Answer the following questions: What is atomicity? What is rusting? Distinguish between physical and chemical changes. What is crystallisation? What is precipitation reaction?

D. Answer the following questions:

1. What is atomicity? Atomicity is the number of atoms present in one molecule of an element. For example, the atomicity of oxygen (O₂) is 2, and the atomicity of phosphorus (P₄) is 4.

2. What is rusting? Rusting is a chemical process where iron or its alloys react with oxygen and moisture (water) to form hydrated iron(III) oxide, commonly known as rust. It is a form of corrosion.

3. Distinguish between physical and chemical changes.

   • Physical changes alter the physical properties of a substance (like shape, size, or state) but do not form new substances. They are often reversible.
   • Chemical changes result in the formation of one or more new substances with different chemical properties. They are generally irreversible.

4. What is crystallisation? How can you get crystals of sugar from its solution?

   • Crystallisation is a process used to separate a pure solid in the form of its crystals from a solution.
   • To get crystals of sugar from its solution, dissolve sugar in hot water to create a saturated solution. Allow the solution to cool slowly and undisturbed. As the solution cools, the sugar's solubility decreases, causing pure sugar crystals to form and separate out.

5. What is precipitation reaction? A precipitation reaction is a chemical reaction occurring in an aqueous solution where two soluble ionic compounds react to form an insoluble product, called a precipitate, which separates from the solution as a solid.

6. What are the two methods of separating a solid from a liquid? Two common methods for separating a solid from a liquid are:

   • Filtration: Used to separate insoluble solids from a liquid.
   • Evaporation: Used to separate a soluble solid from a liquid by heating the solution to vaporize the liquid, leaving the solid behind.

HOTS (Critical Thinking)

• "Cutting of wood and burning of wood," belong to the same kind of changes. True or False? Explain.
  • False.
  • Cutting of wood is a physical change because only the size and shape of the wood change; no new substance is formed.
  • Burning of wood is a chemical change because the wood reacts with oxygen to produce new substances like ash, carbon dioxide, and water vapor.