Olympiad Test: Physical & Chemical Changes - Delhi Police Constable MCQ

Burning of candle wick is an example of chemical change as wick gets burnt and new substance is formed with new property.

On Combustion, fuel is the substance which produces heat and light.

When water freezes it undergoes a physical change and no new substance is formed.

Physical change is a type of change in which only the physical properties of the substance like colour, size, shape, etc, change but no new substance is formed. Example: Conversion of water into steam.

Chemical changes are irreversible and a new product is formed. Example: Digestion of food.
An explanation for correct option:
Melting of iron: When iron is melted only the state of iron changes. The solid iron changes its state of matter as solid turns liquid.
Thus, it's a physical change.
Bending of iron rod: When we bend the iron rod, only the shape of the rod changes, and no new substance is formed. It's a physical change.
Drawing a wire of iron metal: Since iron is a metal it will possess the property of ductility. Hence, a wire can be formed. This does not change the composition of the metal, it only changes its shape. Hence, it's a physical change.
Since melting of iron, bending of iron rod, and drawing a wire of iron metal are physical changes.

Since the law of conservation of mass is valid under all circumstances, hence, mass always remains the same, whether a substance undergoes physical change or chemical change.

Rusting of iron is a chemical change as rust is a new substance having new properties unlike the iron.

D is the correct option.Crystallization is the process by which we can obtain crystals of copper sulphate. In this process, a few drops of dilute sulphuric acid is added to water. The water is then boiled & as it starts boiling, copper sulphate powder is added with stirring. The powder must be added on till the solution becomes saturated.

Chemical changes are characterised by formation of new substance having new properties

The product formed by dissolving a substance into another is called a solution. Solution is a mixture.

Rusting of iron is slow change as it takes few days on the other hand burning of petrol is a fast change as it takes less than a second.

Ripening of fruit is a slow change as it takes time. Ripening of fruit is also desirable as we want to do show.

During chemical changes, change in colour, heat or temperature may takes place 

Metal wires expand on heating and contract on cooling. The electric wires become tight during winter due to expansion and tight during winter due to contraction.

The process of converting gas into liquid is called condensation. Condensation is a physical change.

Sodium Bicarbonate (NaHCO3) is called baking soda because when heated, it releases carbon dioxide gas, which is responsible for the rising of cookies, doughnuts, and bread.

Air is a mixture of gases like nitrogen, oxygen and carbon dioxide along with other gases and other particles.

Tearing of paper in small piece is an example of physical change as no new substance is formed and properties remain same.

Magnesium ribbon burns with brilliant white light to produce magnesium oxide.

Rusting of iron, combustion of magnesium ribbon, burning of a candle are chemical changes while melting of wax is a physical change.

- Burning of Paper: This is a chemical change. Burning transforms paper into ash and gases, and cannot be reversed.

- Cooking of an Egg: Cooking causes permanent denaturation of proteins, making it irreversible.

- Freezing of Water: This is a physical change. Water can be frozen and then melted back to liquid, making it reversible.

- Rusting of Iron: This is a chemical change. Iron reacts with oxygen to form rust, which cannot be easily reversed.

Thus, option C, freezing of water, is the reversible change.