10th Chemistry Important Questions with Answers

Demystify chemical equations and reactions with our expert-compiled guide to 10th Chemistry Important Questions. Scoring high in Chemistry requires a strong grasp of conceptual logic, systematic balancing of reactions, and precise technical terms. This comprehensive archive narrows down the most high-yield topics across your entire syllabus—from acids, bases, and salts to carbon compounds and metallurgy. Whether you are looking to master chemical properties or practice application-based reasoning questions, this resource provides the ultimate blueprint to streamline your revision and secure top marks.

GRADE X

10TH SCIENCE (CHEMISTRY)

Important Questions

1. Define: Relative atomic mass.

Relative atomic mass of an element is the ratio between the average mass of its isotopes to 1/12^th part of the mass of a carbon-12 atom. It is denoted as A_r. It is otherwise called "Standard Atomic Weight".

A_r = Average mass of the isotopes of the element / (1/12 of the mass of one Carbon-12 atom)

2. Write the different types of isotopes of oxygen and its percentage abundance.

Isotope	Mass (amu)	% abundance
₈O¹⁶	15.9949	99.757
₈O¹⁷	16.9991	0.038
₈O¹⁸	17.9992	0.205

3. Define: Atomicity

Atomicity is the number of atoms in a molecule of an element.

4. Give any two examples for heterodiatomic molecules.

Two examples of heterodiatomic molecules:

HCl (Hydrogen chloride)
CO (Carbon monoxide)
HBr (Hydrogen Bromide)
HF (Hydrogen Fluoride)

5. What is Molar volume of a gas?

The molar volume of a gas is the volume occupied by 1 mole of the gas at S.T.P., which is 22.4 litres (22,400 ml).

6. Name the acid that renders aluminium passive. Why?

Acid: Concentrated Nitric Acid (HNO₃)

Reason: It forms a protective layer of Al₂O₃ (aluminium oxide) on the surface, preventing further reaction.

a) Identify the bond between H and F in HF molecule.
Bond: Polar covalent bond

b) What property forms the basis of identification?
Electronegativity difference (H = 2.1, F = 4.0)

c) How does the property vary in periods and in groups?
In periods: Electronegativity increases from left to right.
In groups: Electronegativity decreases from top to bottom.

8. Define the term: Solution

A solution is a homogeneous mixture of two or more substances, where the solute is uniformly distributed within the solvent. Eg: Sea Water

9. What is meant by binary solution?

A binary solution is a solution that consists of two components: one solute and one solvent.

10. Give an example for each:

i) Gas in liquid: Carbon dioxide in water (soda water).

ii) Solid in liquid: Sodium chloride in water.

iii) Solid in solid: Copper dissolved in gold (alloy).

iv) Gas in gas: Mixture of helium and oxygen.

11. What is aqueous and non-aqueous solution? Give an example.

• Aqueous solution: A solution in which water is the solvent (e.g., salt in water).

• Non-aqueous solution: A solution in which the solvent is a liquid other than water (e.g., sulphur in carbon disulphide).

12. Define Volume percentage

Volume percentage is defined as the percentage by volume of solute present in the given volume of the solution.

Volume Percentage = (Volume of solute / Volume of solution) × 100

13. The aquatic animals live more in cold region. Why?

Aquatic animals live more in cold regions because cold water holds more dissolved oxygen compared to warm water. Solubility of oxygen is more in water at low temperature.

14. Define Hydrated salt.

A hydrated salt is a salt that contains a definite number of water molecules (water of crystallization) as part of its crystalline structure.

15. A hot saturated solution of copper sulphate forms crystals as it cools. Why?

As the solution cools, the solubility of copper sulphate decreases, causing the excess solute to crystallize out.

16. Classify the following substances into deliquescent, hygroscopic:

• Deliquescent: Conc. Sulphuric acid, Calcium chloride, Gypsum salt

• Hygroscopic: Copper sulphate pentahydrate, Silica gel

17.

a) What happens when MgSO₄·7H₂O is heated? Write the appropriate equation:
When magnesium sulphate heptahydrate (MgSO₄·7H₂O) is heated, it loses its water of crystallization and becomes anhydrous magnesium sulphate.
Equation: MgSO₄·7H₂O → MgSO₄ + 7H₂O

b) Define Solubility:
Solubility is the amount of solute that can be dissolved in 100 g of solvent to form a saturated solution at a given temperature and pressure.

Solubility of the solute = (Mass of the solute / Mass of the solvent) × 100

18. In what way hygroscopic substances differ from deliquescent substances?

Property	Hygroscopic Substances	Deliquescent Substances
Moisture Absorption	Absorb moisture from the air but do not dissolve.	Absorb moisture from the air and dissolve in it.
Physical State Change	Do not change their physical state on exposure.	Change their physical state (solid to liquid) on exposure.
Form	Can be amorphous solids or liquids.	Typically crystalline solids.
Example	Silica gel, concentrated sulphuric acid.	Sodium hydroxide (NaOH), Calcium chloride (CaCl₂).

19. A solution is prepared by dissolving 45 g of sugar in 180 g of water. Calculate the mass percentage of solute.

Mass of solute (sugar) = 45 g
Mass of solvent (water) = 180 g
Mass Percentage = [Mass of Solute / (Mass of Solute + Mass of Solvent)] × 100
Mass Percentage = [45 / (45 + 180)] × 100 = (45 / 225) × 100 = 20%
The mass percentage of solute (sugar) is 20%.

20. When an aqueous solution of potassium chloride is added to an aqueous solution of silver nitrate, a white precipitate is formed. Give the chemical equation of this reaction.

KCl_(aq) + AgNO_3(aq) → AgCl_(s) + KNO_3(aq)

The white precipitate formed is Silver Chloride. The other product is Potassium Nitrate.

21. Why does the reaction rate of a reaction increase on raising the temperature?

Increasing the temperature of a reaction provides energy to break more bonds and thus it speeds up the reaction.

For Example: Calcium carbonate reacts slowly with hydrochloric acid at room temperature. When the reaction mixture is heated the reaction rate increases.

22. Define combination reaction. Give one example for an exothermic combination reaction.

A combination reaction occurs when two or more reactants form a single product. It is otherwise called 'synthesis reaction' or 'composition reaction'. When a reactant 'A' combines with 'B', it forms the product 'AB'. Most of the combination reactions are exothermic in nature.

Example: H₂ + Cl₂ → 2HCl

23. Differentiate reversible and irreversible reactions

REVERSIBLE REACTION	IRREVERSIBLE REACTION
It can be reversed under suitable conditions.	It cannot be reversed.
Both forward and backward reactions take place simultaneously.	It is unidirectional. It proceeds only in forward direction.
It attains equilibrium.	Equilibrium is not attained.
The reactants cannot be converted completely into products.	The reactants can be completely converted into products.
It is relatively slow.	It is fast.

24. What are called thermolysis reactions?

Thermolysis is a type of decomposition reaction where a compound breaks down into two or more simpler substances upon heating. The molecule is dissociated by absorbing heat. It is also called Thermal Decomposition.

For example: CaCO_3(s) →_(heat) CaO_(s) + CO_2(g). Here, calcium carbonate decomposes into calcium oxide and carbon dioxide when heated.

On heating, mercury (II) oxide is decomposed into mercury metal and oxygen gas: 2HgO_(s) →_(heat) 2Hg_(l) + O_2(g).

25. Name the simplest ketone and give its structural formula.

The simplest ketone is propanone (acetone).

          H   O   H

          |   ||  | 

        H-C - C - C-H

          |       | 

          H       H

26. How is ethanoic acid prepared from ethanol? Give the chemical equation.

Ethanoic acid is prepared on a large scale by the oxidation of ethanol in the presence of alkaline potassium permanganate or acidified potassium dichromate.

CH₃CH₂OH + 2[O] →_{(KMnO₄ / OH^-)} CH₃COOH + H₂O

CH₃CH₂OH + 2[O] →_{(K₂Cr₂O₇ / H⁺)} CH₃COOH + H₂O

27. Give the salient features of "Modern atomic theory".

An atom is no longer indivisible (after the discovery of the electron, proton, and neutron).
Atoms of the same element may have different atomic mass (discovery of isotopes ₁₇Cl³⁵, ₁₇Cl³⁷).
Atoms of different elements may have same atomic masses (discovery of isobars ₁₈Ar⁴⁰, ₂₀Ca⁴⁰).
Atoms of one element can be transmuted into atoms of other elements.
Atoms may not always combine in a simple whole number ratio (E.g. Glucose C₆H₁₂O₆; C:H:O = 6:12:6 or 1:2:1).
Atom is the smallest particle that takes part in a chemical reaction.
The mass of an atom can be converted into energy (E = mc²).

28. A is a reddish brown metal, which combines with O₂ at < 1370 K gives B, a black coloured compound. At a temperature > 1370 K, A gives C which is red in colour. Find A, B and C with reaction.

A: Copper (Cu)
B: CuO (Copper(II) oxide) - black in colour
C: Cu₂O (Copper(I) oxide) - red in colour

Reactions:
Below 1370 K: 2Cu + O₂ → 2CuO (black)
Above 1370 K: 4Cu + O₂ → 2Cu₂O (red)

29. A is a silvery white metal. A combines with O₂ to form B at 800°C, the alloy of A is used in making the aircraft. Find A and B

A: Aluminium (Al)
B: Al₂O₃ (Aluminium oxide)
Reaction: 4Al + 3O₂ →_(800°C) 2Al₂O₃

30. What is rust? Give the equation for formation of rust.

When iron is exposed to moist air, it forms a layer of brown hydrated ferric oxide on its surface. This compound is known as rust and the phenomenon of formation of rust is known as rusting.

4Fe + 3O₂ + xH₂O → 2Fe₂O₃·xH₂O (rust)

31. State two conditions necessary for rusting of iron:

a. Presence of moisture (water)
b. Presence of oxygen

32. Write notes on:

i) Saturated Solution: A solution in which no more solute can be dissolved in a definite amount of solvent at a given temperature is called a saturated solution. Example: 36 g of sodium chloride dissolves in 100 g of water at 25°C to form a saturated solution.

ii) Unsaturated Solution: A solution that contains less solute than a saturated solution at a given temperature is called an unsaturated solution. Example: 20 g of sodium chloride in 100 g of water at 25°C forms an unsaturated solution.

33. Factors Affecting Solubility:

Nature of Solute and Solvent: Polar solutes dissolve in polar solvents (e.g., salt in water); non-polar solutes dissolve in non-polar solvents (e.g., fat in ether).
Effect of Temperature: Solubility of solids in liquids generally increases with temperature, while solubility of gases in liquids decreases.
Effect of Pressure: Increases the solubility of gases in liquids.

34. Action of Heat on Blue Vitriol:

CuSO₄·5H₂O (A) is a blue-colored crystalline salt containing water of crystallization. On heating, it loses its water of crystallization and turns white, forming B (CuSO₄, anhydrous copper sulphate):

CuSO₄·5H₂O →_(heat) CuSO₄ + 5H₂O

When water is added to B (CuSO₄), it turns blue again, reforming A:

CuSO₄ + 5H₂O → CuSO₄·5H₂O

35. Explain the types of double displacement reactions with examples.

When two compounds react, if their ions are interchanged, then the reaction is called double displacement reaction.

Precipitation Reactions: A solid precipitate forms when two solutions of soluble salts are mixed:
Na₂SO_4(aq) + BaCl_2(aq) → BaSO_4(s)↓ + 2NaCl_(aq)

Neutralization Reactions: An acid reacts with a base to produce a salt and water:
HCl_(aq) + NaOH_(aq) → NaCl_(aq) + H₂O_(l)

36. Explain the factors influencing the rate of a reaction:

Nature of reactants: Sodium reacts faster with HCl than with acetic acid because HCl is stronger and more reactive.
Concentration of reactants: Higher concentration causes more frequent collisions, increasing the reaction rate.
Temperature: Raising temperature provides more kinetic energy to break bonds, accelerating the reaction.
Surface Area: Powdered calcium carbonate reacts much faster with HCl than large marble chunks due to greater exposed surface area.
Catalyst: Increases reaction speed without being consumed.
Pressure: For gaseous reactions, higher pressure brings particles closer, resulting in more collisions.

37. Role of pH in everyday life:

• Biology: Blood pH is maintained around 7.4. Basic toothpastes neutralize acid to prevent tooth decay.

• Agriculture: Citrus fruits prefer alkaline soil, potatoes prefer acidic soil, and sugarcane prefers neutral soil.

38. Environmental Impact: Acid rain (pH below 5.6) damages plants, soil, and aquatic ecosystems.

39. Chemical Equilibrium: The state of a reversible reaction where the rates of forward and backward reactions are equal:
Rate of forward reaction = Rate of backward reaction
Properties like pressure, concentration, color, and density remain unchanged over time; it is a dynamic equilibrium.

40. Differentiate soaps and detergents.

Feature	Soap	Detergent
Composition	Sodium salts of long chain fatty acids	Sodium salts of sulphonic acids
In Hard Water	Ineffective due to insoluble scum formation	Effective even in hard water
Biodegradability	Biodegradable	Often non-biodegradable (branched chains)
Source	Derived from natural fats or vegetable oils	Prepared from hydrocarbons of crude oil
Scum Formation	Forms scum in hard water	Does not form scum in hard water
Foaming Capacity	Poor foaming capacity in hard water	Rich foaming capacity

41. Homologous Series & Characteristics:

A homologous series is a group of organic compounds with the same functional group and similar chemical properties, differing by a –CH₂– unit in their molecular formula.

Same general molecular formula (e.g., C_nH_2n+2 for alkanes).
Same functional group and same chemical properties.
Physical properties (boiling point, melting point) show a gradual gradation.
All members can be prepared by common general methods.

42. Systematic IUPAC name of CH₃-CH₂-CH₂-OH:

Step 1: Longest carbon chain has 3 carbons → Root word "Prop"
Step 2: Carbon-carbon single bonds → Primary suffix "ane" → "Propane"
Step 3: Functional group is –OH (alcohol) → Secondary suffix "-ol"
Step 4: Locant number for –OH is 1 → "1-ol"
Step 5: Drop terminal 'e' of propane: Propan-1-ol

43. Manufacture of ethanol from sugarcane (Molasses):

Manufactured by the fermentation of molasses (contains ~30% sucrose):

(i) Dilution: Molasses is diluted to 8–10% sugar concentration.

(ii) Nitrogen source: Fortified with ammonium sulphate if nutrient content is low.

(iii) Yeast addition: Maintained at ~303 K. Invertase and zymase convert sugar to ethanol:
C₁₂H₂₂O₁₁ + H₂O →_(invertase) C₆H₁₂O₆ (glucose) + C₆H₁₂O₆ (fructose)
C₆H₁₂O₆ →_(zymase) 2C₂H₅OH + 2CO₂↑

(iv) Distillation: Fractional distillation of the 'wash' (15–18% alcohol) yields rectified spirit (95.5% ethanol). Refluxing with quicklime produces absolute alcohol (100% pure).

44. Alcohol with molecular formula C₄H₁₀O and locant number 2 for -OH:

(i) Structural formula:

            H   H   H   H

            |   |   |   |

          H-C - C - C - C-H

            |   |   |   |

            H  OH   H   H

(ii) IUPAC Name: Butan-2-ol

(iii) Nature: Saturated (contains only carbon-carbon single bonds).

45. Esterification Reaction:

(i) Compound 'A': Ethanoic acid (CH₃COOH)
(ii) Chemical Equation:
C₂H₅OH + CH₃COOH →_{(conc. H₂SO₄)} CH₃COOC₂H₅ + H₂O
(CH₃COOC₂H₅ is the sweet-smelling ester ethyl ethanoate)
(iii) Process Name: Esterification

46. What is an Amalgam? Give an example.
An alloy of mercury with another metal. Example: Silver-tin amalgam used in dental fillings.

47. Uses of Copper: Electric cables and appliances, utensils, electroplating, and alloyed with gold/silver in jewelry.

48. Uses of Aluminium: Household utensils, overhead transmission cables, aircraft and industrial parts.

49. What is Corrosion?

The gradual destruction of metals by chemical or electrochemical reaction with environmental gases, converting them into oxides, hydroxides, or sulphides.

50. Types of Corrosion:
• Dry/Chemical Corrosion: Occurs at high temperatures in the absence of moisture directly by gases like O₂.
• Wet/Electrochemical Corrosion: Occurs in the presence of moisture/water or electrolyte solutions.

51. Prevention of Corrosion:

Alloying: Combining with other metals (e.g., Stainless Steel).
Galvanization: Coating iron sheets with a thin protective layer of zinc.
Electroplating: Coating a superior metal over another using electric current.
Anodizing: Electrochemical oxidation creating a durable oxide layer (widely used for aluminium).
Cathodic Protection: Attaching a sacrificial anode (more electropositive metal) to protect the primary metal.

52. Alloy and Reasons for Alloying:

An alloy is a homogeneous mixture of two or more metals, or metals with non-metals. Reasons: to modify appearance/color, alter reactivity, lower melting point, increase hardness/tensile strength, and modify electrical resistance.

53. Types and Uses of Iron:
• Pig iron: Pipes, stoves, radiators, railings, manhole covers.
• Steel: Building construction, machinery, TV towers, transmission cables.
• Wrought iron: Springs, anchors, electromagnets.

54. Chief Ore of Iron: Haematite (Fe₂O₃).

55. Applications of Avogadro's Law:

Explains Gay-Lussac's law of combining volumes.
Helps determine the atomicity of elementary gases.
Derives molecular formulas of gases.
Relates relative molecular mass and vapour density: Molecular Mass = 2 × Vapour Density.
Determines the gram molar volume of any gas (22.4 litres at S.T.P.).