15 extra multiple-choice questions for Types of Chemical Reactions (10th Standard Science, Samacheer Kalvi), beyond the ones printed in the textbook — each with the correct option highlighted and a clear, worked explanation. Free to read in English and Tamil.
Q1
When slaked lime is applied to walls during white washing, it reacts slowly with a gas in the air to form a thin, shiny layer. What is the chemical formula of this shiny layer?
- A. \(\text{Ca(OH)}_2\)
- B. \(\text{CaO}\)
- C. \(\text{CaCO}_3\)Correct
- D. \(\text{CaCl}_2\)
Explanation. Slaked lime reacts with atmospheric carbon dioxide to form calcium carbonate, which provides a shiny and white finish to the walls.
Q2
According to the activity of halogens, which of the following single displacement reactions will NOT occur in practice?
- A. 2\(\text{NaCl}_{(aq)} + \text{F}_{2(g)} \rightarrow 2\text{NaF}_{(aq)} + \text{Cl}_{2(g)}\)
- B. 2\(\text{NaF}_{(aq)} + \text{Cl}_{2(g)} \rightarrow 2\text{NaCl}_{(aq)} + \text{F}_{2(g)}\)Correct
- C. 2\(\text{KBr}_{(aq)} + \text{Cl}_{2(g)} \rightarrow 2\text{KCl}_{(aq)} + \text{Br}_{2(g)}\)
- D. 2\(\text{KI}_{(aq)} + \text{Br}_{2(g)} \rightarrow 2\text{KBr}_{(aq)} + \text{I}_{2(g)}\)
Explanation. Fluorine is more active than chlorine. Therefore, chlorine cannot displace fluorine from sodium fluoride, making this displacement reaction non-feasible.
Q3
Based on the activity series of metals, which of the following metals is the most reactive and placed at the top of the series?
- A. Sodium (Na)
- B. Calcium (Ca)
- C. Potassium (K)Correct
- D. Magnesium (Mg)
Explanation. In the reactivity series, potassium is located at the top as the most reactive metal, followed by sodium, lithium, calcium, and others.
Q4
The reaction between silicon dioxide and calcium oxide to form calcium silicate (\(\text{SiO}_{2(s)} + \text{CaO}_{(s)} \rightarrow \text{CaSiO}_{3(s)}\)) is classified under which type of combination reaction?
- A. Element + Element \(\rightarrow\) Compound
- B. Compound + Element \(\rightarrow\) Compound
- C. Compound + Compound \(\rightarrow\) CompoundCorrect
- D. Element + Compound \(\rightarrow\) Element
Explanation. Silicon dioxide and calcium oxide are both compounds. Their reaction to produce calcium silicate is a compound-to-compound combination reaction.
Q5
During the thermal decomposition of mercury (II) oxide, which is a thermolysis reaction, the reactant decomposes into:
- A. Two simpler compounds
- B. A compound and an element
- C. Two elementsCorrect
- D. A compound and water
Explanation. Heating mercury (II) oxide decomposes it into mercury metal and oxygen gas, both of which are elements, illustrating compound-to-elements decomposition.
Q6
Granulated zinc reacts faster with \(2\text{ M}\) hydrochloric acid than with \(1\text{ M}\) hydrochloric acid. This rate increase is directly due to the influence of which factor?
- A. Nature of reactants
- B. Surface area of reactants
- C. Concentration of reactantsCorrect
- D. Temperature of reaction mixture
Explanation. Higher molarity means higher concentration. Increasing reactant concentration increases the number of reacting particles per unit volume, speeding up the reaction.
Q7
In the decomposition of potassium chlorate into potassium chloride and oxygen, manganese dioxide (\(\text{MnO}_2\)) is added. What role does manganese dioxide play in this chemical reaction?
- A. It acts as a reactant and is completely consumed
- B. It acts as an endothermic heat absorber
- C. It acts as a catalyst to increase the reaction rateCorrect
- D. It acts as an inhibitor to prevent the reaction
Explanation. Manganese dioxide serves as a catalyst that accelerates the decomposition of potassium chlorate without undergoing any permanent chemical change or consumption.
Q8
Why is chemical equilibrium referred to as a dynamic equilibrium?
- A. Because the reaction completely stops at equilibrium
- B. Because both forward and backward reactions continue to occur at equal ratesCorrect
- C. Because the concentration of reactants becomes zero
- D. Because the pressure of the system continuously fluctuates
Explanation. Chemical equilibrium is dynamic because both forward and backward reactions proceed at equal rates, maintaining constant concentrations of reactants and products.
Q9
The equilibrium attained during the physical process of melting of ice is known as:
- A. Chemical equilibrium
- B. Dynamic gas equilibrium
- C. Physical equilibriumCorrect
- D. Static solid equilibrium
Explanation. An equilibrium established during physical state changes, such as solid ice transitioning to liquid water, is categorized as a physical equilibrium.
Q10
For a general reaction \(\text{A} \rightarrow \text{B}\), the rate of reaction can be represented by which of the following expressions?
- A. \(\text{Rate} = +\frac{d[\text{A}]}{dt} = -\frac{d[\text{B}]}{dt}\)
- B. \(\text{Rate} = -\frac{d[\text{A}]}{dt} = +\frac{d[\text{B}]}{dt}\)Correct
- C. \(\text{Rate} = -\frac{d[\text{A}]}{dt} = -\frac{d[\text{B}]}{dt}\)
- D. \(\text{Rate} = +\frac{d[\text{A}]}{dt} = +\frac{d[\text{B}]}{dt}\)
Explanation. The negative sign represents the decrease in reactant A's concentration over time, while the positive sign represents product B's increase.
Q11
In the human body, all vital biochemical reactions take place within a narrow, regulated pH range of:
- A. 7.0 to 7.8Correct
- B. 5.6 to 6.0
- C. 8.2 to 9.0
- D. 1.5 to 3.5
Explanation. Human physiological systems maintain a precise pH level, and all crucial biochemical reactions take place between pH 7.0 and 7.8.
Q12
Rainwater is classified as 'acid rain' when its pH value falls below which threshold?
- A. 7.0
- B. 6.5
- C. 5.6Correct
- D. 4.0
Explanation. Normal rainwater has slightly acidic tendencies, but when atmospheric pollutant gases dissolve and lower its pH below 5.6, it is called acid rain.
Q13
What is the pH of a \(1.0 \times 10^{-4}\text{ M}\) solution of nitric acid (\(\text{HNO}_3\))?
- A. 10
- B. 4Correct
- C. 1.4
- D. 14
Explanation. Nitric acid completely dissociates giving \([\text{H}^+] = 10^{-4}\text{ M}\). Using \(\text{pH} = -\log_{10}[\text{H}^+]\), the pH is calculated to be 4.
Q14
Calculate the pH of a \(1.0 \times 10^{-4}\text{ M}\) solution of sodium hydroxide (\(\text{NaOH}\)).
- A. 4
- B. 7
- C. 10Correct
- D. 14
Explanation. NaOH yields \([\text{OH}^-] = 10^{-4}\text{ M}\), so \(\text{pOH} = 4\). Using \(\text{pH} + \text{pOH} = 14\), we get \(\text{pH} = 14 - 4 = 10\).
Q15
On recharging a mobile phone, the chemical reactions that provide energy during discharging are reversed. This process is a practical application of:
- A. Thermal irreversible decomposition
- B. Reversible reactions under suitable conditionsCorrect
- C. Rapid uncontrolled combustion
- D. Unidirectional double displacement
Explanation. Mobile batteries rely on reversible chemical reactions that proceed in the forward direction when discharging, and can be driven backward during recharging.
