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Lessons
- Chapter 1 – Nutrients in plants
- Chapter 2 – Nutrition in Animals
- Chapter 3 – Fibre to Fabric
- Chapter 4 – Heat
- Chapter 5 – Acids bases and salts
- Chapter 6 – Physical and Chemical Changes
- Chapter 7 – Weather, Climate and Adaptations of Animals to Climate
- Chapter 8 – Winds, Storms and Cyclone
- Chapter 9 – Soil
- Chapter 10 – Respiration in Organisms
- Chapter 11 – Transportation in Animals and Plants
- Chapter 12 – Reproduction in Plants
- Chapter 13 – Motion and Time
- Chapter 14 – Electric Current and its Effects
- Chapter 15 – Light
- Chapter 16 – Water: A Precious Resource
- Chapter 17 – Forests: Our Lifeline
- Chapter 18 – Waste water Story
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Question Bank
- Question Bank – Chapter 1 – Nutrients in Plants
- Question Bank – Chapter 2 – Nutrition in Animals
- Question Bank – Chapter 3 – Fibre to Fabric
- Question Bank – Chapter 4 – Heat
- Question Bank – Chapter 5 – Acids, Bases and Salts
- Question Bank – Chapter 6 – Physical & Chemical Changes
- Question Bank – Chapter 7 – Weather, Climate & Adaptations of Animals to Climate
- Question Bank – Chapter 8 – Winds, Storms and Cyclones
- Question Bank – Chapter 9 – Soil
- Question Bank – Chapter 10 – Respiration in Organisms
- Question Bank – Chapter 11 – Transportation in Animals and Plants
- Question Bank – Chapter 12 – Reproduction in Plants
- Question Bank – Chapter 13 – Motion and Time
- Question Bank – Chapter 14 – Electric Current & Its Effects
- Question Bank – Chapter 15 – Light
- Question Bank – Chapter 16 – Water: A Precious Resoure
- Question Bank – Chatper 17 – Forest: Our Life Line
- Question Bank – Chapter 18 – Waste Water Story
Chapter 6 – Physical and Chemical Changes
Chapter 6
Physical and Chemical Changes
- Properties such as shape, size, color and state of a substance are called its physical properties.
- A change in which a substance undergoes a change in its physical properties is called a physical change.
- A physical change is generally reversible, no new substance is formed.
- A change in which one or more new substances are formed is called a chemical change.
- A chemical change is also called a chemical reaction
- Chemical changes are very important in our lives.
- All new substances are formed as a result of chemical changes, if a metal is to be extracted from an ore, such as iron from iron ore, a series of chemical changes will be carried out
- A medicine is the end product of a chain of chemical reactions.
- Useful new materials, such as plastics and detergents, are produced by chemical reactions.
- Indeed, every new material is discovered by studying chemical changes.
- Heat, light or any other radiation ultraviolet, may be given off or absorbed. Sound may be produced.
- A change in smell may take place or a new smell may be given off, color change may take place, gas may be formed after chemical reactions.
- A change is in the rusting of iron a piece of iron left in the open for some time, it acquires a film of brownish substance. This substance is called rust and the process is called rusting
- Rusting is one change that affects iron articles and slowly destroys them.
- Since iron is used in making bridges, ships, cars, truck bodies and many other articles, the monetary loss due to rusting is huge.
- Iron (Fe) + Oxygen (O2, from the air) + water (H2O)—à rust (iron oxide Fe2O3)
- For rusting, the presence of both oxygen and water or vapor is essential.
- One simple way is to apply a coat of paint or grease, these coats should be applied regularly to prevent rusting.
- Another way is to deposit a layer of a metal like chromium or zinc on iron.
- The salt water makes the process of rust formation faster. Therefore, ships suffer a lot of damage from rusting in spite of being solutions.
- Stainless steel is made by mixing iron with carbon and metals like chromium, nickel and manganese. It does not rust.
- This process of depositing a layer of zinc on iron is called galvanisation.
- The iron pipes we use in our homes to carry water are galvanised to prevent rusting.
- Salt can be obtained by the evaporation of sea water.
- The salt obtained in this manner is not pure and its crystals are small.
- The shape of the crystals cannot be seen clearly. The process is of crystallization, is an example of a physical change.
- Changes can be of two types, physical and chemical.
- Physical changes are changes in the physical properties of substances.
- No new substances are formed in these changes. These changes may be reversible.
- In chemical changes new substances are produced.
- Some substances can be obtained in pure state from their solutions by crystallisation.
- The equations here are different from those in mathematics. In equations of this kind, the arrow implies ‘becomes’.
- No attempt should be made to balance chemical equations at this stage.
- You must have heard of the ozone layer in our atmosphere.
- It protects us from the harmful ultraviolet radiation which come from the sun.
- Ozone absorbs this radiation and breaks down to oxygen.
- Oxygen is different from ozone.
- If ultraviolet radiation were not absorbed by ozone, it would reach the earth’s surface and cause harm to us and other life forms.
- Ozone acts as a natural shield against this radiation.
- Stainless steel is made by mixing iron with carbon and metals like chromium, nickel and manganese. It does not rust.
- You know that ships are made of iron and a part of them remains under water.
- On the part above water also, water drops keep clinging to the ship’s outer surface.
- Moreover, the water of the sea contains many salts. The salt water makes the process of rust formation faster.
- In fact, if the content of moisture in air is high, which means if it is more humid, rusting becomes faster.
- Prevent iron articles from coming in contact with oxygen, or water, or both.
- One simple way is to apply a coat of paint or grease.
- In fact, these coats should be applied regularly to prevent rusting.
- Another way is to deposit a layer of a metal like chromium or zinc on iron
Important reactions of this chapter are