Experts have designed these Class 7 Science Notes Chapter 5 Changes Around Us Physical and Chemical Class 7 Notes for effective learning.
Class 7 Science Chapter 5 Changes Around Us Physical and Chemical Notes
Class 7 Changes Around Us Physical and Chemical Notes
Class 7 Science Chapter 5 Notes – Changes Around Us Physical and Chemical Notes Class 7
Physical Changes:
Changes in which physical properties like shape, size and state of substances change, are called physical changes. Example; Melting of butter.
Chemical Changes:
Changes in which one or more new substances are formed, are called chemical changes.
Example: Lime water turns milky on passing carbon dioxide.
Chemical Reaction:
New substances are produced due to rearrangement of atoms. The process is called Chemical Reaction.
Test for Carbon Dioxide:
You can test the carbon dioxide gas by passing it through lime water. Lime water turns milky. Milkiness is due to formation of calcium carbonate. The formula of lime water is Ca(OH)2
The formula of calcium carbonate is CaCO3.
Reaction of Vinegar and Baking soda :
It is a chemical reaction in which new substances and carbon dioxide are produced. The new substances are sodium salt of acetic acid and water.
Example:
Vinegar + Baking (Soda) → Salt + Water + Carbon dioxide
and
Calcium hydroxide + Carbon dioxide → Calcium (carbonate) + Water (lime water)
→ Rusting: Corrosion of iron is called Rusting. It is oxidation of iron as a result of chemical attack: by air and moisture, forming new substance iron oxide.
Fig. Rusted iron nails Rust is chemically iron oxide (brown-coloured substance).
→ Combustion: A chemical reaction in which a substance reacts with oxygen and produces heat and/or light is called Combustion.
Fig. Burning magnesium ribbon
→ Combustible Substances: Substances that undergo combustion reactions are called combustible substances. Example: Paper, cotton, kerosene etc.
→ Essential Conditions for Combustion:
- A combustible substance, also called ‘Fuel’.
- Presence of supporter of combustion—Oxygen.
- Heat that allows the fuel to reach to its ignition temperature.
→ Supporter of Combustion: Substance that causes oxidation of fuel.
→ Fuel: Substance that produces energy in the form of heat and light on burning, e.g. CNG (Compressed Natural Gas).
→ Ignition Temperature: The minimum temperature at which a substance catches fire.
→ Ignition temperature of paper is much lower than coal, so it catches fire almost immediately.
→ Non Supporter of Combustion: Carbon dioxide is used in fire extinguisher as it is non supporter of combustion.
→ Burning of a candle involves both physical and chemical changes.
(a) The melting of wax, its solidification, and formation of wax vapours are physical changes.
(b) Burning of wax vapours to produce carbon dioxide, water vapours, heat and light is a chemical change.
→ Chemical changes are Permanent and Irreversible.
→ Physical changes may be Reversible and Irreversible.
→ Reversible Physical Changes:
Changes in which substances return back to their original size, shape and form.
Example: When water evaporates, it can be condensed back into liquid water as no new substance is formed.
Irreversible Physical Changes:
Changes in which substances cannot return back to their original size, shape and form, though chemical composition of the substance does not change.
For example:
(a) Changing milk into curd.
(b) Decomposition of organic waste into compost.
(c) Cooking of food.
→ Desirable changes are those that are useful in our daily life.
→ Organic Waste: Waste of living organisms (Farmyard waste, Animal Excreta).
→ Undesirable Changes: Undesirable changes are those which are not useful for human beings.
For example:
- Spoiling of food
- Rusting of iron.
→ Rancid: Taste of the food item changes or smell comes out of food due to oxidation of food in air.
→ Slow natural changes:
- Weathering of rocks
- Soil Erosion.
→ Weathering: The process of breaking down large rocks into smaller pieces due to temperature changes, freezing water into cracks of rocks or growing roots of trees into crevices of rocks, leading to formation of soil.
→ Soil Erosion: The process of carrying away of soil from one place to another by natural forces like wind, flowing water, etc.
→ Weathering of rocks leads to formation of soil. It involves both physical and chemical changes.
(a) Weathering is a physical change as appearance of rock changes, though the composition of rock remains same.
(b) Weathering is a chemical change as water or chemicals present in water, can also react with rocks and change their composition.
→ Sediments: Heaps of soil, sand, stones lying at the base of mountains are called sediments.
→ Basalt: An originally black coloured rock, has iron in it. Original black colour of basalt changes from black to red due to the formation of iron oxide on exposure to atmosphere for a long time.
→ Soil erosion during landslide is an example of a physical change.
→ Fireflies are nature’s wonder. Their light is produced by a chemical change. The insects that emit light in dark are called fireflies.
→ Bioluminescence: Production of light (without using heat) in living organisms due to chemical reaction.
→ Physical Changes: The state, shape, form and the size of the substance changes. No new substance is formed.
→ Chemical Changes: Changes in which one or more new substances are formed.
→ Chemical Reaction: Chemical change in which new substances are formed.
→ Combustion: A chemical reaction in which a substance reacts with oxygen and produces heat and/ or light.
→ Fuel: A substance that produces energy in the form of heat and light on burning.
→ Rust: Rust is chemically iron oxide (brown coloured substance).
→ Ignition Temperature: The minimum temperature at which a substance catches fire.
→ Reversible Physical Changes: Changes in which a substance returns back to its original size, shape, form and appearance.
→ Irreversible Physical Changes: Changes in which a substance cannot return back to its original size, shape, form and appearance, though chemical composition of the substance remains same.
→ Desirable Changes: Changes that are useful in our daily life. For example: Formation of compost from organic waste.
→ Undesirable Changes: Changes that are not useful in our daily life, such as corrosion, spoiling of food.
→ Bioluminescence: Production of light in fireflies due to chemical reaction.
→ Supporter of Combustion:
Substance that supports burning reaction. For example: Oxygen.
→ Non Supporter of Combustion: Substance that does not support combustion reaction such as carbon dioxide.
→ Weathering: The process of breaking down large rocks into smaller pieces due to climatic conditions such as temperature changes etc.
→ Soil Erosion: The process of carrying away of soil from one location to another by natural forces like wind, flowing water etc.