CBSE Class 7 Science Chapter 6: Physical and Chemical Changes NCERT Solutions

The classification of the given processes is as follows:

Physical Changes are those where a substance changes its form or appearance but not its chemical composition. No new substance is formed. Examples from the list include:

• (b) Dissolving sugar in water: The sugar dissolves and disperses in water, but it can be recovered by evaporation. Its chemical identity remains the same.
• (d) Melting of wax: Wax changes from solid to liquid upon heating, but it is still wax. It can solidify again upon cooling.
• (e) Beating Aluminium to make Aluminium foil: The shape of aluminium is changed, but it remains aluminium. No new substance is formed.

Chemical Changes are those where a substance undergoes a chemical reaction to form one or more new substances with different properties. Examples from the list include:

• (a) Photosynthesis: Plants convert carbon dioxide and water into glucose and oxygen, forming new substances.
• (c) Burning of coal: Coal reacts with oxygen to produce ash, carbon dioxide, and heat, forming new substances.
• (f) Digestion of food: Food is broken down into simpler substances that the body can absorb, involving chemical reactions.

Here are the true/false answers with corrections for the false statements:

(a) False. Cutting a log of wood into pieces is a physical change because the size and shape of the wood change, but its chemical composition remains the same. No new substance is formed.

(b) False. Formation of manure from leaves is a chemical change because the leaves decompose and transform into new substances with different properties (manure).

(c) True. Iron pipes coated with zinc (galvanized iron) do not get rusted easily because zinc acts as a protective layer and corrodes preferentially, preventing the iron from rusting.

(d) False. Iron and rust are different substances. Rust is primarily hydrated iron(III) oxide, formed when iron reacts with oxygen and moisture.

(e) True. Condensation of steam is a physical change. Steam (gaseous water) changes into liquid water upon cooling, but it remains water (H2O).

Here are the completed statements:

(a) When carbon dioxide is passed through lime water, it turns milky due to the formation of Calcium carbonate.

(b) The chemical name of baking soda is Sodium bicarbonate.

(c) Two methods by which rusting of iron can be prevented are painting and galvanization.

(d) Changes in which only physical properties of a substance change are called physical changes.

(e) Changes in which new substances are formed are called chemical changes.

This is a chemical change because a new substance (gas) is produced, along with other products.

The reaction between baking soda (sodium bicarbonate, NaHCO3) and lemon juice (which contains citric acid) is a chemical reaction. The general equation is:

$$Baking\; Soda\; +\; Lemon\; Juice\; (Acid)\; \to \; Salt\; +\; Carbon\; Dioxide\; +\; Water$$

The bubbles observed are due to the release of carbon dioxide gas (CO2). This formation of new substances (salt, carbon dioxide, and water) signifies a chemical change.

When a candle burns:

Physical Change: The solid wax near the flame melts into liquid wax. This liquid wax then rises up the wick and vaporizes. This melting and vaporization of wax are physical changes because the wax changes its state but not its chemical identity. The vaporized wax can condense back into solid wax upon cooling.

Chemical Change: The vaporized wax burns in the presence of oxygen, reacting to form new substances like carbon dioxide (CO2) and water vapor (H2O), along with heat and light. This combustion process is a chemical change.

Another example of a process involving both physical and chemical changes is cooking an egg:

Physical Change: The egg white and yolk change from a liquid to a semi-solid state upon heating. Their texture and appearance change.

Chemical Change: The heat causes chemical reactions within the proteins of the egg. These reactions alter the structure of the proteins, changing their properties permanently. The cooked egg cannot be turned back into a raw egg.