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Changes Around Us: Physical and Chemical Class 7 Science Notes

Changes Around Us: Physical and Chemical

Changes happen all the time around us. We can observe these changes through our senses – sight, smell, touch, hearing, and taste.

Physical Change

A physical change is a change in the physical properties of a substance or object, such as its shape, size, or state (solid, liquid, gas), without changing the substance itself. No new substance is formed.

Example

Folding paper, inflating a balloon, and crushing chalk are examples of physical changes. The paper is still paper, the balloon is still rubber, and the chalk is still chalk, even though their shapes or sizes have changed. Melting ice is also a physical change; the water changes from solid to liquid, but it is still

H_2O

.

Chemical Change

A chemical change is a change in which one or more new substances are formed. This involves a chemical reaction, which can be represented by a chemical equation.

Example

When air is blown into lime water, the lime water turns milky due to the formation of calcium carbonate, a new substance. This indicates a chemical change.

The chemical equation for this reaction is:

Calcium hydroxide + Carbon dioxide $\rightarrow$ Calcium carbonate + Water (Lime water) $\qquad \qquad \qquad \qquad \qquad$ (Insoluble substance)

Example

Mixing vinegar (acetic acid) and baking soda (sodium hydrogen carbonate) produces carbon dioxide, which can be tested by passing it through lime water (which will turn milky). This is also a chemical change.

The chemical equation for this reaction is:

Vinegar + Baking soda $\rightarrow$ Carbon dioxide + Other substances

Some Other Processes Involving Chemical Changes

Rusting

Rusting is a chemical change where iron reacts with oxygen and water to form a new substance called rust (iron oxide). Rust is a brown-colored substance that weakens the iron.

Combustion

Combustion is a chemical reaction in which a substance reacts with oxygen and produces heat and/or light. Substances that undergo combustion are called combustible substances. Examples of combustible substances include wood, paper, cotton, and kerosene.

Example

Burning a magnesium ribbon is a chemical change because it forms a new substance, magnesium oxide, and produces heat and light.

The reaction can be represented as:

$\underset{\substack{\text { Magnesium } \\ \text { (Ribbon) }}}{\text { Oxygen }} \rightarrow \underset{\text { (Air) }}{\text { Magnesium oxide }}+\text { Heat }+ \text { Light }$

For combustion to occur, there are three requirements:

A combustible substance (fuel)
Oxygen
Heat to reach its ignition temperature

The ignition temperature is the minimum temperature at which a substance catches fire.

Example

A paper won't burn unless its ignition temperature is reached. Focusing sunlight on paper with a magnifying glass increases the paper's temperature to the ignition point, causing it to burn.

Note

Oxygen is essential for combustion. If a candle is covered with a glass tumbler, it will stop burning after some time because the oxygen supply is cut off.

Example

Fireflies produce light through a chemical change called bioluminescence, which is light production without heat in living organisms.

Physical and Chemical Changes in the Same Process

Physical and chemical changes can occur simultaneously.

Example

When a candle burns, the wax melts (physical change), travels up the wick, and evaporates (physical change). The wax vapor then burns (chemical change) to produce a flame.

Are Changes Permanent?

Some changes can be reversed, while others cannot.

Example

Melting ice can be reversed by refreezing the water. Boiling water can be reversed by condensing the steam back into liquid water. These are reversible changes.

Example

Chopping vegetables or making popcorn are irreversible changes because you cannot return the vegetables or corn to their original state.

Are All Changes Desirable?

Some changes are useful and desirable, while others are not.

Example

Changing milk into curd, ripening fruits, and cooking food are desirable changes.

Example

Rusting of iron and decay of food are undesirable changes. However, the decomposition of food can be useful in converting food waste into compost, so a change can be undesirable in some situations but desirable in others.

Increased consumption of fuels and drying of paint can have long-term environmental impacts, such as increased carbon dioxide in the atmosphere and atmospheric pollution.

Some Slow Natural Changes

Weathering of Rocks

Weathering is the process by which rocks are broken down into smaller pieces through physical and chemical changes.

Physical changes Temperature changes, growing roots of trees, and freezing of water can break rocks.
Chemical changes Water or chemicals in water can react with the rocks.

Example

Basalt, a black-colored rock containing iron, can chemically change to produce a red-colored layer (iron oxide) when exposed to water or air over a long time.

Erosion

Erosion is the process by which rock pebbles, soil, and sediments are broken down and moved from one location to another by natural forces like wind and flowing water.

Example

A landslide is an example of erosion, a physical change.

Rocks and pebbles appear smoother due to constant erosion by flowing water. Sediments settle at the bottom of oceans or lakes, harden over time, and become new rocks. These changes often take thousands of years and cannot be reversed.

Changes Around Us: Physical and ChemicalClass 7 Science Notes

Changes Around Us: Physical and Chemical

Physical Change

Chemical Change

Some Other Processes Involving Chemical Changes

Rusting

Combustion

Physical and Chemical Changes in the Same Process

Are Changes Permanent?

Are All Changes Desirable?

Some Slow Natural Changes

Weathering of Rocks

Erosion