Physical & Chemical Changes - UPSC Sci Tech Free MCQ Test with solutions

• Melting of ice is a process where solid ice turns into liquid water.
• This change is classified as a physical change because:


• No new substances are created; water remains water in different states (solid vs. liquid).
• The chemical composition (H2O) stays the same before and after the melting process.
• Physical changes can often be reversed, such as freezing water back into ice.

.Answer: B) A new substance is formed.

• A chemical change results in the formation of one or more new substances with different properties from the original substances.
• This process involves breaking and forming chemical bonds, leading to a change in the chemical composition.
• Examples include rusting of iron, burning of wood, and baking a cake.
• Unlike physical changes, chemical changes are usually not easily reversible and often involve energy changes.

• Formation of Calcium Carbonate: When carbon dioxide gas is passed through lime water, a chemical reaction occurs where the carbon dioxide reacts with the calcium hydroxide in the lime water to form calcium carbonate.


• Lime Water Turns Milky: The formation of calcium carbonate results in the lime water turning milky or cloudy in appearance.


• Both of These: Therefore, when carbon dioxide gas is passed through lime water, both the formation of calcium carbonate and the milky appearance of the lime water are observed as a result of the chemical reaction.

Melting of wax is a reversible change while burning of matchstick, changing of milk into curd and germination of seed is chemical change.

Crystals of copper sulphate are prepared by the method of crystallization. It is an example of physical change.

So option D is correct answer. 

1. Galvanisation

Explanation: Galvanisation is a process used to prevent rusting by coating iron or steel with a layer of zinc. The zinc layer acts as a protective barrier, preventing moisture and oxygen from reaching the metal surface and causing rust.

• Sedimentation and crystallization are unrelated to rust prevention.

Rusting of iron is a chemical change because it happens due to a chemical reaction between iron and air in presence of moisture.

Rusting of iron is an example of a chemical change because it involves a chemical reaction between iron, oxygen, and moisture to form a new substance, iron oxide (rust). This process changes the chemical composition of the iron, and the change is irreversible under normal conditions.

In contrast:

• Heating of an iron rod (1) is a physical change as it only increases the temperature of the iron without changing its chemical structure.
• Melting of wax (2) is also a physical change as it changes the state of the wax from solid to liquid without altering its chemical composition.
• Woollen yarn to knitted sweater (4) is a physical change as it involves changing the shape and form of the yarn without changing its chemical properties.

The burning of magnesium ribbon in air is an example of a chemical and exothermic change. This is because when magnesium burns in the presence of oxygen, it forms magnesium oxide and releases energy in the form of heat and light. This release of energy is characteristic of an exothermic reaction. Additionally, from magnesium oxide, we cannot revert to its original state, which confirms the chemical change.

To prevent rusting of iron articles, one simple way is to apply a coat of paint or grease regularly. This helps to protect the iron from coming in contact with oxygen or water, which are essential for rust formation.

• A chemical change involves a transformation that alters the substance's chemical structure.
• This change is typically permanent; the original substance cannot be easily recovered.
• During a chemical change, energy is often absorbed or released, indicating the process is not energy-neutral.
• Unlike physical changes, chemical changes result in new substances with different properties and compositions.