Exercise Solutions (From Screenshot Page 94)

Q1: Fill in the blanks.

1. A permanent change in which new substances are formed is called a chemical change.
2. A symbolic representation of a chemical reaction is called a chemical equation.
3. Substances that undergo chemical change are called reactants.
4. New substances formed in a chemical reaction are called products.
5. A force that holds atoms together in molecules or compounds is called a chemical bond.
6. A bond formed by the complete transfer of electrons is an ionic bond.
7. A bond formed by the mutual sharing of electrons is a covalent bond.
8. Positively charged ions are called cations.
9. Negatively charged ions are called anions.
10. The combining capacity of an element is called its valency.
11. Ionic compounds generally have high melting and boiling points.
12. Covalent compounds generally have low melting and boiling points.

Q2: Match the pairs.

(Note: As an AI, I cannot create interactive matching. I will provide the correct pairs.)

• Chemical change - Burning of paper
• Reactants - Left side of equation
• Products - Right side of equation
• Ionic bond - Electron transfer
• Covalent bond - Electron sharing
• Cation - Positive ion
• Anion - Negative ion
• Valency - Combining capacity
• NaCl - Ionic compound
• H₂O - Covalent compound

Q3: Give scientific reasons.

1. Ionic compounds conduct electricity in molten state or in aqueous solution.
   Reason: In the solid state, ions in an ionic compound are held in fixed positions by strong electrostatic forces, so they are not free to move and cannot conduct electricity. However, when an ionic compound is melted or dissolved in water, the strong electrostatic forces holding the ions together are overcome, allowing the ions to become mobile and free to move. These free-moving ions act as charge carriers, enabling the molten compound or its aqueous solution to conduct electricity.
2. Covalent compounds do not conduct electricity.
   Reason: Covalent compounds are formed by the mutual sharing of electrons between atoms, resulting in the formation of neutral molecules. Unlike ionic compounds, they do not contain free ions or free electrons that can move and carry electric charge. Since the conduction of electricity requires the presence of mobile charged particles, covalent compounds generally do not conduct electricity.
3. Atoms form chemical bonds.
   Reason: Atoms form chemical bonds to achieve a stable electronic configuration, typically by having a complete outermost electron shell. This stable configuration (usually 8 electrons, known as the octet rule, or 2 for the first shell, known as the duplet rule) makes them chemically unreactive, similar to noble gases. Atoms achieve this stability by gaining, losing, or sharing electrons with other atoms.
4. Chemical equations must be balanced.
   Reason: Chemical equations must be balanced to obey the Law of Conservation of Mass. This fundamental law states that mass can neither be created nor destroyed in a chemical reaction. Therefore, the total number of atoms of each element on the reactant side (left side of the equation) must be exactly equal to the total number of atoms of that same element on the product side (right side of the equation). Balancing ensures that matter is conserved during the reaction.

Q4: Answer the following questions.

1. Explain chemical change with examples.
   Answer: A chemical change is a permanent and irreversible process that results in the formation of one or more new substances with entirely different chemical compositions and properties from the original substances. This change involves the breaking of existing chemical bonds and the formation of new ones, leading to a rearrangement of atoms. Examples:
   • Burning of paper: Paper burns to form ash, carbon dioxide, and water vapor, all new substances.
   • Rusting of iron: Iron reacts with oxygen and moisture to form iron oxide (rust), a new substance.
   • Cooking food: Raw ingredients undergo chemical reactions to form cooked food with different tastes, textures, and chemical compositions.
   • Digestion of food: Complex food molecules are broken down into simpler ones through chemical reactions aided by enzymes.
2. Explain ionic bond with examples.
   Answer: An ionic bond (or electrovalent bond) is a type of chemical bond formed by the complete transfer of one or more electrons from a metal atom to a nonmetal atom. This transfer leads to the formation of oppositely charged ions: the metal atom loses electrons to become a positively charged cation, and the nonmetal atom gains electrons to become a negatively charged anion. These oppositely charged ions are then held together by strong electrostatic forces of attraction. Example: Formation of Sodium Chloride (NaCl):
   • Sodium (Na) atom (electronic configuration 2, 8, 1) loses its one valence electron to achieve a stable octet, forming a positively charged sodium ion (Na⁺).
   • Chlorine (Cl) atom (electronic configuration 2, 8, 7) gains this one electron to complete its octet, forming a negatively charged chloride ion (Cl⁻).
   • The strong electrostatic attraction between Na⁺ and Cl⁻ ions forms the ionic bond in NaCl.
3. Explain covalent bond with examples.
   Answer: A covalent bond is a type of chemical bond formed by the mutual sharing of one or more pairs of electrons between two nonmetal atoms. Atoms share electrons to achieve a stable electronic configuration, typically an octet (8 electrons) in their outermost shell (or a duplet of 2 electrons for hydrogen and helium). This sharing allows both atoms to effectively have a complete outer shell. Examples:
   • Formation of Hydrogen molecule (H₂): Each hydrogen atom has 1 electron. They share one pair of electrons, forming a single covalent bond (H-H), and each atom achieves a stable duplet.
   • Formation of Methane (CH₄): Carbon has 4 valence electrons and needs 4 more to complete its octet. Each of the four hydrogen atoms has 1 electron and needs 1 more for a duplet. Carbon shares one electron with each of the four hydrogen atoms, forming four single covalent bonds.
   • Oxygen molecule (O₂): Each oxygen atom needs 2 electrons. They share two pairs of electrons, forming a double covalent bond (O=O).
4. What is valency? How is it used to write chemical formulas?
   Answer: Valency: Valency is the combining capacity of an element. It represents the number of electrons an atom gains, loses, or shares to achieve a stable electronic configuration (a complete outermost shell, usually an octet or a duplet for hydrogen/helium). Using Valency to Write Chemical Formulas (Criss-Cross Method):
   1. Write the symbols of the elements (or polyatomic ions) side by side.
   2. Write their valencies (combining capacities) as superscripts above their respective symbols.
   3. Criss-cross the valencies, meaning the valency of the first element becomes the subscript of the second, and the valency of the second element becomes the subscript of the first.
   4. If the valencies are the same, they cancel out (subscript 1 is usually omitted). If they have a common factor, simplify the ratio.
   Example: Writing the formula for Aluminium Oxide
   • Symbols: Al, O
   • Valencies: Al (3), O (2)
   • Criss-cross: Al₂O₃
   So, the chemical formula for Aluminium Oxide is Al₂O₃.

Q5: Differentiate between.

1. Ionic Bond and Covalent Bond
   

   Feature	Ionic Bond	Covalent Bond
   Formation	Complete transfer of electrons	Mutual sharing of electrons
   Between	Metal and Nonmetal	Two Nonmetals
   Result	Formation of ions (cations & anions)	Formation of neutral molecules
   Conductivity	Conducts in molten/aqueous state	Does not conduct electricity
   Melting/Boiling Points	High	Low

2. Reactants and Products
   

   Feature	Reactants	Products
   Definition	Substances that undergo chemical change	New substances formed as a result of chemical change
   Position in Equation	Written on the left side of the arrow (→)	Written on the right side of the arrow (→)
   Chemical Identity	Original substances	Chemically different from reactants