × 2 = 1000 g = 1 kg 15 Answer: D Option A: New products, elemental bromine and silver are formed. Option B: Reddish-brown bromine gas is formed at the anode. Option C: Silvery molten metal is formed at the negative terminal. Option D: The volume of molten lead( II) bromide decreases over time as reddish - brown gas escapes from the set up.

16 Answer: D The enthalpy change (∆H) of the backward reaction is +23.0 kJ/mol. Hence, the activation energy of the backward reaction is the sum of the activation energy and the enthalpy change of the backward reaction. Ea of the backward reaction = 23 + 5.4 = +28.4 kJ/mol 17 Answer: B In an exothermic reaction, the energy given out when bonds are formed is greater than the energy absorbed to break the bonds. 18 Answer: A Option A: Energy change when one mole of potassium chloride dissolves in water = + 720 4 39+35.5 = +13.4 kJ/mol Option B: Powdered potassium chloride will dissolve in a faster rate but the energy change will be the same. Option C: Since the energy change is positive , it is an endothermic reaction. The energy level of the products is higher than the reactants. Option D: Since it is an endothermic reaction, the temperature of the reaction mixture after the reaction will be lower than water before the experiment. The temperature change, Tfinal – Tinitial, will be negative. 19 Answer: B Statement 1: the reaction time remained constant at 10 s when 0.92 mol/dm 3 to 1 mol/dm3 of sulfuric acid was used. Hence, when 1.5 mol/dm 3 of sulfuric acid was used, the approximate reaction time is also 10 s. Statement 2: the rate of reaction i s inversely proportional to reaction time. Since the reaction time decreases with increasing concentration, the rate of reaction increases with increasing concentration. Statement 3: s ince sulfuric acid is in excess, and the same mass of magnesium is used in each experiment, the same volume of hydrogen gas would be produced in each experiment.

20 Answer: A In step 1: Bromine oxidised sulfur in SO2 from an oxidation state of +4 in SO2 to +6 in H2SO4. Sulfur dioxide reduced bromine in Br2 from an oxidation state of 0 in Br2 to -1 in HBr. Hence Br2 is the oxidising agent and SO2 is the reducing agent. In step 2: Hydrogen bromide reduced chlorine in C l2 from an oxidation state of 0 in Cl2 to an oxidation state of −1 in 2HCl. Chlorine oxidised bromine in HBr from an oxidation state of −1 in HBr to 0 in Br2. Hence Cl2 is the oxidising agent and HBr is the reducing agent. 21 Answer: B Hydrogen sulfide acts as an acid and reacts with lead carbonate to form a salt, water, and carbon dioxide gas. This reaction is not a redox reaction. 22 Answer: A Sodium hydroxide, being an alkali, has the highest pH compared to the other solutions which are acids. Sulfuric acid, being a dibasic acid, has the highest concentration of H+ ions, so it has the lowest pH. Ethanoic acid, being a weak acid, has a lower c oncentration of H+ ions compared to hydrochloric acid. Hence, it has a higher pH than hydrochloric acid. 23 Answer: C Option A: Copper does not react with acid. Option B: Lead(II) sulfate is an insoluble salt. Option C: Magnesium chloride can be prepared by reacting magnesium with hydrochloric acid or magnesium carbonate and hydrochloric acid. Option D: Sodium carbonate is soluble in water. 24 Answer: A Gas Q is an element that can be obtained by the cracking of crude oil. 25 Answer: A Option A: The reactivity of halogens decreases down the group. It is less able to be reduced down the group. Thus, the ability as an oxidizing agent decreases down the group. Option B, C, D: The boiling points, colour intensity and density increases down the group.

26 Answer: D T is magnesium, U is aluminium, V is silicon, W is phosphorus. Melting point - V > W: Silicon, having a giant molecular structure, has a higher melting point than W, which has a simple molecular structure. Electrical conductivity – T > W: Magnesium, having a giant metallic structure, is able to conduct electricity unlike W, phosphorus, which has a simple molecular structure. Forms basic oxides – T: Magnesium reacts with oxygen to form magnesium oxide, which is basic as it reacts with acids. 27 Answer: B Iron is magnetic – it can be obtained through Step 1 Aluminium is lighter than chromium and copper. Transition metals such as chromium and copper has high density. Thus aluminium can be obtained through Step 2. 28 Answer: B Option 1: electrolysis of molten metal oxide enables metal X to be obtained at the cathode. Option 2: metal X is below carbon in the reactivity series; it can be obtained by reduction of metal oxide with carbon