1. A Substance May Change in Appearance: Physical Changes

Changes are an integral part of our environment. When a change occurs in the physical appearance of an object without altering its internal chemical structure, it is categorized as a Physical Change.

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Understanding Physical Properties

To identify a physical change, we look for modifications in the physical properties of a substance. These properties include:

• Shape: Folding a paper into different forms or rolling dough.
• Size: Cutting a long string into smaller bits or chopping vegetables.
• State of Matter: Change from solid to liquid (melting) or liquid to gas (boiling).
• Volume: Compressing or inflating a balloon.

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Key Characteristics of Physical Changes

Most physical changes follow a specific set of rules that distinguish them from other transformations:

• No New Substance: The most vital point is that no new chemical substance is created. The paper remains paper, and the water remains water.
• Reversibility: Many physical changes can be reversed to get the original substance back.
  • Example: Steam can be condensed back into liquid water.
  • Example: An inflated balloon returns to its original shape when air is released.
• Irreversible Physical Changes: Not all physical changes are reversible. For instance, once you chop a potato, you cannot merge the pieces back into a whole potato, yet it is still a physical change because it is still a potato.

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Tabular Summary of Physical Activities

Activity	Observation	Nature of Change
Melting of Ghee/Ice	Solid turns to Liquid	Reversible Physical Change
Crushing Chalk	Solid piece turns to Powder	Irreversible Physical Change
Folding Paper	Change in Shape/Size	Reversible Physical Change
Stretching Rubber Band	Change in Length	Reversible Physical Change

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2. Formation of New Substances: Chemical Changes

When the internal identity of a substance is transformed and new substances are created, the process is known as a Chemical Change or a Chemical Reaction.

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The Test for Carbon Dioxide ($CO_2$)

One of the most important chemical reactions in middle school science involves Lime Water (Calcium Hydroxide). This is used as a standard test to detect the presence of Carbon Dioxide.

• The Process: When we blow air through a straw into lime water, the gas reacts with the liquid.
• Observation: The clear lime water turns milky or cloudy.
• Scientific Reason: A new, white, insoluble substance called Calcium Carbonate is formed, which settles in the water.

Chemical Equation for the Process

The reaction is represented as follows:

$\text{Calcium hydroxide (Lime water)} + \text{Carbon dioxide} \rightarrow \text{Calcium carbonate} + \text{Water}$

$\text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O}$

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Common Kitchen Reactions: Vinegar and Baking Soda

In many Indian households, Baking Soda (Sodium hydrogen carbonate) and Vinegar are used. When these two are mixed, a chemical change occurs immediately:

• Fizzing Sound: A distinct hissing or bubbling sound is heard.
• Gas Production: Bubbles of Carbon Dioxide are produced rapidly.
• Verification: If this gas is passed through lime water, it will turn the lime water milky, proving it is indeed $CO_2$.

Economic Context

If a student buys a small $200 \text{ml}$ bottle of vinegar for $\text{₹} \ 45 \text{/}$ and a packet of baking soda for $\text{₹} \ 10 \text{/}$, the total cost for this experiment would be:

$\text{Total Cost} = 45 + 10 = \text{₹} \ 55 \text{/}$

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Evidence of a Chemical Change

Unlike physical changes, chemical changes often provide sensory evidence that a reaction has taken place:

• Formation of Gas: Seen as bubbles or felt as pressure.
• Heat or Light: Some reactions release energy (like burning).
• Change in Color: A permanent shift in appearance.
• Change in Smell: Like the foul odor of decaying food or the strong smell of a ripening banana.

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3. Combustion and the Requirements for Fire

Combustion is a specialized chemical reaction where a substance reacts with atmospheric Oxygen to release energy in the form of heat and/or light. It is a fundamental process used in everything from lighting a chulha to powering vehicles.

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The Burning of Magnesium Ribbon

A classic demonstration of combustion is the burning of a magnesium ribbon. When ignited, it reacts with oxygen to produce a dazzling white light and a white powdery substance.

• Reactants: Magnesium ($Mg$) and Oxygen ($O_2$).
• Product: Magnesium Oxide ($MgO$), which is chemically distinct from the metal.
• Chemical Equation:

$\text{Magnesium} (2Mg) + \text{Oxygen} (O_2) \rightarrow \text{Magnesium Oxide} (2MgO) + \text{Heat} + \text{Light}$

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The Fire Triangle: Three Essentials

For combustion to occur, three specific factors must be present simultaneously. This is often represented as the Fire Triangle:

1. Fuel (Combustible Substance): Any material that can burn, such as wood, paper, kerosene, or LPG.
2. Oxygen: The component of air that supports burning. Without it, the flame is extinguished.
3. Heat (Ignition Temperature): The energy required to raise the temperature of the fuel.

Ignition Temperature

The minimum temperature at which a substance catches fire is called its Ignition Temperature.

• A matchstick provides immediate heat because its temperature is higher than the paper's ignition point.
• Magnifying Glass Experiment: Focusing sunrays onto a spot on a paper generates concentrated heat. Once the paper reaches its ignition temperature, it starts to smoke and catch fire without a flame being touched to it.

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Combustible vs Non-Combustible Substances

Not all materials can undergo combustion. The following table classifies common materials found in an Indian household:

Material	Combustible?
Dry Wood / Paper	Yes
Iron Nails / Stones	No
Cotton Cloth	Yes
Glass Pieces	No
Kerosene / LPG	Yes

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Bioluminescence: Light Without Heat

In nature, chemical changes can also produce light without heat. This is called Bioluminescence.

• A common example in Indian villages is the Firefly (known locally as Jugnu).
• The light emitted by a firefly is the result of a complex chemical reaction inside its body.

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4. Reversibility and Desirability of Changes

All changes around us can be evaluated based on two criteria: whether they can be undone and whether they provide a benefit to society.

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Reversible and Irreversible Changes

Changes are categorized based on the ability to obtain the original substance back:

• Reversible Changes: These can be undone. For example, melting of ice into water can be reversed by freezing it again. Similarly, melting of ghee is a reversible physical change.
• Irreversible Changes: These cannot be undone. Once you make popcorn from corn or cook a chapati, you cannot get the original grain back. Ripening of a mango is also an irreversible chemical change.

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Desirability of Changes

The impact of a change determines its desirability:

• Desirable Changes: Beneficial transformations like the conversion of milk into curd, ripening of fruits, and cooking of food.
• Undesirable Changes: Harmful transformations such as rusting of iron, spoiling of food, or environmental pollution caused by burning excessive fuel.

Contextual Desirability

A change can be desirable or undesirable depending on the situation:

• Decomposition: It is undesirable when it happens to fresh food in our kitchen. However, it is highly desirable in a compost pit where it converts organic waste into Khaad (manure).

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5. Slow Natural Changes: Weathering and Erosion

Some changes happen so slowly that they take thousands of years to complete. These changes, primarily Weathering and Erosion, are responsible for shaping the Earth's surface and forming soil.

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Weathering of Rocks

Weathering is the process of breaking down large rocks into smaller pieces called sediments. It involves two mechanisms:

• Physical Weathering: Occurs due to temperature fluctuations, freezing of water in rock cracks, or roots of trees pushing through stones. No new substance is formed here.
• Chemical Weathering: Occurs when the minerals in rocks react with water or air. For example, Basalt (a black rock) contains iron. When exposed to moist air over long periods, it reacts to form Iron Oxide, giving the rock a reddish appearance.

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Erosion

Erosion is the physical movement of rock particles, soil, and sediments from one place to another. The primary agents of erosion are:

• Flowing Water: Constant movement of water in rivers makes pebbles appear smoother.
• Wind: Moves fine sand and soil across plains.
• Landslides: A rapid form of physical change where gravity moves large amounts of rock and soil.

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Summary of Changes: Conservation of Mass

In every change, whether physical or chemical, a fundamental law of science applies. Even if the appearance or state of the matter changes, the total mass remains the same. This is known as the Law of Conservation of Mass.

The mathematical logic can be expressed as:

$\text{Total Mass of Reactants} = \text{Total Mass of Products}$

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