CBSE Class 10 Science
Free chapter-wise CBSE Class 10 Science important questions with board-style answers, explanations and application practice across Chemistry, Biology and Physics.
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Subject
Science
Questions
35
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Chapter-wise Science Practice
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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.
Answer
Bacteria produce acids that lower the pH of the mouth below about . Basic toothpaste helps neutralise these acids.
Solution
Tooth enamel begins to dissolve in an acidic medium. Brushing removes food particles and the basic nature of toothpaste helps restore a safer pH.
Answer
. Carbon dioxide is released.
Solution
On heating, sodium hydrogen carbonate decomposes into sodium carbonate, water and carbon dioxide. The carbon dioxide can turn limewater milky.
Answer
In the solid state, ions are fixed in position. In the molten state, ions are free to move and carry electric charge.
Solution
Electrical conduction requires mobile charged particles. Heating breaks the rigid ionic lattice, allowing and ions to move.
Answer
A brown copper coating forms and the blue solution becomes green. .
Solution
Iron is more reactive than copper, so it displaces copper from copper sulphate solution.
Answer
Saturated compounds contain only single bonds; unsaturated compounds contain double or triple bonds. Unsaturated compounds decolourise bromine water.
Solution
Multiple bonds react with bromine by addition, removing its colour. Saturated compounds generally do not show this reaction under the same conditions.
Answer
Soap reacts with calcium and magnesium ions to form insoluble scum. Detergents form soluble compounds and remain effective in hard water.
Solution
The hydrophilic end of soap combines with or to produce insoluble salts. Detergent salts do not form this insoluble precipitate readily.
Answer
Loss of water vapour from leaves creates a suction force called transpiration pull, which helps draw water upward through xylem.
Solution
Evaporation lowers the water potential of leaf cells. The continuous water column in xylem experiences tension, pulling water from the roots.
Answer
. Chlorophyll absorbs light energy.
Solution
Carbon dioxide and water are converted into glucose and oxygen using light energy captured by chlorophyll in chloroplasts.
Answer
A reflex arc is coordinated mainly through the spinal cord, reducing the time required for the response.
Solution
Sensory neurons carry the impulse to the spinal cord, which sends an immediate motor response. The brain receives the information after the action begins.
Answer
DNA copying transfers body design information to offspring. Small copying errors create variations.
Solution
Reproduction needs a copy of genetic information, but biochemical copying is not absolutely error-free. These variations may help organisms adapt.
Answer
Genotypic ratio: . Phenotypic ratio: tall : dwarf.
Solution
The four combinations are , , and . Since is dominant, three plants are tall and one is dwarf.
Answer
, , so . The image forms at the centre of curvature.
Solution
Using , . Hence .
Answer
A concave lens corrects myopia by diverging incoming rays so that the eye lens forms the image on the retina.
Solution
In myopia, distant images form in front of the retina. A suitable concave lens reduces the converging power of the eye system and shifts the image back onto the retina.
Answer
.
Solution
Ohm's law gives . Rearranging, . Substitution gives a resistance of .
Answer
Parallel connection gives each appliance its rated voltage and independent operation. A fuse melts during excessive current and breaks the circuit.
Solution
In parallel, switching off one appliance does not stop the others. A fuse wire of suitable rating heats up and melts when current exceeds the safe limit.
Answer
Point the right thumb in the direction of current; the curled fingers show the direction of magnetic field lines.
Solution
The rule relates the direction of conventional current to the circular magnetic field around the conductor. Reversing current reverses the field direction.
Answer
The concentration increases because the pesticide is not broken down and accumulates through repeated consumption. This is biomagnification.
Solution
A top consumer receives the chemical stored in many organisms below it. Since the substance is non-biodegradable, its concentration becomes highest at the top trophic level.
