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Very Short Answers - Atoms and Molecules, Science, Class 9 Class 9 Notes | EduRev

Class 9 Science by VP Classes

Class 9 : Very Short Answers - Atoms and Molecules, Science, Class 9 Class 9 Notes | EduRev

The document Very Short Answers - Atoms and Molecules, Science, Class 9 Class 9 Notes | EduRev is a part of the Class 9 Course Class 9 Science by VP Classes.

All you need of Class 9 at this link: Class 9

Q1. Define law of conservation of mass.
Ans. In a chemical reaction mass can neither be created nor destroyed.
E.g., 2Na + Cl₂ → 2NaCl ₂ × 23 + 2 × 35.5 → 2(23 + 35.5)

Q2. Explain law of constant proportion.
Ans. In a chemical substance the elements are always present in definite proportions by mass.
E.g., In water, the ratio of the mass of hydrogen to the mass of oxygen H : O is always 1 : 8

Q3. Who coined the term atom?
Ans. John Dalton coined the term atom.

Q4. Define atom.
Ans. The smallest particle of matter, which can take part in a chemical reaction is called atom.

Q5. Define molecule.
Ans. The smallest particle of an element or compound which can exist independently is called molecule.

Q6. Define atomicity.
Ans. The number of atoms constituting a molecule is known as its atomicity.

Q7. What is atomic mass unit?
Ans. The sum of the atomic masses of all the atoms in a molecule of the substance is expressed in atomic mass unit.
E.g., H₂O = (1 × 2) + 16 = 18 amu

Q8. How do atoms exist?
Ans. Atoms exist in the form of atom, molecule or ions.

Q9. Give the atomicity of phosphorous and nitrogen.
Ans. The atomicity of phosphorus is P₄ i.e., 4.
The atomicity of nitrogen is N₂ i.e., 2.

Q10. What is an ion?
Ans. Charged atom is called as an ion. The ion can be positively charged called cation or negatively charged called anion.

Q11. Give one example of cation and anion.
Ans. Cation ⇒ Na⁺ Anion ⇒ Cl⁻

Q12. Give one difference between cation and anion.
Ans. Cations are positively charged ion.
Anions are negatively charged ion.

Q13. Give the chemical formula for ammonium sulphate.
Ans. Ammonium sulphate NH₄⁺ SO₄²⁻
Chemical formula → (NH₄)₂SO₄.

Q14. What is Avogadro’s constant?
Ans. The Avogadro’s constant (6.022 × 10²³) is defined as the number of atoms that are present in exactly 12 g of carbon–12.

Q15. Find the molecular mass of H₂O.
Ans. Molecular mass of H₂O = (2 × 1) + (16) = 2 + 16 = 18 u

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