Chemical Reactions and Equations — Class 10 Chemistry NCERT Solutions (Free)
Free step-by-step NCERT solutions for Class 10 Chemistry chapter "Chemical Reactions and Equations" — 10 important questions with detailed answers for CBSE board exam preparation.
TL;DR: Free step-by-step NCERT solutions for Class 10 Chemistry chapter "Chemical Reactions and Equations" — 10 important questions with detailed answers for…
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NCERT Solutions for Class 10 Chemistry — Chemical Reactions and Equations. Step-by-step answers to all 10 textbook questions from this chapter, written for CBSE board preparation and free to use.
Chemical Reactions and Equations — All 10 Questions Solved
Q1. What are the four main observations that indicate a chemical reaction has taken place? Give one example for each.
A chemical reaction is a process that involves the rearrangement of the molecular or ionic structure of a substance, as opposed to a change in physical form. We can identify a chemical reaction by observing certain changes. The four main observations are:
1. Change in State: Reactants might be liquids and products might be gases, or vice-versa.
* Example: When a candle burns, solid wax melts (physical change) and then reacts with oxygen to produce carbon dioxide (gas) and water vapour (gas), indicating a change from solid to gaseous products.
`C_25H_52 (s) + 38O_2 (g) \rightarrow 25CO_2 (g) + 26H_2O (g)`
2. Change in Colour: The colour of the substance might change during the reaction.
* Example: When an iron nail is dipped in copper sulphate solution, the blue colour of copper sulphate solution fades and turns greenish due to the formation of iron sulphate, and a reddish-brown deposit of copper forms on the iron nail.
`Fe (s) + CuSO_4 (aq, blue) \rightarrow FeSO_4 (aq, greenish) + Cu (s, reddish-brown)`
3. Evolution of a Gas: A gas might be released, often seen as bubbles.
* Example: When zinc granules react with dilute sulphuric acid, hydrogen gas is evolved, which can be seen as bubbles.
`Zn (s) + H_2SO_4 (aq) \rightarrow ZnSO_4 (aq) + H_2 (g)`
4. Change in Temperature: The reaction mixture might become hotter (exothermic) or colder (endothermic).
* Example: When quicklime (calcium oxide) reacts vigorously with water, a large amount of heat is released, making the mixture hot (exothermic reaction).
`CaO (s) + H_2O (l) \rightarrow Ca(OH)_2 (aq) + Heat`
Q2. Balance the following chemical equation and identify the type of reaction: `Al (s) + O_2 (g) \rightarrow Al_2O_3 (s)`
To balance the chemical equation, we need to ensure that the number of atoms of each element is the same on both the reactant and product sides (Law of Conservation of Mass).
The given unbalanced equation is:
`Al (s) + O_2 (g) \rightarrow Al_2O_3 (s)`
Step 1: List the number of atoms of each element on both sides.
| Element | Reactant Side | Product Side |
| :------ | :------------ | :----------- |
| Al | 1 | 2 |
| O | 2 | 3 |
Step 2: Balance the element that appears in the fewest compounds first. Oxygen often takes a bit more effort due to its even/odd count.
Let's start with Oxygen. We have 2 oxygen atoms on the reactant side and 3 on the product side. To balance them, find the least common multiple, which is 6.
Multiply O2 by 3 and Al2O3 by 2:
`Al (s) + 3O_2 (g) \rightarrow 2Al_2O_3 (s)`
Step 3: Re-evaluate the number of atoms.
| Element | Reactant Side | Product Side |
| :------ | :------------ | :----------- |
| Al | 1 | 2 x 2 = 4 |
| O | 3 x 2 = 6 | 2 x 3 = 6 |
Step 4: Balance Aluminium atoms.
Now, Aluminium is unbalanced. We have 1 Al on the reactant side and 4 Al on the product side. Multiply Al on the reactant side by 4:
`4Al (s) + 3O_2 (g) \rightarrow 2Al_2O_3 (s)`
Step 5: Final check of all atoms.
| Element | Reactant Side | Product Side |
| :------ | :------------ | :----------- |
| Al | 4 | 4 |
| O | 6 | 6 |
All atoms are balanced. So, the balanced chemical equation is:
`4Al (s) + 3O_2 (g) \rightarrow 2Al_2O_3 (s)`
Type of Reaction:
In this reaction, two or more reactants (Aluminium and Oxygen) combine to form a single product (Aluminium oxide). Therefore, this is a Combination Reaction.
Q3. What is a decomposition reaction? Explain with an example where heat is used to carry out the decomposition.
A decomposition reaction is a type of chemical reaction in which a single compound breaks down into two or more simpler substances (elements or simpler compounds). These reactions often require energy in the form of heat, light, or electricity to proceed.
Example using heat (Thermal Decomposition):
When calcium carbonate (limestone) is heated, it decomposes into calcium oxide (quicklime) and carbon dioxide gas.
Equation:
`CaCO_3 (s) \xrightarrow{\text{Heat}} CaO (s) + CO_2 (g)`
Explanation:
1. Reactant: Calcium carbonate (`CaCO_3`) is a single compound.
2. Energy Input: Heat energy is supplied to `CaCO_3`.
3. Products: Upon heating, `CaCO_3` breaks down into two simpler compounds:
* Calcium oxide (`CaO`): A solid white substance (quicklime), which is used in cement manufacturing.
* Carbon dioxide (`CO_2`): A gas, which is released.
This reaction is a classic example of thermal decomposition and is industrially important for the production of quicklime.
Q4. What happens when an iron nail is placed in a copper sulphate solution? Write the balanced chemical equation and identify the type of reaction.
When an iron nail is placed in a copper sulphate solution, the following observations and chemical changes occur:
Observations:
1. The blue colour of the copper sulphate solution starts to fade and gradually turns greenish.
2. A reddish-brown layer gets deposited on the surface of the iron nail.
Explanation:
Iron is more reactive than copper. According to the reactivity series, a more reactive metal can displace a less reactive metal from its salt solution. In this case, iron (Fe) displaces copper (Cu) from copper sulphate (`CuSO_4`) solution.
The iron metal forms iron(II) sulphate (`FeSO_4`), which is greenish in colour and dissolves in the solution. The displaced copper metal gets deposited on the iron nail, forming a reddish-brown layer.
Balanced Chemical Equation:
`Fe (s) + CuSO_4 (aq) \rightarrow FeSO_4 (aq) + Cu (s)`
* `Fe (s)`: Solid iron (iron nail)
* `CuSO_4 (aq)`: Aqueous copper sulphate solution (blue)
* `FeSO_4 (aq)`: Aqueous iron(II) sulphate solution (greenish)
* `Cu (s)`: Solid copper (reddish-brown deposit)
Type of Reaction:
Since one element (Iron) displaces another element (Copper) from its compound, this reaction is a Displacement Reaction.
Q5. Define precipitation reaction. Give an example of a precipitation reaction, write its balanced chemical equation, and mention the colour of the precipitate formed.
Precipitation Reaction:
A precipitation reaction is a type of double displacement reaction where two aqueous solutions react to form an insoluble solid product called a precipitate. This precipitate separates from the solution, often appearing as a cloudy or solid substance settling at the bottom.
Example:
When an aqueous solution of Barium Chloride (`BaCl_2`) is mixed with an aqueous solution of Sodium Sulphate (`Na_2SO_4`), a white precipitate of Barium Sulphate (`BaSO_4`) is formed along with Sodium Chloride (`NaCl`) solution.
Balanced Chemical Equation:
`BaCl_2 (aq) + Na_2SO_4 (aq) \rightarrow BaSO_4 (s) \downarrow + 2NaCl (aq)`
Explanation:
1. Reactants: Barium chloride (`BaCl_2`) and Sodium sulphate (`Na_2SO_4`) are both soluble ionic compounds dissolved in water.
2. Ion Exchange: In the reaction, the positive ion (`Ba^{2+}`) from barium chloride exchanges with the positive ion (`Na^+`) from sodium sulphate.
3. Products:
* Barium ions (`Ba^{2+}`) combine with sulphate ions (`SO_4^{2-}`) to form Barium Sulphate (`BaSO_4`). Barium sulphate is insoluble in water, so it forms a solid precipitate.
* Sodium ions (`Na^+`) combine with chloride ions (`Cl^-`) to form Sodium Chloride (`NaCl`), which remains dissolved in water.
Colour of the precipitate formed: The precipitate formed, `BaSO_4`, is white in colour.
Q6. Identify the substance oxidised, substance reduced, oxidising agent, and reducing agent in the following reaction: `CuO (s) + H_2 (g) \rightarrow Cu (s) + H_2O (l)`
Let's analyse the given reaction step-by-step to identify oxidation, reduction, and the agents involved.
`CuO (s) + H_2 (g) \rightarrow Cu (s) + H_2O (l)`
Step 1: Understand Oxidation and Reduction in terms of Oxygen/Hydrogen.
* Oxidation: Gain of oxygen or loss of hydrogen.
* Reduction: Loss of oxygen or gain of hydrogen.
Step 2: Identify what happens to each reactant.
* For CuO: It loses oxygen to become `Cu`.
`CuO \rightarrow Cu` (Loss of oxygen)
Therefore, `CuO` is reduced.
* For H_2: It gains oxygen to become `H_2O`.
`H_2 \rightarrow H_2O` (Gain of oxygen)
Therefore, `H_2` is oxidised.
Step 3: Identify the Oxidising Agent and Reducing Agent.
* Oxidising Agent: The substance that causes oxidation (i.e., it itself gets reduced).
Since `CuO` is reduced, `CuO` is the oxidising agent.
* Reducing Agent: The substance that causes reduction (i.e., it itself gets oxidised).
Since `H_2` is oxidised, `H_2` is the reducing agent.
Summary:
* Substance Oxidised: `H_2`
* Substance Reduced: `CuO`
* Oxidising Agent: `CuO`
* Reducing Agent: `H_2`
Q7. What is corrosion? Describe two methods to prevent corrosion of iron (rusting).
Corrosion:
Corrosion is a natural process that gradually destroys materials (usually metals) by chemical and/or electrochemical reaction with their environment. It is the process by which metals are eaten up gradually by the action of air, moisture, or a chemical (like an acid) on their surface. The most common example of corrosion is the rusting of iron.
Rusting of Iron:
When iron articles are exposed to moist air (presence of both oxygen and water) for a prolonged period, a reddish-brown flaky substance called rust (`hydrated iron(III) oxide`, `Fe_2O_3 \cdot xH_2O`) is formed on their surface. This process is called rusting.
Methods to prevent corrosion of iron (rusting):
Corrosion causes significant economic loss, so preventing it is crucial. Here are two common methods:
1. Painting/Greasing/Oiling:
* Method: Applying a coat of paint, grease, or oil on the surface of iron articles. This creates a barrier between the metal surface and the atmospheric oxygen and moisture. Since both oxygen and moisture are necessary for rusting, their absence prevents the reaction.
* Example: Iron gates, railway coaches, and car bodies are painted regularly to prevent rusting.
2. Galvanisation:
* Method: This is a method of protecting steel and iron from rusting by coating them with a thin layer of a more reactive metal, usually zinc. Zinc is more reactive than iron, so it readily reacts with oxygen and moisture in the air to form a protective layer of zinc oxide or zinc carbonate, which prevents further corrosion. Even if the zinc coating is broken, the zinc continues to protect the iron because zinc is more reactive and gets corroded preferentially (this is called cathodic protection).
* Example: Iron sheets used for roofing, water pipes, and automobile parts are often galvanised.
Q8. What is rancidity? List two methods to prevent rancidity in food items.
Rancidity:
Rancidity is the process of oxidation of fats and oils in food items when they are exposed to air for a long time. This oxidation leads to a change in the smell and taste of the food, making it unpleasant to eat. The food becomes 'rancid'.
Explanation:
Food items containing fats and oils (like chips, snacks, fried items) react with oxygen in the air over time. This reaction, known as oxidative rancidity, breaks down the fats and oils into smaller, volatile compounds (like aldehydes and ketones) which are responsible for the unpleasant smell and sour/bitter taste.
Methods to prevent rancidity in food items:
To prevent or slow down the process of rancidity and preserve food items, several methods are employed:
1. Adding Antioxidants:
* Method: Antioxidants are substances that prevent or slow down oxidation. They are often added to food items containing fats and oils. They act by reacting with oxygen before the fats and oils do, thus protecting the food.
* Example: Common antioxidants used are Butylated Hydroxyanisole (BHA) and Butylated Hydroxytoluene (BHT). They are often added to packaged snacks like chips and biscuits.
2. Packaging in Nitrogen Gas:
* Method: Food items (especially fried snacks like potato chips) are often packaged in airtight bags flushed with an inert gas like nitrogen instead of air. Nitrogen prevents the oxygen from coming into contact with the fats and oils in the food. Since oxygen is removed, the oxidation process is significantly slowed down, thereby preventing rancidity.
* Example: Bags of potato chips are filled with nitrogen gas to keep them fresh and crunchy for a longer period.
Q9. In the electrolysis of water, name the gas collected at the anode and cathode. Write the balanced chemical equation for the decomposition reaction.
Electrolysis of Water:
Electrolysis of water is a decomposition reaction where electric current is passed through water to break it down into its constituent elements, hydrogen and oxygen. This experiment typically uses two electrodes (anode and cathode) immersed in water, connected to a power source.
Gases collected at the electrodes:
1. At the Cathode (negative electrode): Hydrogen gas (`H_2`) is collected. This is because positively charged hydrogen ions (`H^+`) (formed from water) are attracted to the negative cathode, where they gain electrons and form hydrogen gas.
2. At the Anode (positive electrode): Oxygen gas (`O_2`) is collected. This is because negatively charged hydroxide ions (`OH^-`) (formed from water) are attracted to the positive anode, where they lose electrons and form oxygen gas and water.
Note: The volume of hydrogen gas collected at the cathode is double the volume of oxygen gas collected at the anode, because water (`H_2O`) contains two parts hydrogen and one part oxygen by volume.
Balanced Chemical Equation for the decomposition reaction:
`2H_2O (l) \xrightarrow{\text{Electric Current}} 2H_2 (g) + O_2 (g)`
* `H_2O (l)`: Liquid water
* `H_2 (g)`: Hydrogen gas
* `O_2 (g)`: Oxygen gas
Q10. Why is it important to balance chemical equations? Explain with reference to a fundamental law of chemistry.
It is very important to balance chemical equations because of a fundamental principle of chemistry known as the Law of Conservation of Mass.
Law of Conservation of Mass:
This law, proposed by Antoine Lavoisier, states that mass can neither be created nor destroyed in a chemical reaction. In simpler terms, the total mass of the reactants must be equal to the total mass of the products. This implies that the total number of atoms of each element before the reaction must be equal to the total number of atoms of each element after the reaction.
Why balancing is important (Connecting to the law):
1. Adherence to the Law of Conservation of Mass: An unbalanced chemical equation shows only the chemical formulas of reactants and products but does not reflect the actual quantitative relationship between them. If the number of atoms of each element is not equal on both sides of the equation, it would imply that atoms are either being created or destroyed during the reaction, which contradicts the Law of Conservation of Mass. Balancing ensures that this law is obeyed.
2. Correct Stoichiometry (Quantitative Relationships): A balanced chemical equation provides the correct stoichiometric ratios (mole ratios) between reactants and products. This information is crucial for:
* Calculating reactant and product amounts: In industries or laboratories, chemists need to know the exact amount of reactants required to produce a certain amount of product, or vice-versa. An unbalanced equation would lead to incorrect calculations and wasted materials.
* Predicting reaction yields: Knowing the balanced equation allows us to predict the theoretical yield of a product from a given amount of reactants.
3. Accurate Representation of the Reaction: A balanced equation accurately represents the actual chemical change taking place, showing the exact number of molecules or atoms participating in the reaction.
Example:
Consider the reaction of hydrogen and oxygen to form water:
* Unbalanced: `H_2 (g) + O_2 (g) \rightarrow H_2O (l)`
* Here, on the reactant side, there are 2 oxygen atoms, but on the product side, there is only 1. This implies that one oxygen atom has been destroyed, which is impossible according to the Law of Conservation of Mass.
* Balanced: `2H_2 (g) + O_2 (g) \rightarrow 2H_2O (l)`
* Now, we have 4 hydrogen atoms and 2 oxygen atoms on both the reactant and product sides. The number of atoms of each element is conserved, thus obeying the Law of Conservation of Mass. This equation correctly shows that two molecules of hydrogen react with one molecule of oxygen to produce two molecules of water.
Chemical Reactions and Equations — Practice MCQs with Answers
Attempt these 20 multiple-choice questions after working through the solutions above. Each carries the correct option and the reasoning behind it, so a wrong answer tells you which idea to revisit.
MCQ 1. Which of the following observations helps determine if a chemical reaction has taken place?
- A. Change in state only
- B. Change in colour only
- C. Evolution of gas only
- D. All of the above
Answer: D. All of the above — Chemical reactions are often characterized by observable changes such as a change in state, colour, temperature, evolution of a gas, or formation of a precipitate. Therefore, all these observations can indicate a chemical reaction.
MCQ 2. Which of the following equations represents a balanced chemical equation?
- A. Fe + H₂O → Fe₃O₄ + H₂
- B. 3Fe + 4H₂O → Fe₃O₄ + H₂
- C. 3Fe + 4H₂O → Fe₃O₄ + 4H₂
- D. Fe + H₂O → Fe₃O₄ + 4H₂
Answer: C. 3Fe + 4H₂O → Fe₃O₄ + 4H₂ — A balanced chemical equation has an equal number of atoms of each element on both sides of the equation. In option (c), there are 3 Fe atoms, 8 H atoms, and 4 O atoms on both the reactant and product sides.
MCQ 3. When calcium oxide reacts vigorously with water, it produces slaked lime. This reaction is an example of:
- A. Decomposition reaction
- B. Combination reaction
- C. Displacement reaction
- D. Double displacement reaction
Answer: B. Combination reaction — In a combination reaction, two or more reactants combine to form a single product. Here, calcium oxide (CaO) and water (H₂O) combine to form calcium hydroxide (Ca(OH)₂), which is slaked lime.
MCQ 4. On heating lead(II) nitrate crystals, a brown gas 'X' is evolved. The gas 'X' is:
- A. Nitrogen gas
- B. Oxygen gas
- C. Nitrogen dioxide
- D. Sulphur dioxide
Answer: C. Nitrogen dioxide — When lead(II) nitrate (Pb(NO₃)₂) is heated, it decomposes to form lead(II) oxide (PbO), oxygen gas (O₂), and nitrogen dioxide gas (NO₂). Nitrogen dioxide is a reddish-brown gas.
MCQ 5. Which of the following reactions is a displacement reaction?
- A. 2H₂ + O₂ → 2H₂O
- B. CaCO₃ → CaO + CO₂
- C. Fe + CuSO₄ → FeSO₄ + Cu
- D. NaCl + AgNO₃ → AgCl + NaNO₃
Answer: C. Fe + CuSO₄ → FeSO₄ + Cu — In a displacement reaction, a more reactive element displaces a less reactive element from its compound. Here, iron (Fe) is more reactive than copper (Cu), so it displaces copper from copper sulfate (CuSO₄).
MCQ 6. A student adds barium chloride solution to sodium sulfate solution. What is the observation and type of reaction?
- A. Colourless solution, combination reaction
- B. White precipitate, double displacement reaction
- C. Black precipitate, decomposition reaction
- D. Evolution of gas, displacement reaction
Answer: B. White precipitate, double displacement reaction — When barium chloride (BaCl₂) reacts with sodium sulfate (Na₂SO₄), a white precipitate of barium sulfate (BaSO₄) is formed along with sodium chloride (NaCl). This is a double displacement (precipitation) reaction.
MCQ 7. In the reaction: CuO + H₂ → Cu + H₂O, which substance is oxidized?
- A. CuO
- B. H₂
- C. Cu
- D. H₂O
Answer: B. H₂ — Oxidation is the gain of oxygen or loss of hydrogen. Here, hydrogen (H₂) gains oxygen to form water (H₂O), hence H₂ is oxidized.
MCQ 8. The chemical formula for rust is:
- A. FeO
- B. Fe₂O₃
- C. Fe(OH)₃
- D. Fe₂O₃.xH₂O
Answer: D. Fe₂O₃.xH₂O — Rust is hydrated iron(III) oxide, which means iron oxide with water molecules associated with it. Its chemical formula is Fe₂O₃.xH₂O, where 'x' represents a variable number of water molecules.
MCQ 9. Which of the following methods is NOT used to prevent rancidity?
- A. Adding antioxidants
- B. Storing food in airtight containers
- C. Storing food in the presence of oxygen
- D. Flushing with nitrogen gas
Answer: C. Storing food in the presence of oxygen — Rancidity is caused by the oxidation of fats and oils in food. Storing food in the presence of oxygen would accelerate this oxidation, thus not preventing rancidity. Antioxidants, airtight containers, and nitrogen gas all help reduce exposure to oxygen.
MCQ 10. Identify the oxidizing agent in the following reaction: MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂
- A. MnO₂
- B. HCl
- C. MnCl₂
- D. Cl₂
Answer: A. MnO₂ — An oxidizing agent is a substance that oxidizes another substance and itself gets reduced. In this reaction, MnO₂ causes HCl to lose hydrogen (getting oxidized to Cl₂) and MnO₂ itself loses oxygen (getting reduced to MnCl₂).
MCQ 11. Silver bromide is used in black and white photography because it undergoes:
- A. Combination reaction
- B. Decomposition reaction by light
- C. Displacement reaction
- D. Double displacement reaction
Answer: B. Decomposition reaction by light — Silver bromide (AgBr) is sensitive to light. When exposed to light, it decomposes into silver (Ag) and bromine (Br₂). This photochemical decomposition reaction is the basis for black and white photography.
MCQ 12. Which of the following statements is correct about the reaction between calcium oxide and water?
- A. It is an endothermic reaction and releases heat.
- B. It is an exothermic reaction and absorbs heat.
- C. It is an endothermic reaction and absorbs heat.
- D. It is an exothermic reaction and releases heat.
Answer: D. It is an exothermic reaction and releases heat. — The reaction between calcium oxide (quicklime) and water to form calcium hydroxide (slaked lime) is a combination reaction that releases a significant amount of heat, hence it is an exothermic reaction.
MCQ 13. Which of the following reactions involves both combination and decomposition processes?
- A. 2H₂O → 2H₂ + O₂
- B. 2KClO₃ → 2KCl + 3O₂
- C. Photosynthesis
- D. None of the above
Answer: C. Photosynthesis — Photosynthesis is a complex process where light energy is used to convert carbon dioxide and water into glucose and oxygen. While it involves the formation of larger molecules (combination), it also involves the breakdown of water (decomposition) and is not a simple combination or decomposition reaction in isolation.
MCQ 14. The symbol (aq) in a chemical equation represents:
- A. A liquid state
- B. An aqueous solution
- C. A gaseous state
- D. A solid state
Answer: B. An aqueous solution — The symbol (aq) stands for 'aqueous', meaning the substance is dissolved in water, forming a solution.
MCQ 15. Which of the following metals cannot displace hydrogen from dilute acids?
- A. Zinc (Zn)
- B. Iron (Fe)
- C. Copper (Cu)
- D. Magnesium (Mg)
Answer: C. Copper (Cu) — According to the reactivity series, metals more reactive than hydrogen can displace it from dilute acids. Copper is less reactive than hydrogen, hence it cannot displace hydrogen from dilute acids.
MCQ 16. When ferrous sulfate crystals are heated, they lose water and then decompose. What are the products of the decomposition of anhydrous ferrous sulfate?
- A. Fe₂O₃, SO₂, SO₃
- B. FeO, SO₂, SO₃
- C. Fe, SO₂, SO₃
- D. FeSO₄, O₂
Answer: A. Fe₂O₃, SO₂, SO₃ — Upon strong heating, anhydrous ferrous sulfate (FeSO₄) decomposes to form ferric oxide (Fe₂O₃), sulfur dioxide (SO₂), and sulfur trioxide (SO₃). The colour changes from green to reddish-brown.
MCQ 17. When copper is exposed to moist air for a long time, it acquires a dull green coating. This coating is due to the formation of:
- A. Copper oxide
- B. Copper sulfide
- C. Basic copper carbonate
- D. Copper nitrate
Answer: C. Basic copper carbonate — Copper corrodes in moist air containing carbon dioxide to form a dull green layer of basic copper carbonate (a mixture of copper hydroxide and copper carbonate, Cu(OH)₂.CuCO₃).
MCQ 18. Which type of reaction is primarily responsible for the formation of a precipitate?
- A. Combination reaction
- B. Decomposition reaction
- C. Displacement reaction
- D. Double displacement reaction
Answer: D. Double displacement reaction — Double displacement reactions often involve the exchange of ions between two compounds, leading to the formation of an insoluble product known as a precipitate. These are specifically called precipitation reactions.
MCQ 19. Which of the following statements about chemical equations is incorrect?
- A. They represent the reactants and products of a chemical reaction.
- B. They can be balanced by altering the subscripts of the elements.
- C. They provide information about the states of reactants and products.
- D. They follow the Law of Conservation of Mass.
Answer: B. They can be balanced by altering the subscripts of the elements. — Chemical equations are balanced by adjusting the coefficients in front of the formulas, not by altering the subscripts. Altering subscripts changes the chemical identity of the substances.
MCQ 20. What does a chemical equation symbolically represent?
- A. Physical changes
- B. Mixtures
- C. Chemical reactions
- D. Elements and compounds
Answer: C. Chemical reactions — A chemical equation uses chemical formulas and symbols to represent the reactants and products involved in a chemical reaction, showing the transformation of substances.
How to Use These Solutions
Attempt each question yourself first and only then compare with the worked answer. Marks in the CBSE board exam are awarded for the METHOD as much as the final result, so reproduce the steps rather than memorising the last line. Where a solution states a law, a formula or a definition, learn that wording — examiners look for it.
Related practice for this chapter: chapter MCQs, previous-year questions, revision notes and sample papers.