Changes Around Us: Physical and Chemical - A Substance May Change in Appearance and Become a New Substance!
Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
A physical change is a change in which a substance undergoes a change in its physical properties like shape, size, color, or state. Generally, no new substance is formed and the change is reversible.
A chemical change is a change in which one or more new substances are formed. These are also called chemical reactions and are usually irreversible.
Common signs of a chemical change include the evolution of gas, change in color, production of heat or light, and the formation of a precipitate.
Magnesium burning in air is a chemical change where magnesium reacts with oxygen to form magnesium oxide (). When is dissolved in water, it forms magnesium hydroxide (), which is basic in nature.
Rusting of iron occurs when iron () reacts with oxygen () and water () to form iron oxide (). Both moisture and oxygen are essential for rusting.
Galvanization is the process of depositing a layer of zinc () on iron to prevent it from rusting.
Crystallization is a physical change used to obtain large crystals of pure substances from their solutions (e.g., obtaining pure copper sulphate crystals).
📐Formulae
💡Examples
Problem 1:
What happens when an iron nail is placed in a solution of copper sulphate ()? Write the observations and the type of change.
Solution:
The blue color of the copper sulphate solution changes to green, and a brown deposit of copper is formed on the iron nail. This is a chemical change.
Explanation:
The iron () reacts with the copper sulphate () to form iron sulphate (), which is green, and displaces copper (), which appears as a brown coating on the nail:
Problem 2:
When vinegar and baking soda are mixed, a gas is evolved that turns lime water milky. Identify the gas and the substances involved.
Solution:
The gas evolved is Carbon Dioxide (). The lime water turns milky due to the formation of Calcium Carbonate ().
Explanation:
Vinegar (Acetic acid) reacts with baking soda (Sodium hydrogen carbonate) to produce . When passes through lime water (), the following reaction occurs: