Q1: What happens to the total mass when salt dissolves in water? (a) The mass increases (b) The mass decreases (c) The mass remains unchanged (d) The mass becomes zero
Solution:
Ans: (c) Explanation: During physical changes like dissolving, no new substance forms and total mass stays constant. The solution's mass equals the combined masses of salt and water.
Q2: Which scientist proposed the Law of Conservation of Mass in 1789? (a) John Dalton (b) Joseph Louis Proust (c) Antoine Lavoisier (d) Raja Ramanna
Solution:
Ans: (c) Explanation: Antoine Lavoisier, known as the Father of Modern Chemistry, proposed the Law of Conservation of Mass stating matter cannot be created or destroyed.
Q3: In water, what is the mass ratio of hydrogen to oxygen? (a) 1 : 16 (b) 1 : 8 (c) 2 : 8 (d) 8 : 1
Solution:
Ans: (b) Explanation: According to the Law of Constant Proportions, water always contains hydrogen and oxygen in a fixed mass ratio of 1:8 regardless of its source.
Q4: Which type of bond is formed when atoms share electrons? (a) Ionic bond (b) Covalent bond (c) Metallic bond (d) Hydrogen bond
Solution:
Ans: (b) Explanation: A covalent bond forms when atoms share electrons to achieve stability. Examples include hydrogen molecule (H₂) and oxygen molecule (O₂) formations.
Q5: What is the molecular mass of water (H₂O)? (Atomic mass: H = 1 u, O = 16 u) (a) 16 u (b) 17 u (c) 19 u (d) 18 u
Solution:
Ans: (d) Explanation: Molecular mass of water equals the sum of atomic masses: (1 u × 2) + (16 u × 1) = 18 u.
Fill in the Blanks
Q1: The Law of Conservation of Mass states that matter can neither be created nor _____ in a chemical reaction.
Solution:
Ans: destroyed
Q2: A positively charged ion is called a _____.
Solution:
Ans: cation
Q3: Oxygen molecule (O₂) contains a _____ covalent bond.
Solution:
Ans: double
Q4: Ionic compounds conduct electricity in _____ or molten state.
Solution:
Ans: aqueous
Q5: The formula unit mass is the mass of the simplest whole number ratio of ions in a(n) _____ compound.
Solution:
Ans: ionic
True or False
Q1: In an open system, when baking soda reacts with vinegar, the measured mass appears to decrease because carbon dioxide gas escapes.
Solution:
Ans: True Explanation: Gas escapes in open systems, making the measured mass seem lower. In closed systems, total mass remains constant as gas is trapped.
Q2: Atoms with more than 4 valence electrons tend to donate electrons to achieve stability.
Solution:
Ans: False Explanation: Atoms with more than 4 valence electrons tend to gain or share electrons, while those with less than 4 tend to donate electrons.
Q3: Covalent compounds are generally soluble in water but insoluble in kerosene and petrol.
Solution:
Ans: False Explanation: Covalent compounds like camphor and naphthalene are insoluble in water but soluble in kerosene and petrol, unlike ionic compounds which dissolve in water.
Q4: John Dalton postulated that atoms of a given element are identical in mass and chemical properties.
Solution:
Ans: True Explanation: This is one of Dalton's postulates stating all atoms of the same element have identical mass and chemical properties distinguishing them from other elements.
Q5: Ionic compounds have lower melting and boiling points compared to covalent compounds.
Solution:
Ans: False Explanation: Ionic compounds have high melting and boiling points due to strong electrostatic forces, while covalent compounds usually have lower melting and boiling points.
Match the Following
Column A
Column B
1. Law proposed by Antoine Lavoisier
A. Sharing of electrons between atoms
2. Covalent bond formation
B. Negatively charged ion
3. Anion
C. Transfer of electrons forming oppositely charged ions
4. Ionic bond formation
D. Law of Conservation of Mass
5. Law of Constant Proportions
E. Proposed by Joseph Louis Proust
Solution:
Ans:
1 - D: Antoine Lavoisier proposed the Law of Conservation of Mass in 1789 stating matter cannot be created or destroyed in reactions.
2 - A: Covalent bonds form when atoms share valence electrons to achieve stable electronic configuration, like in hydrogen or chlorine molecules.
3 - B: An anion is a negatively charged ion formed when an atom gains electrons, such as chloride ion (Cl⁻) with 18 electrons.
4 - C: Ionic bonds form through electron transfer where one atom loses electrons forming cations and another gains forming anions, like in NaCl.
5 - E: Joseph Louis Proust proposed elements in compounds are always present in fixed mass ratios regardless of source or preparation method.
Short Answer Questions
Q1: How does the Law of Conservation of Mass apply to chemical reactions? Explain with reference to a closed system.
Solution:
Ans: The Law of Conservation of Mass states that matter cannot be created or destroyed during chemical reactions. In closed systems, total mass before reaction equals total mass after reaction. When baking soda reacts with vinegar producing carbon dioxide, if conducted in a closed container with a balloon, the gas is trapped and total mass remains unchanged, verifying this law.
Q2: Explain how a covalent bond is formed in a hydrogen molecule (H₂).
Solution:
Ans: Hydrogen atoms have one electron in their K-shell and need one more to become stable with a duplet configuration. Two hydrogen atoms share one electron each to complete their shells. This shared pair of electrons forms a single covalent bond between them. The resulting hydrogen molecule is represented as H-H or H₂ and exists as a stable entity.
Q3: Describe the formation of sodium chloride (NaCl) through ionic bonding.
Solution:
Ans: Sodium has one valence electron which it loses to achieve stability, forming a positively charged cation (Na⁺) with 11 protons and 10 electrons. Chlorine has seven valence electrons and gains one electron to complete its octet, forming a negatively charged anion (Cl⁻) with 17 protons and 18 electrons. The electrostatic attraction between oppositely charged Na⁺ and Cl⁻ ions creates an ionic bond.
Q4: What is the Law of Constant Proportions? Provide an example to support your answer.
Solution:
Ans: The Law of Constant Proportions, proposed by Joseph Louis Proust, states that elements in any chemical compound are always present in a fixed ratio by mass, regardless of source or preparation method. For example, water always contains hydrogen and oxygen in the mass ratio of 1:8. Thus, 9 grams of water will always decompose to give 1 gram of hydrogen and 8 grams of oxygen.
Q5: Why do ionic compounds conduct electricity in aqueous or molten state but not in solid state?
Solution:
Ans: In solid state, ions in ionic compounds are fixed in their crystal lattice positions and cannot move freely, preventing electrical conductivity. However, in aqueous solution or molten state, these ions become free to move. The movement of charged ions allows the flow of electric current, enabling ionic compounds to conduct electricity under these conditions but not when solid.
Long Answer Questions
Q1: Analyze how Dalton's Atomic Theory explains the Law of Conservation of Mass and the Law of Constant Proportions in chemical reactions.
Solution:
Ans: Dalton's Atomic Theory explains both laws through its postulates. It states atoms are indivisible and cannot be created or destroyed during chemical reactions, only rearranged. This directly explains the Law of Conservation of Mass as total mass remains unchanged when atoms simply rearrange. The theory also states atoms combine in simple whole number ratios and that the relative number and kinds of atoms in a compound remain constant. This explains the Law of Constant Proportions, as fixed atom ratios mean fixed mass ratios in compounds regardless of source.
Q2: Compare the properties of ionic and covalent compounds in terms of solubility, electrical conductivity, and melting points.
Solution:
Ans:
Solubility: Ionic compounds like sodium chloride dissolve in water but not in kerosene or petrol, while covalent compounds like camphor dissolve in kerosene and petrol but not in water.
Electrical conductivity: Ionic compounds conduct electricity in aqueous or molten state due to free ions, but not in solid state. Covalent compounds generally do not conduct electricity even in solution as they lack free ions.
Melting and boiling points: Ionic compounds have high melting and boiling points due to strong electrostatic forces, whereas covalent compounds usually have lower melting and boiling points.
Q3: Justify why atoms combine to form molecules and explain the two main ways atoms achieve stability when combining.
Solution:
Ans: Atoms combine to form molecules because they seek to achieve a stable electronic configuration with complete outermost shells (octet for most, duplet for hydrogen and helium). When atoms combine, the total energy of the system becomes lower than individual atoms' energies, making the arrangement more stable. Atoms achieve stability through two main methods: sharing electrons to form covalent bonds (like in H₂ and O₂ molecules), or transferring electrons to form ionic bonds where one atom loses electrons becoming a cation and another gains electrons becoming an anion (like in NaCl).
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