Our surroundings are constantly undergoing various **changes**. These changes can involve one or more substances and might alter their appearance, properties, or even their fundamental nature.

Examples of common changes include dissolving sugar in water, making curd from milk, milk turning sour, or stretching a rubber band.

Changes can be broadly categorised into two main types: **physical changes** and **chemical changes**.

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Physical Changes

A **physical change** is a change that affects the **physical properties** of a substance, such as its shape, size, colour, or state (solid, liquid, gas). In a physical change, the substance itself remains the same; **no new substance is formed**.

Physical changes are generally **reversible**, meaning the original substance can often be recovered.

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Activity 6.1

Cutting a piece of paper into smaller pieces changes the paper's **size and shape**. However, each piece is still **paper**. Even though the original single piece cannot be perfectly reassembled, the fundamental material (paper) has not changed its chemical composition or properties. This is a physical change.

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Activity 6.2

Crushing a piece of chalk into dust changes its **size and form** (from solid lump to powder). Adding water to make a paste and reshaping it into a chalk piece, which dries, is a physical change. Although the form changes (solid $\rightarrow$ powder $\rightarrow$ paste $\rightarrow$ solid), the substance is still **chalk**. This change is reversible as the dust can be used to create a piece of chalk again.

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Activity 6.3

Taking ice (solid water) and melting a part of it by placing the tumbler in the sun results in a mixture of ice and water (liquid water). This is a change of **state** from solid to liquid. Placing the tumbler in a freezing mixture (ice and salt) cools the water down, causing it to freeze and turn back into solid ice. This change is **reversible** and involves only a change in the state of water.

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Activity 6.4

Boiling water produces steam (water vapour, gaseous state). Holding an inverted pan over the steam collects water droplets on the cool inner surface of the pan (condensation), showing the change of state from gas back to liquid. This demonstrates the change of state from liquid to gas and back again, a **reversible** physical change.

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Activity 6.5

Heating the tip of a hacksaw blade changes its **colour** (e.g., to reddish or bluish) due to the heat. When the blade is removed from the flame and cools down, its colour returns to the original state. This temporary change in colour on heating, which is reversed on cooling, is a physical change affecting the appearance, but not the composition, of the metal blade.

These activities illustrate changes where only physical properties (shape, size, state, colour) are altered, and the original substance remains chemically the same. These changes are typically reversible.

Chemical Change

A **chemical change** involves a fundamental alteration in the composition of a substance. In a chemical change, one or more **new substances** with different properties are formed. Chemical changes are often **irreversible**.

A chemical change is also known as a **chemical reaction**.

A common example of a chemical change is the **rusting of iron**. When iron is exposed to air and moisture, a brownish flaky substance called **rust** forms on its surface. Rust is chemically different from iron. The formation of rust is a chemical change.

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Activity 6.6

Burning a magnesium ribbon in air is a chemical change. Magnesium (a metal) reacts with oxygen from the air when heated:

$\textsf{Magnesium (Mg)} + \textsf{Oxygen (O}_2\textsf{)} \rightarrow \textsf{Magnesium oxide (MgO)}$

Magnesium burns with a brilliant white light, producing a white powdery substance called **magnesium oxide** (ash). This ash is a **new substance** with properties different from magnesium. This burning is a chemical change, accompanied by the production of heat and light.

Further, mixing the magnesium oxide ash with water results in another chemical change:

$\textsf{Magnesium oxide (MgO)} + \textsf{Water (H}_2\textsf{O)} \rightarrow \textsf{Magnesium hydroxide [Mg(OH)}_2\textsf{]}$

**Magnesium hydroxide** is a **new substance** formed. Testing this solution with litmus paper shows it is **basic** in nature (turns red litmus blue).

Thus, burning magnesium and dissolving its ash in water both involve the formation of new substances and are chemical changes.

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Activity 6.7

Mixing a solution of copper sulphate (blue vitriol), which is typically blue, with an iron object like a nail or shaving blade. A chemical reaction occurs:

$\textsf{Copper sulphate solution (blue)} + \textsf{Iron (Fe)} \rightarrow \textsf{Iron sulphate solution (green)} + \textsf{Copper (Cu)}$

The original blue colour of the copper sulphate solution gradually changes to a greenish colour due to the formation of **iron sulphate**, a **new substance**. A reddish-brown deposit forms on the surface of the iron nail or blade, which is **copper**, another **new substance**. Since new substances are formed, this is a chemical change.

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Activity 6.8

Mixing vinegar (acetic acid solution) with baking soda (sodium hydrogencarbonate) produces a rapid reaction:

$\textsf{Vinegar} + \textsf{Baking soda} \rightarrow \textsf{Carbon dioxide gas} + \textsf{Other substances}$

A hissing sound is heard, and bubbles of a gas are produced. This gas is **carbon dioxide**. The formation of a gas is often an indicator of a chemical change.

Passing this produced carbon dioxide gas through fresh lime water (calcium hydroxide solution) causes another chemical reaction:

$\textsf{Carbon dioxide (CO}_2\textsf{)} + \textsf{Lime water [Ca(OH)}_2\textsf{]} \rightarrow \textsf{Calcium Carbonate (CaCO}_3\textsf{)} + \textsf{Water (H}_2\textsf{O)}$

The lime water turns milky or cloudy due to the formation of tiny particles of **calcium carbonate**, a **new substance**. Turning lime water milky is a standard test for the presence of carbon dioxide gas.

Activities 6.6, 6.7, and 6.8 all demonstrate the formation of one or more new substances, confirming they are chemical changes.

Chemical changes are fundamental to many natural processes (digestion, ripening, fermentation) and technological advancements (creation of new materials like plastics and detergents). Understanding chemical changes is key to discovering new substances and processes.

Besides the formation of new substances, chemical changes can be accompanied by other observable effects:

• Release or absorption of **heat**, **light**, or other forms of **radiation**.
• Production of **sound**.
• Change in **smell** or production of a new smell.
• Change in **colour**.
• Formation of a **gas**.

Burning of any substance (coal, wood, leaves) is a chemical change that produces heat and new substances (ash, gases). Explosion of fireworks is a chemical change involving heat, light, sound, and polluting gases. Food spoilage is a chemical change producing foul smells and new substances. Colour changes in cut fruits or vegetables (like apple turning brown) are also often due to chemical changes forming new substances.

Neutralisation reactions (like acid + base $\rightarrow$ salt + water) also involve the formation of new substances (salt and water) and are chemical changes, often accompanied by heat evolution.

Rusting Of Iron

**Rusting** is a specific type of chemical change that occurs when iron reacts with oxygen and water, forming iron oxide (rust). It is a significant problem causing extensive damage to iron structures and objects.

The chemical equation for rusting is:

$\textsf{Iron (Fe)} + \textsf{Oxygen (O}_2\textsf{)} + \textsf{water (H}_2\textsf{O)} \rightarrow \textsf{rust (iron oxide, Fe}_2\textsf{O}_3\textsf{)}$

For rusting to occur, the presence of both **oxygen** (from air) and **water** (or water vapour/moisture) is necessary. Rusting happens faster in environments with high moisture content (humidity), such as coastal areas, and is further accelerated by the presence of salts (e.g., in seawater).

To prevent rusting, iron articles must be protected from contact with oxygen and/or water. Common methods include:

• Applying a coat of **paint** or **grease** regularly.
• Depositing a layer of another metal, such as **chromium** or **zinc**, on the iron surface. Coating iron with zinc is called **galvanisation**.

The Iron Pillar near Qutub Minar in Delhi is a historical example of ancient Indian metal technology, known for its remarkable resistance to rusting despite being over 1600 years old.

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Ozone Layer: The ozone layer in the atmosphere is important because it absorbs harmful ultraviolet (UV) radiation from the sun. This absorption causes ozone ($O_3$) to break down into oxygen ($O_2$). This breaking down of ozone into oxygen is a **chemical change**, as a new substance (oxygen) is formed from ozone. The ozone layer acts as a natural protective shield, preventing damaging UV radiation from reaching the Earth's surface.

Crystallisation

**Crystallisation** is a process used to obtain pure solid crystals of a substance from its solution. It is an example of a **physical change** because it involves a change in the state or form of the substance (from dissolved in solution to solid crystalline form), but the substance itself does not change chemically.

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Activity 6.9

Preparing crystals of copper sulphate:

Procedure: 1. Take water in a beaker and add a few drops of dilute sulfuric acid (to ensure clear solution). 2. Heat the water. While it is boiling or very hot, slowly add copper sulphate powder, stirring continuously. 3. Continue adding copper sulphate until no more powder dissolves (the solution is saturated). 4. Filter the hot, saturated solution to remove any undissolved impurities. 5. Allow the filtered solution to cool down slowly without disturbing it. 6. As the solution cools, crystals of pure copper sulphate will gradually form and settle down.

The copper sulphate crystals formed are a pure solid form of the same substance that was dissolved in the solution. No new substance is created during this process, making it a physical change.

In summary, changes around us can be classified as either physical (altering form/appearance, usually reversible, no new substance) or chemical (forming new substances, often irreversible, potentially accompanied by heat, light, gas, colour, smell changes).

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