Notes-NCERT-Class-7-Science-Curiosity-Chapter-5-Changes Around Us: Physical and Chemical-CBSE

Topics to be Learn : Physical Change Chemical Change Processes Involving Chemical Changes Reversible Changes & Irreversible Changes Desirable Changes & Undesirable Changes Slow Natural Changes

A Substance May Change in Appearance but Remain the Same! :

Physical Property : Physical properties are the features of a substance like:

• Shape
• Size
• Colour
• State (solid, liquid, gas)

Physical Change : A physical change is a change in which:

• No new substance is formed
• Only physical properties (appearance) change

Examples:

• Melting of ice
• Freezing of water
• Breaking of glass
• Boiling of water
• Folding of paper
• Mixing sand with water

Characteristics of Physical Changes :

• No new substance is formed
• Change usually temporary and reversible and can return to original form easily. Example: Ice → Water → Ice
• Very little heat or energy is used or released

Experimental Evidence:

• Paper Manipulation: Folding paper to create objects (origami) changes the shape, but unfolding it returns the original sheet.
• Balloon Dynamics: Inflating a balloon changes its size and shape; allowing air to escape returns it to its uninflated state.
• State Alterations: Water exists as a solid (ice), liquid (water), and gas (vapor). Transitioning between these states is a physical change.
• Mechanical Alterations: Crushing chalk into powder or chopping vegetables changes dimensions but not the material itself.

A Substance May Change in Appearance and Not Remain the Same! :

Chemical Change :  A chemical change, or chemical reaction, occurs when two or more substances react to form a new substance with entirely different chemical properties.

Characteristics of Chemical Changes

• New substance is formed : The new substance has different properties
• Energy change occurs : Heat, light, or radiation may be released or absorbed
• Sound may be produced : Example: crackers bursting
• Change in smell : A new smell may be produced
• Change in colour : Substance may change its colour
• Gas may be formed : Bubbles or gases can be seen
• Permanent change : Cannot be reversed easily

Examples of Chemical Changes

• Burning of substances : Burning of coal, wood, or leaves. New substances like ash and gases are formed
• Fireworks (crackers) : Produce heat, light, sound, and gases. Air pollution is also caused
• Spoilage of food : Food gives a bad smell. New substances are formed
• Apple turning brown : Cut apple reacts with oxygen in air. A new substance is formed, changing colour.
• Neutralisation reaction : Acid + Base → Salt + Water. New substances are formed
• Photosynthesis in plants : Plants make food (glucose) using: Carbon dioxide, Water, Sunlight and chlorophyll. Oxygen is also released
• Digestion of food : Food breaks down into simpler substances. These can be absorbed by the body
• Carbon dioxide with lime water : Forms a white substance (calcium carbonate). Lime water turns milky

Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

(Lime water)                                    (Insoluble substance)

 Experimental Evidence:

• Gas Production: Mixing vinegar (acid) with baking soda (sodium hydrogen carbonate) produces a fizzing sound and creates carbon dioxide.
• Precipitation/Cloudiness: Blowing exhaled air (containing carbon dioxide) into lime water (calcium hydroxide) creates a white, insoluble substance called calcium carbonate, turning the liquid milky.
• Oxidation (Rusting): The reaction of iron with environmental factors to form iron oxide, a new brown-colored substance.

Importance of Chemical Changes :

Chemical changes are very important in our daily life because they help in making new and useful substances.

• Metals like iron are obtained from their ores through chemical changes
• Medicines are produced through a series of chemical reactions
• Materials like plastics and detergents are made using chemical reactions

Some Other Processes Involving Chemical Changes :

Rusting : Rusting is a chemical change that occurs when iron objects are exposed to moist air.

• Process: Iron reacts with oxygen and water to form a brown-coloured substance called rust (iron oxide).
• Chemical Formula: Fe₂O₃ (Iron Oxide).
• Prevention: Rusting can be prevented by painting, oiling, or scrubbing with sandpaper. It is accelerated by moisture, air, and salt.

Combustion : Combustion is a chemical reaction where a substance reacts with oxygen to produce heat and light.

Requirements: Three conditions are necessary for combustion:

• A combustible substance (fuel).
• Ignition Temperature: The minimum temperature at which a substance catches fire.

Example: Burning a magnesium ribbon produces a white powder (magnesium oxide) and intense heat/light.

Reaction: Magnesium + Oxygen → Magnesium Oxide + Heat + Light

Know This : Fireflies : Fireflies are the insects that can glow in dark due to light produced by some chemical changes in their bodies. This light producing process without heat in living organisms is called as bioluminescence.

Can Physical and Chemical Changes Occur in the Same Process?

Some processes exhibit both physical and chemical changes simultaneously.

Burning a Candle:

• Physical Change: The melting of wax, its solidification, and evaporation.
• Chemical Change: The burning of the wax vapor or the wick, which produces carbon dioxide and heat.

Cooking of Food :

• Physical Change: Change in texture, color, and state (e.g., water evaporating, softening of vegetables).
• Chemical Change: Breakdown of complex molecules (like proteins and carbohydrates) into simpler ones. New substances with different taste and aroma are formed.

Are Changes Permanent? :

• Reversible Changes are those changes where we can get back the original substance or the changes that can be reversed. e.g. Melting of ice, boiling of water, etc.
• Irreversible Changes are those change where we can not get back the original substance or the changes that cannot be reversed. e.g. Cutting of vegetables, making popcorn, etc.

Are All Changes Desirable? :

(i) Desirable Changes : Changes that are useful and beneficial in our daily life and have a positive impact is a desirable changes.

Examples:

• Milk changing into curd
• Ripening of fruits
• Cutting of fruits
• Cooking of food

(ii) Undesirable Changes : Changes that are harmful or not useful is a undesirable changes.

Examples: Rusting of iron, Decay (spoilage) of food

Note : Some changes can be both desirable and undesirable depending on the situation

Example: Decomposition of food:

• Undesirable when food spoils
• Desirable when it helps in making compost from waste

Whether a change is desirable or not depends on its use and situation.

Some Slow Natural Changes :

(i) Weathering Changes: Weathering is a slow natural process in which large rocks break down into smaller pieces called sediments. These sediments collect at the base of mountains and help in the formation of soil.

Weathering involves both physical and chemical changes.

• In physical change, rocks break into smaller pieces without forming any new substance. This can happen due to temperature changes, growing roots of trees, or freezing of water in the cracks of rocks.
• In chemical change, rocks react with water or chemicals present in water, leading to the formation of new substances. For example, a black-coloured rock called basalt can change into a red layer due to long exposure to air and water.

(ii) Erosion: Erosion is the process in which rocks, soil, and sediments are broken down and carried away from one place to another by natural forces like wind and flowing water. It is mainly a physical change.

• Examples of erosion include landslides and the wearing away of rocks by wind and water. Due to continuous erosion by flowing water, river rocks and pebbles become smooth. When the speed of water or wind decreases, the sediments settle down and, over time, harden to form new rocks.

Weathering and erosion are slow natural processes that continuously change the Earth’s surface and help in soil and rock formation.

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