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CBSE Class 9  >  Class 9 Notes  >  Science   >  Unit Test: Atoms and Molecules

Unit Test: Atoms and Molecules

Time: 1 hour
M.M. 30
Attempt all questions. 
Questions 1 to 5 carry 1 mark each. 
Questions 6 to 8 carry 2 marks each. 
Questions 9 to 11 carry 3 marks each. 
Questions 12 & 13 carry 5 marks each.

Q1. Who proposed the concept of indivisible particles called atoms? (1 Mark)

Q2. Define an atomic mass unit. (1 Mark)

Q3. What is the valency of the sodium ion? (1 Mark)

Q4. Which law states that elements in a compound are present in definite proportions by mass? (1 Mark)

Q5. What is the atomicity of the oxygen molecule (O2)? (1 Mark)

Q6. State the Law of Conservation of Mass with an example. (2 Marks)

Q7. Differentiate between an atom and a molecule. (2 Marks)

Q8. Why do we use brackets while writing formulae of compounds like Ca(OH)₂ and not in NaOH? (2 Marks)

Q9. Explain Dalton's atomic theory postulates that support the Law of Conservation of Mass and Law of Definite Proportions. (3 Marks)

Q10. Calculate the ratio by number of atoms for ammonia (NH3) using atomic masses. (3 Marks)

Q11. Write the chemical formulae for: (a) Aluminium oxide, (b) Calcium hydroxide, (c) Sodium sulphate. (3 Marks)

Q12. Explain how to write the chemical formula of a compound using valency, with examples of magnesium chloride and ammonium sulphate. (5 Marks)

Q13. Calculate the formula unit mass of: 
(a) NaCl,
 (b) CaCO3, 
Also, verify the Law of Conservation of Mass using the reaction: 5.3 g sodium carbonate + 6 g acetic acid → 8.2 g sodium acetate + 2.2 g carbon dioxide + 0.9 g water. (5 Marks)

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FAQs on Unit Test: Atoms and Molecules

1. What are atoms and molecules, and how do they differ?
Ans. Atoms are the basic units of matter and the defining structure of elements. Each atom consists of a nucleus, containing protons and neutrons, surrounded by electrons. Molecules, on the other hand, are formed when two or more atoms bond together chemically. The key difference is that atoms are single units, while molecules are combinations of atoms.
2. What are the different types of bonds that form between atoms to create molecules?
Ans. The main types of bonds that form between atoms to create molecules are ionic bonds, covalent bonds, and metallic bonds. Ionic bonds occur when electrons are transferred from one atom to another, resulting in charged ions. Covalent bonds form when atoms share electrons. Metallic bonds involve a sea of delocalized electrons shared among a lattice of metal atoms.
3. Can you explain the law of conservation of mass in the context of chemical reactions?
Ans. The law of conservation of mass states that in a closed system, the mass of the reactants must equal the mass of the products in a chemical reaction. This means that atoms are neither created nor destroyed during the reaction; they are simply rearranged to form new substances. This principle is fundamental in balancing chemical equations.
4. What is the significance of the atomic number and mass number in understanding elements?
Ans. The atomic number is significant because it defines the element and determines its position in the periodic table. It represents the number of protons in the nucleus of an atom. The mass number, on the other hand, is the total number of protons and neutrons in the nucleus. Together, these numbers help identify isotopes of an element and provide insights into its chemical behavior.
5. How do we represent chemical reactions, and why is it important to balance them?
Ans. Chemical reactions are represented using chemical equations, which show the reactants on the left side and the products on the right side, separated by an arrow. Balancing chemical equations is important because it reflects the law of conservation of mass, ensuring that the number of atoms of each element is the same on both sides of the equation. This helps to accurately depict the reaction and predict the amounts of reactants and products involved.
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