CBSE CLASS 10 SCIENCE
Syllabus-aligned Chemical Reactions and Equations questions with clear answers, explanations and application-focused practice for CBSE Class 10 Science.
Questions
18
Chapter
Chemical Reactions
Resource
Important Questions
Answer
A chemical equation is balanced to satisfy the law of conservation of mass. The balanced equation is .
Solution
Count each element on both sides and choose coefficients so that the number of atoms is equal. Three iron atoms, four water molecules and four hydrogen molecules give the required balance.
Answer
The browning is oxidation. It can be slowed by adding lemon juice or keeping the cut surface away from air.
Solution
Oxygen in air reacts with substances in the apple. Lemon juice provides an acidic medium and antioxidants, while limiting contact with air reduces the rate of oxidation.
Answer
Combination reaction.
Solution
Two reactants combine to form a single product, so this is a combination reaction.
Answer
. It is a photochemical decomposition reaction.
Solution
Silver chloride breaks into silver and chlorine when exposed to sunlight. One compound gives two simpler substances.
Answer
The blue colour fades and reddish-brown copper is deposited. .
Solution
Zinc is more reactive than copper and displaces it from copper sulphate. Colourless zinc sulphate remains in solution and copper is deposited.
Answer
. It is a double displacement and precipitation reaction.
Solution
The ions exchange partners and insoluble yellow lead iodide separates as a precipitate.
Answer
Respiration releases energy as glucose is oxidised. Glucose + oxygen gives carbon dioxide + water + energy.
Solution
The chemical energy stored in glucose is released during its oxidation in cells, making respiration exothermic.
Answer
It is an endothermic reaction and absorbs heat. .
Solution
Heat supplies the energy needed to break calcium carbonate into calcium oxide and carbon dioxide.
Answer
Hydrogen is oxidised to water and copper(II) oxide is reduced to copper.
Solution
Hydrogen gains oxygen, so it is oxidised. Copper(II) oxide loses oxygen, so it is reduced. The reaction is a redox reaction.
Answer
Moisture and dissolved salts near the sea speed up corrosion. Painting, oiling, galvanising or alloying can prevent rusting.
Solution
Rusting needs oxygen and water; salts increase the conductivity of the moisture film. A protective coating or zinc layer keeps iron from contact with air and water.
Answer
Nitrogen reduces contact with oxygen and slows oxidation of oils, preventing rancidity.
Solution
Rancidity is oxidation of fats and oils, producing unpleasant smell and taste. Replacing oxygen with an inert gas delays this reaction.
Answer
An insoluble solid appears in the solution.
Solution
A precipitate is an insoluble product that separates from a solution during a chemical reaction.
Answer
There are two oxygen atoms on the left but one on the right. The balanced equation is .
Solution
The coefficients are adjusted so that hydrogen and oxygen atoms are equal on both sides, satisfying conservation of mass.
Answer
The black substance is copper(II) oxide. .
Solution
Copper combines with oxygen on heating to form black copper(II) oxide. Copper is oxidised in this reaction.
Answer
Both the Assertion and Reason are true, and the Reason correctly explains the Assertion.
Solution
Combustion releases energy to the surroundings. The reaction is , so both statements are correct.
Answer
It is both a combination and an oxidation reaction.
Solution
Two reactants form one product, so it is combination. Magnesium also gains oxygen, so it is oxidation.
Answer
Hydrogen is released: .
Solution
Zinc displaces hydrogen from hydrochloric acid. The gas burns with a characteristic pop sound.
Answer
Magnesium: combination and oxidation; silver chloride: photochemical decomposition; iron and copper sulphate: displacement.
Solution
is combination and oxidation. in sunlight is decomposition. is displacement.
