Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
A Physical Change is a temporary change where no new substance is formed, and the chemical composition of the original substance remains the same. Most physical changes are reversible, such as the melting of ice: .
A Chemical Change is a permanent change in which one or more new substances with entirely different properties are formed. These are usually irreversible, such as the burning of coal: .
Characteristics of Physical Changes: No change in the mass of the individual substance, no production of energy (usually), and the change is generally reversible by reversing conditions.
Characteristics of Chemical Changes: New substances are formed, energy is usually evolved or absorbed (in the form of heat or light), and the total mass of reactants equals the total mass of products, though the original substance cannot be easily recovered.
Rusting of Iron: This is a slow chemical change that occurs when iron is exposed to moisture and oxygen. The chemical formula for rust (hydrated iron(III) oxide) is .
Crystallization: A physical change used to obtain a pure solid in the form of crystals from its solution.
Galvanization: A process used to prevent the chemical change of rusting by coating iron with a layer of Zinc ().
📐Formulae
💡Examples
Problem 1:
When a Magnesium () ribbon is burnt in air, a white ash is formed. Identify the type of change and write the chemical equation.
Solution:
Chemical Change. Equation:
Explanation:
This is a chemical change because a new substance, Magnesium Oxide (), is formed which has different properties from Magnesium and Oxygen. It cannot be reversed back to Magnesium metal easily.
Problem 2:
Classify the following as Physical or Chemical changes: (a) Dissolving sugar in water, (b) Digestion of food, (c) Glowing of an electric bulb.
Solution:
(a) Physical Change, (b) Chemical Change, (c) Physical Change.
Explanation:
(a) Sugar and water can be separated by evaporation. (b) Food is broken down into simpler substances like glucose () via enzymes, which is irreversible. (c) The filament glows due to heat but no new substance is formed; it returns to its original state when the current is switched off.
Problem 3:
Explain the process of rusting using a chemical reaction.
Solution:
Rusting is a chemical change represented by:
Explanation:
Iron () reacts with Oxygen () from the air and moisture () to form a flaky brown substance called Rust (). Both air and water are essential for this change to occur.