LAQ for Chapter 5 Changes Around Us: Physical and Chemical Class 7 Science NCERT

Answer

A change in which a substance undergoes a change in its physical properties is called physical change, physical properties involves shape, size, colour and state of a substance. It is generally reversible; in physical change no new substance is formed. For example: evaporation, condensation etc.
A change in which one or more new substances are formed is called chemical change. Chemical change is irreversible. It is also called chemical reaction; in this type of change a new substance is formed. Example—Burning of coal, photosynthesis.
The difference between physical change and chemical change is as follows-Physical change
(1) No new substance is formed. A substance undergoes a change in its physical properties.
(2) Physical change is generally reversible.
Example—Melting of ice, lighting of bulb Chemical Change
(1) One or more new substances are formed by chemical reaction.
(2) Chemical change is irreversible.
Example—burning of coal, photosynthesis

Answer

The process of rusting occurs when iron reacts with oxygen and water, forming a reddish-brown substance known as rust (iron oxide). This reaction is accelerated in humid conditions where moisture is abundant. To prevent rusting, consider the following methods:
• Galvanisation: This involves coating iron with a layer of zinc, which protects it from moisture and oxygen.
• Painting or Greasing: Applying a coat of paint or grease creates a barrier that prevents contact with air and water.
• Using Stainless Steel: Stainless steel, made by mixing iron with chromium and other metals, is resistant to rust.

Regular maintenance of protective coatings is essential to ensure long-lasting protection against rusting. 

Answer

A chemical change occurs when the composition of a substance is altered, resulting in the formation of one or more new substances. This process is essential in our daily lives for several reasons:
• New substances are created through chemical changes, such as during the digestion of food and the ripening of fruits.
• Medicines are produced as a result of complex chemical reactions.
• Many useful materials, including plastics and detergents, are formed through chemical processes.
• Every new material discovered is linked to the study of chemical changes.

In summary, chemical changes are vital as they lead to the creation of essential substances and materials that support life and industry. 

Answer

Rusting of an iron object occurs more rapidly in coastal areas than in desert regions due to the following reasons:
• Coastal areas have higher levels of moisture in the air because of the nearby sea.
• Deserts, on the other hand, have dry air with little to no moisture.
• Both air and moisture are essential for rusting to take place.
• Thus, the increased humidity in coastal areas leads to faster rusting compared to the arid conditions of deserts.