Changes Around Us: Physical and Chemical - A Substance May Change in Appearance but Remain the Same!
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, and state. In such a change, no new substance is formed. Examples include melting of ice ( solid to liquid) and stretching a rubber band.
A Chemical Change (or Chemical Reaction) is a change in which one or more new substances are formed. These are usually irreversible. Signs of a chemical change include the evolution of gas, change in color, production of heat or light, or a change in smell.
Rusting is a chemical change that affects iron objects and slowly destroys them. For rusting to occur, the presence of both Oxygen () and water vapor () is essential.
Galvanization is the process of depositing a layer of a metal like Zinc () or Chromium () on iron to prevent it from rusting.
Crystallization is a process used to obtain large crystals of pure substances from their solutions. It is considered a physical change because no new chemical substance is formed during the process.
📐Formulae
💡Examples
Problem 1:
When a magnesium ribbon is burnt in air, a powdery ash is formed. If this ash is dissolved in water and tested with red litmus paper, what is the observation?
Solution:
The red litmus paper turns blue.
Explanation:
Burning magnesium is a chemical change: . The ash is Magnesium Oxide (). When dissolved in water, it forms Magnesium Hydroxide: . Magnesium Hydroxide is a base, which turns red litmus blue.
Problem 2:
Describe the change in color when an iron nail is dropped into a Copper Sulphate () solution.
Solution:
The blue color of the solution changes to green, and a brown deposit appears on the nail.
Explanation:
This is a chemical change due to the reaction between Iron () and Copper Sulphate. Iron, being more reactive, displaces copper: . The green color is due to the formation of Iron Sulphate, and the brown deposit is Copper.
Problem 3:
Why is the process of setting curd from milk considered a chemical change?
Solution:
It is a chemical change because a new substance (Lactic acid) is formed and it is irreversible.
Explanation:
In curdling, the properties of the original substance (milk) are changed permanently by the action of bacteria (). We cannot get milk back from curd, which is a key characteristic of a chemical change.