CBSE Class 7 Science Revision Notes Chapter 6

Class 7 Science Chapter 6 Notes

CBSE Class 7 Science Revision Notes Chapter 6 – Physical and Chemical Changes

Extramarks’ Class 7 Science Chapter 6 Notes are concise and ideal for quick revisions before exams. Curated as per the latest CBSE guidelines, these notes help students gain a clear understanding of various concepts in this chapter while studying for the exam. 

The notes offer a deeper understanding of the changes a substance undergoes in its physical attributes and the formation of new chemical compounds. Class 7 Science Chapter 6 Notes also describe many techniques to prevent iron from rusting.

Revision Notes for CBSE Class 7 Science Chapter 6

Access Class 7 Science Chapter 6 – Physical and Chemical Changes

There are two types of changes that can occur in a substance, namely, physical and chemical changes.

Physical alterations refer to modifications in a substance’s physical attributes. These modifications are not likely to result in the creation of some new compounds. These modifications might be alterable. For example, lighting an electric bulb, squashing a can, tearing paper, and mixing sand and water.

Chemical alterations take place when the chemical composition and substance characteristics are modified. Advanced chemicals are formed as an outcome of chemical reactions. This is a stable and irreversible transformation. For example, lighting a candle, making curd from milk, and ripening fruits.

Chemical Reactions in the Real World:

  1. Iron Rusting: The conversion of iron into iron oxide is known as iron rusting. When iron comes in contact with water and oxygen, a brownish film is formed on the surface of the iron articles. It is a common oxidation process in nature.

Prevention of Rusting: 

  • By painting: The surface of the iron objects can be coated with paint to avoid rusting.
  • By oiling and greasing: The iron objects can be properly greased to prevent rusting.
  • By chromium plating: Depositing a layer of a metal like chromium on iron is another way of preventing the iron from rusting.
  • By galvanising: The process of depositing a layer of zinc on any object made of iron is called galvanising. It helps prevent rusting.
  1. Cooking of Food: While cooking food, some complex carbohydrates, lipids, and protein molecules are broken down into smaller molecules. It is considered to be a disintegration process. Food that is well-cooked is simpler to digest than food that is not cooked.
  2. Decay of Organic Substances:  Microorganisms like fungi and bacteria produce enzymes that further break down complex chemical materials into smaller components.

Class 7 Science Chapter 6 Notes 

Class 7 Science Chapter 6 Notes will give students an edge in their exam preparations. These Physical and Chemical Changes Notes are written in an easy-to-understand format for students to easily revise concepts before exams. These Class 7 Science Chapter 6 Notes are prepared according to the revised CBSE guidelines, so students can access updated study materials directly from the Extramarks website.

Physical and Chemical Changes Class 6 Notes

Physical Changes: Substances are said to undergo physical changes when there are changes in their physical attributes like size, state, shape and colour. There is no new substance formed in these changes. All the physical changes are usually momentary changes that can easily be turned back to their original state of existence.

It is important to conduct a few experiments to understand the physical changes of a substance, in terms of its physical attributes. If some form of energy such as heat is used or is exposed to a substance, it is absorbed or developed. For instance, water can transform its state depending on the energy it is exposed to from solid to liquid and liquid to gas.

Crystallisation: Seawater is salty as it has numerous dissolved salts in it. Evaporation, a process by which salt is obtained by boiling seawater, can be used to obtain these salts. These salt crystals are tiny and not visible. Moreover, the salt obtained by this process is not pure. Therefore, the process of crystallisation can be used to attain giant crystals of pure substances from their solutions.

The method of extracting crystals from a hot concentrated solution of a substance after cooling it is known as crystallisation. This is a chief example of a physical change.

Chemical Changes: Class 7 Science Chapter 6 Notes contain a detailed explanation of chemical changes with chemical equations which are known as chemical reactions. When two substances react with each other chemically to produce a new substance with totally different chemical properties, a chemical change occurs. The invention of new substances that we use in various fields of our life is due to chemical changes or chemical reactions.

There are many examples of chemical changes. The most common one is the rusting of iron. Metals such as iron or steel items kept open in the environment will gradually get rusted. It builds a layer of coating of a brownish film known as rust, and the process is known as rusting. Generally, the gates of farms and parks, and benches kept in gardens, are made of iron. Most of the iron items kept in an open place get rusted over a long period due to moisture present in the air. Agricultural tools, kitchen tools, etc., also form a layer of rust. Rust is a different substance that builds up on the surface of the iron due to the climatic conditions the iron-made object is kept.

Rusting of Iron: When an iron object is kept in a moist environment, it reacts chemically with oxygen and water present in the air which forms a reddish-brown flaky substance known as rust. The equation can present the process of rusting as follows:

Fe (Iron) + O₂ (Oxygen) + H₂O(Water) → Fe₂O₃(Iron oxide) (Rust)

Rusting happens due to many environmental factors such as the presence of both moisture and oxygen. The more moist and humid the environment, the faster the rusting takes place. The rust settles and forms a brownish layer on the surface of the iron, leading to substantial loss. Iron is used in making ships, cars, bridges, truck bodies, and other several items.

FAQs (Frequently Asked Questions)

1. Explain the process of rusting.

When a piece of iron is left in the open for a long period of time, it forms a film of brownish substance. This brownish substance is called rust. Thus, this process is called rusting.

The rusting process is expressed by the equation below:

Iron (Fe) + Oxygen (O2) + Water (H2O) → Rust (Fe2O3)

2. Describe the process of crystallisation.

Crystallisation is the process by which salts are removed from their solution. It is a method for purifying seawater or separating crystals from impure substances. Large crystals of pure substances can be set up from their solutions.

The salt obtained by the evaporation of seawater is contaminated and its crystals are minute. The crystal’s shape is not visible. This is why crystallisation is used.

3. What is galvanisation?

The process of placing a layer of zinc on iron is called galvanisation. Applying a coating of metal like chromium or zinc on iron is another way of preventing contact with oxygen, water, etc. The iron pipes used in homes to carry water are galvanised to avoid rusting.