Important Questions For All Chapters – Science Class 9
Short Questions
1. Who postulated that matter is made up of very small particles called ‘Parmanu’?
Answer: Maharishi Kanad postulated that matter is made up of very small particles called ‘Parmanu’.
2. What is the Law of Conservation of Mass?
Answer: The Law of Conservation of Mass states that mass can neither be created nor destroyed in a chemical reaction.
3. Who stated the Law of Definite Proportions?
Answer: The Law of Definite Proportions was stated by Joseph L. Proust.
4. What are the building blocks of all matter?
Answer: The building blocks of all matter are atoms.
5. What is the unit of atomic radius?
Answer: Atomic radius is measured in nanometres (nm).
6. What is the full form of IUPAC?
Answer: The full form of IUPAC is International Union of Pure and Applied Chemistry.
7. What is one atomic mass unit equal to?
Answer: One atomic mass unit is equal to one-twelfth the mass of one atom of carbon-12.
8.What is the combining power of an element called?
Answer: The combining power of an element is called its valency.
9. What is a molecule?
Answer: A molecule is the smallest particle of an element or compound capable of independent existence.
10. What is a polyatomic ion?
Answer: A polyatomic ion is a group of atoms carrying a charge.
Long Questions
1. Explain the Law of Conservation of Mass with an example.
Answer: The Law of Conservation of Mass states that mass can neither be created nor destroyed in a chemical reaction. For example, when sodium carbonate reacts with acetic acid, the total mass of the reactants equals the total mass of the products — proving the law.
2. State and explain the Law of Constant Proportions.
Answer: The Law of Constant Proportions, stated by Proust, says that in a chemical substance, elements are always present in definite proportions by mass. For example, in water, hydrogen and oxygen are always present in the ratio 1:8 by mass.
3. Write any six postulates of Dalton’s Atomic Theory.
Answer:
(i) All matter is made of tiny particles called atoms.
(ii) Atoms are indivisible and indestructible.
(iii) Atoms of the same element are identical in mass and properties.
(iv) Atoms of different elements have different masses and properties.
(v) Atoms combine in simple whole-number ratios to form compounds.
(vi) The relative number and kinds of atoms in a compound are constant.
4. How big are atoms? Explain with examples.
Answer: Atoms are extremely small; millions of atoms stacked together make a layer barely as thick as a sheet of paper. For example, the radius of a hydrogen atom is 10⁻¹⁰ m, and that of a water molecule is about 10⁻⁹ m.
5. Explain how modern symbols of elements are written.
Answer: Modern symbols are approved by IUPAC. They are formed using one or two letters of the element’s English name. The first letter is capitalized and the second, if present, is in lowercase. For example, Hydrogen (H), Aluminium (Al), and Cobalt (Co).
6. What is atomic mass and how is it measured?
Answer: The atomic mass is the average mass of an atom compared to one-twelfth of the mass of a carbon-12 atom. It is measured in atomic mass units (u).
7. Differentiate between atoms and molecules.
Answer: Atoms are the smallest particles of an element that cannot usually exist independently, whereas molecules are groups of two or more atoms chemically bonded together and capable of independent existence.
8. What are molecules of elements and compounds?
Answer: Molecules of elements contain atoms of the same element (e.g., O₂, N₂), while molecules of compounds contain atoms of different elements combined in definite proportions (e.g., H₂O, CO₂).
9. Define ions and explain their types with examples.
Answer: Ions are charged species formed when atoms gain or lose electrons. Positively charged ions are cations (e.g., Na⁺), and negatively charged ions are anions (e.g., Cl⁻). Groups of atoms carrying a charge are polyatomic ions (e.g., SO₄²⁻).
10. What is valency? Explain with an example.
Answer: Valency is the combining capacity of an element. For example, hydrogen has a valency of 1 and oxygen has a valency of 2, so they combine in a 2:1 ratio to form water (H₂O).
11. Explain the rules for writing chemical formulae.
Answer:
(i) The valencies of ions must balance.
(ii) The metal’s symbol is written first.
(iii) When multiple ions are present, brackets are used with subscripts.
Example: Ca(OH)₂, Na₂CO₃.
12. What is molecular mass? How is it calculated?
Answer: The molecular mass is the sum of the atomic masses of all atoms in a molecule.
Example: For water (H₂O), molecular mass = (2 × 1) + 16 = 18 u.
13. What is formula unit mass? Give an example.
Answer: Formula unit mass is the sum of atomic masses of all atoms in a formula unit of an ionic compound.
Example: NaCl → (23 + 35.5) = 58.5 u.
14. State what you have learnt from this chapter.
Answer:
- Mass is conserved during chemical reactions.
- Elements in compounds combine in fixed proportions.
- Atoms and molecules are the smallest units of matter.
- Chemical formulae show the number and type of atoms.
- Valency and charge determine how atoms combine.
15. Explain the steps to write the formula of magnesium chloride.
Answer:
(i) Write the symbols and valencies: Mg²⁺ and Cl⁻.
(ii) Criss-cross the valencies: Mg¹Cl₂.
(iii) Simplify to get the formula MgCl₂.
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