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CSCA Chemistry December 2025 Past Paper

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CSCA Chemistry December 2025 Past Paper · #1· Topic: 物理变化与化学变化
Which of the following changes is a chemical change? ( )
  • Correct answer: Iron rusting
  • Activated carbon adsorbing pigment
  • Concentrated hydrochloric acid volatilizing
  • Ice melting into water
Show solution(Answer: A)
Answer: A The defining feature of a chemical change is that a new substance is formed. Rusting turns iron into new substances (iron oxides/hydroxides). Adsorption by activated carbon, volatilization of concentrated hydrochloric acid and melting of ice only change the physical state or dispersion; no new substance is formed.
CSCA Chemistry December 2025 Past Paper · #2· Topic: 化学式与化合价
Which of the following compound names is correct? ( )
  • CO - carbon dioxide
  • Correct answer: FeCO₃ - iron(II) carbonate
  • Na₂CO₃ - baking soda
  • CuCl - copper chloride
Show solution(Answer: B)
Answer: B FeCO₃ is made of Fe²⁺ and CO₃²⁻ ions, so it is iron(II) carbonate. The other names are wrong: CO is carbon monoxide, not carbon dioxide; Na₂CO₃ is soda ash, not baking soda (baking soda is NaHCO₃); CuCl is copper(I) chloride, not copper chloride.
CSCA Chemistry December 2025 Past Paper · #4· Topic: 元素符号
Which chemical notation is written correctly? ( )
  • 2 hydrogen molecules: 2H
  • 4 water molecules: 4H₂O₂
  • 5 sodium ions: 5Na
  • Correct answer: 3 oxygen atoms: 3O
Show solution(Answer: D)
Answer: D Atoms, molecules and ions are written differently: - Atoms: the element symbol, so 3 oxygen atoms is 3O. - Molecules: the formula with subscripts, so 2 hydrogen molecules is 2H₂ and 4 water molecules is 4H₂O (H₂O₂ is hydrogen peroxide). - Ions: the charge must be shown, so 5 sodium ions is 5Na⁺. Only "3 oxygen atoms: 3O" is correct.
CSCA Chemistry December 2025 Past Paper · #5· Topic: 化学物质的分类方法
Which of the following substances is a pure substance? ( )
  • Hydrochloric acid solution
  • A mixture of ice and water
  • Air
  • Saturated sodium chloride solution
Show solution(Answer: B)
Answer: B A pure substance contains only one substance. A mixture of ice and water has a solid and a liquid phase, but both are H₂O, so it is a pure substance. Hydrochloric acid, air and saturated salt solution all contain several components and are mixtures.
CSCA Chemistry December 2025 Past Paper · #6· Topic: 同素异形体
Which pair of substances are allotropes of each other? ( )
  • Correct answer: White phosphorus and red phosphorus
  • Ice and dry ice
  • Water and hydrogen peroxide
  • Oxygen gas and liquid oxygen
Show solution(Answer: A)
Answer: A Allotropes are different structural forms of the same element. White phosphorus and red phosphorus are both made only of P with different structures. Ice and dry ice are different compounds (H₂O and CO₂), water and hydrogen peroxide are different compounds, and oxygen gas and liquid oxygen are the same substance in different states.
CSCA Chemistry December 2025 Past Paper · #7· Topic: 化学式与化合价
In NH₄NO₃, the oxidation numbers of N are ( )
  • -3, +3
  • Correct answer: -3, +5
  • +1, +5
  • +1, -1
Show solution(Answer: B)
Answer: B NH₄NO₃ contains two different N atoms: - In NH₄⁺, H is +1, so 4 H give +4; the ion's charge is +1: N + 4 = +1, N = −3. - In NO₃⁻, O is −2, so 3 O give −6; the ion's charge is −1: N − 6 = −1, N = +5. The oxidation numbers are −3 and +5.
CSCA Chemistry December 2025 Past Paper · #8· Topic: 溶液的含义和组成
Which of the following is a solution? ( )
  • Distilled water
  • Correct answer: Normal saline (physiological saline)
  • Peanut oil
  • Cement slurry
Show solution(Answer: B)
Answer: B A solution is a homogeneous, stable mixture. Normal saline is an aqueous solution of NaCl. Distilled water is a pure substance, peanut oil is not a solution of a solute in a solvent, and cement slurry is a suspension, not homogeneous.
CSCA Chemistry December 2025 Past Paper · #9· Topic: 焰色试验
In the flame test with sodium chloride, the flame color is ( )
  • Pale white
  • Correct answer: Yellow
  • Light blue
  • Green
Show solution(Answer: B)
Answer: B Na⁺ gives a bright yellow flame in the flame test (a standard fact to memorize).
CSCA Chemistry December 2025 Past Paper · #10· Topic: 非电解质
Which of the following is a non-electrolyte? ( )
  • Sodium acetate
  • Correct answer: Sucrose
  • Sodium hydroxide
  • Sulfuric acid
Show solution(Answer: B)
Answer: B A non-electrolyte does not ionize in water and does not conduct electricity. Sucrose dissolves as molecules and produces no ions. Sodium acetate, sodium hydroxide and sulfuric acid are all electrolytes.
CSCA Chemistry December 2025 Past Paper · #11· Topic: 电解质物质类别判断
Among the following substances, which is an electrolyte? ( )
  • Correct answer: NaCl
  • Metallic aluminum
  • Sucrose
  • Carbon dioxide
Show solution(Answer: A)
Answer: A An electrolyte is a compound that ionizes and conducts when dissolved in water or molten: NaCl → Na⁺ + Cl⁻. Aluminum conducts but is an element, not a compound; sucrose does not ionize; CO₂ is molecular (the small amount of H₂CO₃ it forms in water is what ionizes, so CO₂ itself is not counted as an electrolyte).
CSCA Chemistry December 2025 Past Paper · #12· Topic: 氧化还原反应定义、本质及特征
Which of the following reactions is a redox reaction? ( )
  • CaCO₃ △ CaO + CO₂↑
  • HCl + NaOH = NaCl + H₂O
  • Correct answer: Fe + CuSO₄ = FeSO₄ + Cu
  • CO₂ + H₂O = H₂CO₃
Show solution(Answer: C)
Answer: C In Fe + CuSO₄ = FeSO₄ + Cu, Fe(0) → Fe²⁺ loses electrons and is oxidized, and Cu²⁺ → Cu(0) gains electrons and is reduced. A reaction in which oxidation states change is a redox reaction. In the other three reactions no element changes its oxidation state.
CSCA Chemistry December 2025 Past Paper · #13· Topic: 常见氧化剂与还原剂
Which of the following is commonly used as an oxidizing agent? ( )
  • Iron
  • Carbon monoxide
  • Correct answer: Potassium permanganate
  • Hydrogen gas
Show solution(Answer: C)
Answer: C In KMnO₄, Mn is in the +7 oxidation state and is very easily reduced, so potassium permanganate is a common strong oxidizing agent. Iron, carbon monoxide and hydrogen are reducing agents.
CSCA Chemistry December 2025 Past Paper · #14· Topic: 同素异形体
Which of the following pairs are allotropes of each other? ( )
  • Correct answer: diamond and graphite
  • CH₄ and C₄H₈
  • 12C and 14C
  • CO and CO₂
Show solution(Answer: A)
Answer: A Diamond and graphite are the classic allotropes: both are pure carbon with different structures. ¹²C and ¹⁴C are isotopes, not allotropes; CH₄ and C₄H₈, and CO and CO₂, are different compounds.
CSCA Chemistry December 2025 Past Paper · #15· Topic: 盐溶液的酸碱性及原因
Which aqueous solution of the following compounds is acidic? ( )
  • NaCl
  • NaOH
  • Na₂CO₃
  • Correct answer: NH₄Cl
Show solution(Answer: D)
Answer: D In water, NH₄⁺ (the conjugate acid of the weak base NH₃) hydrolyzes and releases H⁺, so NH₄Cl solution is acidic. NaCl solution is neutral; NaOH and Na₂CO₃ solutions are basic.
CSCA Chemistry December 2025 Past Paper · #16· Topic: 溶液的酸碱性与 pH
When phenolphthalein is added to a solution, it turns red. The solution is ( )
  • Correct answer: Basic
  • Neutral
  • Acidic
  • Cannot be determined
Show solution(Answer: A)
Answer: A Phenolphthalein is colorless in acidic and neutral solutions and turns red in basic solutions. A red color means the solution is basic.
CSCA Chemistry December 2025 Past Paper · #18· Topic: 物质的量浓度的含义
From 1 L of 1 mol·L-1 NaOH solution, 50 mL is taken out. The molar concentration of the 50 mL NaOH solution is ( )
  • 0.02 mol·L-1
  • Correct answer: 1 mol·L-1
  • 0.2 mol·L-1
  • 2 mol·L-1
Show solution(Answer: B)
Answer: B The solution is uniform, so removing part of it removes solute and solvent in the same proportion and the concentration does not change. The 50 mL portion is still 1 mol·L⁻¹.
CSCA Chemistry December 2025 Past Paper · #19· Topic: 化学性质及用途
Which statement about the uses of substances is incorrect? ( )
  • O₂ supports combustion
  • Correct answer: CO can be used to extinguish fires
  • Carbon can be used as a fuel
  • 75% ethanol can be used for disinfection
Show solution(Answer: B)
Answer: B CO is flammable and toxic, so it cannot be used to extinguish fires; CO₂ is the gas used for that (it does not burn and is denser than air). The other statements are correct.
CSCA Chemistry December 2025 Past Paper · #20· Topic: 元素周期表结构分析
The position of 16S in the periodic table is ( )
  • Period 3, Group IVA
  • Correct answer: Period 3, Group VIA
  • Period 2, Group IVA
  • Period 2, Group VA
Show solution(Answer: B)
Answer: B Atomic number 16 is sulfur (electron configuration 2, 8, 6): 3 electron shells means Period 3, and 6 valence electrons means Group VIA (Group 16).
CSCA Chemistry December 2025 Past Paper · #21· Topic: 浓硫酸的吸水性
A beaker of concentrated sulfuric acid is left in air. After some time, what change occurs? ( )
  • The mass of the solvent decreases
  • Correct answer: The mass of the solution increases
  • The mass of the solute increases
  • The mass of the solution decreases
Show solution(Answer: B)
Answer: B Concentrated sulfuric acid is strongly hygroscopic: left in air, it absorbs water vapor. The solvent mass and therefore the solution mass increase (and the concentration decreases); the solute mass does not change.
CSCA Chemistry December 2025 Past Paper · #22· Topic: 同系物的判断
Which of the following pairs of substances are homologues? ( )
  • CH₃CH₃ and HC≡CH
  • CH₃CH=CH₂ and CH₃CH₃
  • CH₃CH=CHCH₃ and CH₃C=CCH₃
  • Correct answer: CH₃CH₃ and CH₃CH₂CH₃
Show solution(Answer: D)
Answer: D. Ethane and propane are both alkanes and differ by one CH₂ group, so they are homologues.
CSCA Chemistry December 2025 Past Paper · #23· Topic: 四种基本反应类型
Which statement about reaction types is correct? ( )
  • MnO₂ + 4HCl(conc.) = MnCl₂ + Cl₂↑ + 2H₂O is a combination reaction
  • Fe₂O₃ + 3CO = 2Fe + 3CO₂ is a single-displacement reaction
  • 3Fe + 2O₂ = Fe₃O₄ is a decomposition reaction
  • Correct answer: HClO + NaOH = NaClO + H₂O is a double-displacement reaction
Show solution(Answer: D)
Answer: D HClO + NaOH = NaClO + H₂O is an acid–base neutralization, which is a double-displacement reaction. The others are mislabeled: MnO₂ + concentrated HCl is a redox reaction, not a combination; Fe₂O₃ reduced by CO is a redox reaction but not a single displacement (CO is a compound); 3Fe + 2O₂ = Fe₃O₄ is a combination reaction, not a decomposition.
CSCA Chemistry December 2025 Past Paper · #24· Topic: 化学平衡状态的判断方法
When a reaction reaches equilibrium, which of the following must be equal in the reaction system? ( )
  • The time atoms spend in reactant molecules equals the time in product molecules
  • Reactant concentrations equal product concentrations
  • Correct answer: Forward reaction rate equals reverse reaction rate
  • The mass of reactants equals the mass of products
Show solution(Answer: C)
Answer: C Chemical equilibrium is dynamic: the forward rate equals the reverse rate, so macroscopic properties stay constant while the reaction continues. Reactant and product concentrations (or masses) are constant at equilibrium but need not be equal.
CSCA Chemistry December 2025 Past Paper · #25· Topic: 氧化物
Which of the following substances is an oxide? ( )
  • Correct answer: H₂O
  • NaOH
  • O₂
  • KClO₃
Show solution(Answer: A)
Answer: A An oxide is a compound of two elements, one of which is oxygen. H₂O fits. NaOH is a base, O₂ is an element, and KClO₃ is a salt of an oxyacid (three elements).
CSCA Chemistry December 2025 Past Paper · #26· Topic: 催化剂对化学反应速率的影响
Which statement about catalysts is incorrect? ( )
  • Before and after the reaction, the catalyst’s chemical nature does not change
  • Before and after the reaction, the catalyst’s mass changes
  • Some reactions may use different catalysts
  • A catalyst can change the reaction rate
Show solution(Answer: B)
Answer: B A catalyst only changes the reaction rate; its mass and chemical properties are the same before and after the reaction. Saying that its mass changes is incorrect.
CSCA Chemistry December 2025 Past Paper · #27· Topic: 共价键
Which group of substances contains the same type of chemical bond(s) throughout? ( )
  • NaCl and HCl
  • Correct answer: O₂ and H₂
  • HNO₃ and NH₄NO₃
  • NaOH and KCl
Show solution(Answer: B)
Answer: B O₂ and H₂ are both nonmetal molecules held together only by covalent bonds. NaCl is ionic while HCl is covalent; HNO₃ is covalent while NH₄NO₃ has ionic and covalent bonds; NaOH has ionic and covalent bonds while KCl is only ionic.
CSCA Chemistry December 2025 Past Paper · #29· Topic: 硫酸根离子的检验
To a certain solution, BaCl₂ solution is added and a white precipitate forms. After adding nitric acid, the precipitate does not dissolve. Which statement is correct? ( )
  • The solution must contain SO42−\mathrm{SO_4^{2-}}
  • The solution may contain CO32−\mathrm{CO_3^{2-}}
  • The solution must contain Ag+
  • Correct answer: The solution may contain SO42−\mathrm{SO_4^{2-}} or Ag+
Show solution(Answer: D)
Answer: D BaCl₂ solution brings in both Ba²⁺ and Cl⁻, so a white precipitate that does not dissolve in dilute nitric acid has two possible sources: - SO₄²⁻ in the solution: Ba²⁺ + SO₄²⁻ → BaSO₄↓; - Ag⁺ in the solution: Ag⁺ + Cl⁻ → AgCl↓. Both BaSO₄ and AgCl are insoluble in dilute nitric acid, so we can only say the solution may contain SO₄²⁻ or Ag⁺. A and C are wrong: either ion could cause what is observed, so neither "must" be present. B is wrong: BaCO₃ would dissolve in nitric acid and give off CO₂, which contradicts "the precipitate does not dissolve". So the answer is D.
CSCA Chemistry December 2025 Past Paper · #30· Topic: 四种基本反应类型
The reaction between hydrochloric acid and sodium hydroxide is a ( )
  • Decomposition reaction
  • Correct answer: Neutralization reaction
  • Combination reaction
  • Displacement reaction
Show solution(Answer: B)
Answer: B Hydrochloric acid and sodium hydroxide react as H⁺ + OH⁻ → H₂O, an acid–base neutralization reaction (also a double-displacement reaction).
CSCA Chemistry December 2025 Past Paper · #31· Topic: 元素周期表结构分析
Which group of elements belongs to the same main group? ( )
  • Correct answer: Hydrogen, lithium, sodium
  • Carbon, oxygen, sulfur
  • Hydrogen, carbon, nitrogen
  • Sodium, magnesium, aluminum
Show solution(Answer: A)
Answer: A Lithium and sodium are in Group 1 (IA), and hydrogen is placed at the top of Group IA, so hydrogen, lithium and sodium belong to the same main group. The other sets span several groups.
CSCA Chemistry December 2025 Past Paper · #32· Topic: 微粒半径大小的比较方法
Which trend in atomic radii is correct? ( )
  • B < Al < Mg < Na
  • Al < B < Mg < Na
  • Al < B < Na < Mg
  • B < Al < Na < Mg
Show solution(Answer: A)
Answer: A Atomic radius trends: - Across a period from left to right the radius decreases: Na > Mg > Al. - Down a group the radius increases: B (Period 2) < Al (Period 3). Together: B < Al < Mg < Na.
CSCA Chemistry December 2025 Past Paper · #34· Topic: 物质的量浓度与溶液稀释的计算
At 25°C, 100 mL of 0.05 mol·L-1 H₂SO₄ solution is concentrated by evaporating water to 10 mL. The pH of the solution is ( ).
  • 1
  • 2
  • 14
  • Correct answer: 0
Show solution(Answer: D)
Answer: D Evaporating from 100 mL to 10 mL reduces the volume 10-fold, so the concentration rises 10-fold: 0.05 mol·L⁻¹ → 0.5 mol·L⁻¹. Treating H₂SO₄ as a fully ionized diprotic strong acid, [H⁺] = 2 × 0.5 = 1.0 mol·L⁻¹ and pH = 0. (Strictly, the second ionization is incomplete, but the pH is still very close to 0; among the choices the answer is 0.)
CSCA Chemistry December 2025 Past Paper · #35· Topic: 化学反应速率计算
In a 2 L closed container, the reaction 2SO₂(g) + O₂(g) → 2SO₃(g) occurs. In 10 s, the amount of SO₂ decreases from 1 mol to 0.6 mol. The reaction rate v(O₂) is ( )
  • 0.02 mol·L-1·s-1
  • Correct answer: 0.01 mol·L-1·s-1
  • 0.2 mol·L-1·s-1
  • 0.04 mol·L-1·s-1
Show solution(Answer: B)
Answer: B In 10 s, SO₂ falls from 1.0 mol to 0.6 mol, a decrease of 0.4 mol; in a 2 L vessel Δc(SO₂) = 0.4 / 2 = 0.2 mol·L⁻¹. v(SO₂) = 0.2 / 10 = 0.02 mol·L⁻¹·s⁻¹. From the coefficients 2SO₂ : 1O₂, v(O₂) = v(SO₂) / 2 = 0.01 mol·L⁻¹·s⁻¹, option B.
CSCA Chemistry December 2025 Past Paper · #37· Topic: 温度对化学平衡移动的影响
After the following reactions reach equilibrium, in which case will BOTH decreasing pressure OR increasing temperature shift the equilibrium toward the forward direction? ( )
  • N₂(g) + 3H₂(g) ⇌ 2NH₃(g) (forward reaction is exothermic)
  • 2NO₂(g) ⇌ N₂O₄(g) (forward reaction is exothermic)
  • 3O₂(g) ⇌ 2O₃(g) (forward reaction is endothermic)
  • Correct answer: C(s) + CO₂(g) ⇌ 2CO(g) (forward reaction is endothermic)
Show solution(Answer: D)
Answer: D Both changes must shift the equilibrium forward: - Lowering the pressure shifts it toward the side with more moles of gas. - Raising the temperature shifts it in the endothermic direction. C(s) + CO₂(g) ⇌ 2CO(g) is endothermic in the forward direction and has 2 mol of gas on the right versus 1 mol on the left, so both changes favor the forward reaction.
CSCA Chemistry December 2025 Past Paper · #38· Topic: 取代反应
Which of the following reactions is a substitution reaction? ( )
  • Ethene reacting with hydrogen chloride
  • Benzene reacting with hydrogen
  • Correct answer: Methane reacting with chlorine
  • Ethene reacting with water
Show solution(Answer: C)
Answer: C The typical substitution reaction is an alkane with a halogen under light: CH₄ and Cl₂ give CH₃Cl and HCl (and further substitution products). Ethene with HCl or water, and benzene with hydrogen, are addition reactions.
CSCA Chemistry December 2025 Past Paper · #39· Topic: 化学物质的分类方法
Atomic masses: H:1, O:16, Na:23, Cu:64 Which statement about organic compounds is correct? ( )
  • Organic compounds contain only carbon and oxygen
  • Organic substances cannot be synthesized artificially
  • All organic compounds exhibit isomerism
  • Correct answer: Carbon-containing compounds can be inorganic or organic
Show solution(Answer: D)
Answer: D Carbon-containing compounds include both organic and inorganic substances (e.g. CO₂ and carbonates are inorganic). The other statements are wrong: organic compounds can be synthesized artificially, not all organic compounds have isomers, and organic compounds contain other elements besides C and O (such as H and N).
CSCA Chemistry December 2025 Past Paper · #40· Topic: 化学方程式中物质的量的运用
Hydrogen is produced by splitting water using solar energy. If 0.02 mol of water is decomposed, which statement is correct? ( )
  • The volume of H₂ produced is 0.224 L (at STP)
  • The amount of H₂ produced is 0.01 mol
  • Correct answer: The amount of H₂ produced is 0.02 mol
  • The mass of H₂ produced is 0.02 g
Show solution(Answer: C)
Answer: C 2H₂O → 2H₂ + O₂. The coefficients show that 2 mol of water gives 2 mol of hydrogen, so decomposing 0.02 mol of water gives 0.02 mol of H₂ (0.448 L at STP, 0.04 g).
CSCA Chemistry December 2025 Past Paper · #42· Topic: 温度对化学平衡移动的影响
For the reaction C(s)+H₂O(g)⇌CO(g)+H₂(g), the equilibrium shifts in the reverse direction under which condition? ( )
  • Increase temperature, decrease pressure
  • Increase temperature, increase pressure
  • Decrease temperature, decrease pressure
  • Correct answer: Decrease temperature, increase pressure
Show solution(Answer: D)
Answer: D C(s) + H₂O(g) ⇌ CO(g) + H₂(g) is endothermic in the forward direction and has 2 mol of gas on the right versus 1 mol on the left. To shift it in reverse: lowering the temperature favors the exothermic (reverse) direction, and raising the pressure favors the side with fewer gas molecules (the left). So "decrease temperature, increase pressure" is correct.
CSCA Chemistry December 2025 Past Paper · #43· Topic: 醛类
Which liquid can react with freshly prepared Cu(OH)₂ to form a red Cu₂O precipitate? ( )
  • CH₃COOH
  • CH₃COOCH₂CH₃
  • Correct answer: CH₃CHO
  • CH₃CH₂OH
Show solution(Answer: C)
Answer: C When heated, freshly prepared Cu(OH)₂ is reduced by an aldehyde to a brick-red Cu₂O precipitate. CH₃CHO is an aldehyde; the carboxylic acid, ester and alcohol do not give this reaction.
CSCA Chemistry December 2025 Past Paper · #45· Topic: 化学方程式中物质的量的运用
Using CO to reduce a mixture of CuO and Cu powder, 23.4 g of the mixture reacts completely to yield 20.2 g of pure copper powder. The mass of CuO in the original mixture is ( )
  • 20 g
  • 32 g
  • 40 g
  • Correct answer: 16 g
Show solution(Answer: D)
Answer: D CuO + CO → Cu + CO₂. The mass fraction of Cu in CuO is 64 / 80 = 0.8. Let the mass of CuO be x: the original Cu is 23.4 − x and the Cu produced is 0.8x. (23.4 − x) + 0.8x = 20.2 → x = 16 g.
CSCA Chemistry December 2025 Past Paper · #46· Topic: 弱电解质在水溶液中的电离平衡
In a chemistry lab, students perform the following operations. Which operation can achieve the stated purpose? Operation - Purpose 1 Add excess concentrated NaOH solution to ZnCl₂ solution to prepare Zn(OH)₂ precipitate 2 Heat AlCl₃ solution to dryness to obtain anhydrous AlCl₃ 3 Add dilute nitric acid solution, then filter, wash, and dry to remove CuO mixed in Cu powder 4 Bubble CO₂ into C₆H₅ONa solution; the solution turns cloudy to compare the acidity of carbonic acid and phenol
  • Correct answer: Operation 4 and its purpose
  • Operation 1 and its purpose
  • Operation 3 and its purpose
  • Operation 2 and its purpose
Show solution(Answer: A)
Answer: A Only operation ④ achieves its purpose: CO₂ dissolves to form H₂CO₃, which turns phenoxide into sparingly soluble phenol (the solution turns cloudy), showing that carbonic acid is stronger than phenol. ① Excess NaOH dissolves Zn(OH)₂; ② evaporating AlCl₃ solution to dryness hydrolyzes it; ③ dilute nitric acid also dissolves Cu, so it cannot remove only the CuO.
CSCA Chemistry December 2025 Past Paper · #47· Topic: 物质检验和鉴别的实验方案设计
A white powder is composed of two substances. To identify them, the following experiments are performed: 1 Add a small amount of the sample to excess water: the solid partially dissolves. Then add excess dilute HCl: bubbles form and the solid dissolves completely. 2 Add a small amount of the sample to excess dilute H₂SO₄: bubbles form; after shaking, some solid remains. The white powder could be ( ).
  • AgNO₃ and Na₂CO₃
  • NaOH and CuSO₄
  • CaCO₃ and BaCO₃
  • Correct answer: Na₂CO₃ and BaCO₃
Show solution(Answer: D)
Answer: D The sample only partly dissolves in water, so it contains one soluble substance (Na₂CO₃) and one insoluble one (BaCO₃). With dilute hydrochloric acid it bubbles and dissolves completely: carbonates release CO₂ with acid, and BaCO₃ dissolves in hydrochloric acid. With dilute sulfuric acid a solid remains, because insoluble BaSO₄ forms.
CSCA Chemistry December 2025 Past Paper · #48· Topic: 原电池正负极判断
Scientists developed a room-temperature Na-O₂ rechargeable battery. NaClO₄ is dissolved in an organic solvent as the electrolyte. Metallic sodium and Pt/C-loaded carbon cloth are used as electrode materials. The overall cell reaction is: O₂ + 2Na = Na₂O₂. Which statement is correct? ( )
  • During discharge, ClO₄- moves toward the positive electrode
  • During discharge, O₂ is released; during charge, O₂ is absorbed
  • During discharge, the negative electrode reaction is: Na+ + e- → Na
  • Correct answer: During charge, the positive electrode reaction is: Na₂O₂ - 2e- → 2Na+ + O₂
Show solution(Answer: D)
Answer: D Discharge: Na is oxidized at the negative electrode, Na − e⁻ → Na⁺, and O₂ is reduced to form Na₂O₂. Charging reverses this: Na₂O₂ loses electrons at the positive electrode and releases O₂, so the positive electrode reaction is Na₂O₂ − 2e⁻ → 2Na⁺ + O₂.

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