Look around you, and you will notice that changes are happening constantly. Ice melts into water, cold water warms up, flower buds open into flowers, and bananas develop brown spots and change smell as they ripen. These are just a few examples of the many transformations occurring in our daily lives and in nature.

Activity 5.1 asks us to reflect on various changes, such as melting ice, chopping vegetables, boiling water, making popcorn, cutting paper, adding beetroot extract to water, burning wood, drying clothes, and shaping dough. In each case, something about the object or substance is altered – perhaps its size, shape, state, or smell. Our senses help us observe these changes.

How can we make sense of this vast array of changes? Can we group them into categories based on what happens to the substance involved?

S.No.	Change	Observation(s)
1.	Melting ice cubes	Solid ice turns into liquid water. Shape/state changes.
2.	Chopping vegetables	Large pieces are cut into smaller pieces. Size/shape changes.
3.	Boiling water	Liquid water turns into water vapour (gas). State changes.
4.	Making popcorn from corn	Hard kernels pop into fluffy popcorn. Appearance, texture, sometimes smell/taste change.
5.	Cutting a piece of paper	A larger sheet becomes smaller pieces. Size/shape changes.
6.	Adding beetroot extract to water	Water changes colour. Colour changes.
7.	Burning wood	Wood turns into ash, smoke, gases. Original substance disappears, new substances form.
8.	Drying wet clothes	Water evaporates from clothes. State of water changes, clothes become dry.
9.	Making small balls of dough	Dough is shaped into balls. Shape changes.
10.	Rolling small balls of dough into chapatis	Dough balls are flattened into round shapes. Shape changes.

A Substance May Change In Appearance But Remain The Same!

Activity 5.2: Let Us Create And Discuss

Consider simple activities like folding paper into various shapes, inflating and deflating a balloon, or crushing a piece of chalk into powder.

• When you fold paper into a shape, you can unfold it and get the original paper back.
• When you inflate a balloon and let the air out, the balloon returns to its original uninflated state. However, if you prick an inflated balloon with a pin, it bursts, and you cannot get the original uninflated balloon back in one piece.
• When you crush chalk into powder, it's still chalk, just in a different form (smaller particles). While you might not easily re-form it into the exact original piece, the material itself hasn't changed into something else.

In the first two examples (paper folding and deflating a balloon), the substance (paper, balloon rubber) remains the same, even though its shape or size changes. Crushing chalk also involves a change in size and form, but the powdered substance is still chalk.

These examples illustrate physical changes. In a physical change, only the physical properties of a substance change, such as its shape, size, colour, or state (solid, liquid, gas). No new substance is formed during a physical change. Changes of state, like ice melting into water or water boiling into steam, are also physical changes because the substance is still water, just in a different physical form.

A Substance May Change In Appearance And Not Remain The Same!

Some changes are more profound; they result in the formation of entirely new substances with different properties.

Activity 5.3: Let Us Explore

Let's compare blowing air (exhaling) into tap water versus blowing air into lime water (a solution of calcium hydroxide). When you blow into tap water, you just see bubbles of air passing through. However, when you blow into lime water, it turns cloudy or milky.

This milky appearance in lime water is a visual cue that a new substance has been formed. The carbon dioxide gas in your exhaled breath reacts with the calcium hydroxide in the lime water. This reaction produces calcium carbonate, a white substance that doesn't dissolve in water, making the solution milky.

Such changes, where one or more new substances are created, are called chemical changes. Chemical changes occur through processes known as chemical reactions. The reaction between carbon dioxide and lime water is a chemical reaction, which can be represented simply as:

$ \text{Calcium Hydroxide (Lime Water)} + \text{Carbon Dioxide} \rightarrow \text{Calcium Carbonate (Insoluble)} + \text{Water} $

The milky appearance when lime water reacts with a gas is a common test for the presence of carbon dioxide.

Activity 5.4: Let Us Experiment

Mixing vinegar (acetic acid) with baking soda (sodium hydrogen carbonate) also demonstrates a chemical change.

When these two substances are mixed, you observe vigorous fizzing and the formation of gas bubbles. Passing this gas through freshly prepared lime water turns the lime water milky, confirming that the gas produced is carbon dioxide.

This reaction is also a chemical change because new substances are formed. A simplified representation is:

$ \text{Vinegar} + \text{Baking Soda} \rightarrow \text{Carbon Dioxide} + \text{Other Substances} $

If you mix baking soda with just water, you might see some dissolving, but you wouldn't observe the same vigorous fizzing and gas production. This difference highlights that the reaction between vinegar and baking soda involves a chemical transformation, unlike simply dissolving baking soda in water (which is generally a physical change).

Some Other Processes Involving Chemical Changes

Many processes we observe around us are examples of chemical changes.

Rusting

As discussed in the previous chapter, rusting of iron is a classic example of a chemical change. When iron is exposed to moist air, it reacts with oxygen and water to form a new substance called rust (iron oxide), which is different from iron. The formation of this new substance confirms that rusting is a chemical change.

Combustion

Combustion, or burning, is another common chemical change. When a substance burns, it reacts rapidly with oxygen from the air, producing heat and often light. The original substance is transformed into entirely new substances, such as ash, smoke, and gases.

For instance, burning a magnesium ribbon involves magnesium reacting with oxygen to form magnesium oxide (a white powder). This is accompanied by the release of heat and light, characteristic features of combustion.

$ \text{Magnesium} + \text{Oxygen} \rightarrow \text{Magnesium Oxide} + \text{Heat} + \text{Light} $

Substances that burn are called combustible substances (e.g., wood, paper, kerosene, cotton). The process of burning is a chemical change because new substances are formed.

Activity 5.5: Let Us Investigate

To understand what is needed for combustion, we can light two identical candles. Leave one burning in the open air and cover the other with a glass tumbler.

The candle in the open continues to burn because it has a continuous supply of air (specifically, oxygen). The candle covered by the glass tumbler stops burning after a short while. This happens because the limited amount of air inside the tumbler is quickly used up in the combustion process, and no fresh air is available. This experiment demonstrates that a supply of air (oxygen) is essential for combustion.

The presence of carbon dioxide inside the glass after the candle stops burning can be confirmed by adding lime water, which would turn milky. Carbon dioxide is a product of the candle wax (carbon) reacting with oxygen during burning.

Safety Tip: To extinguish a fire on a person's clothes, wrap them in a blanket or thick cloth (never synthetic, as it can melt). This cuts off the air supply, stopping the combustion.

Fascinating Fact: The light emitted by fireflies is a result of a chemical reaction within their bodies, a phenomenon called bioluminescence. This light production occurs without significant heat, unlike the combustion of a candle.

Is The Presence Of Air Enough For Combustion?

While a combustible substance and oxygen are necessary for burning, they are not always sufficient. For example, a piece of paper can remain in the air indefinitely without catching fire.

Something else is needed to start the combustion process.

Activity 5.6: Let Us Investigate

Hold a piece of paper with tongs and bring a lighted matchstick near it. The paper quickly catches fire because the matchstick's flame is hot enough.

Alternatively, you can use a magnifying glass to focus sunlight onto a piece of paper, creating a small, bright, hot spot. If you hold the magnifying glass steadily, the paper will eventually start to smoke and catch fire, even without a direct flame.

This experiment shows that a substance needs to be heated to a certain temperature before it will burn. This minimum temperature required for a substance to catch fire is called its ignition temperature.

The matchstick's flame is already above the paper's ignition temperature, so it ignites quickly. Focusing sunlight concentrates heat, gradually raising the paper's temperature until it reaches its ignition point, at which point it starts burning.

Therefore, three conditions are necessary for combustion to occur, often represented as the "Fire Triangle":

1. A Combustible substance (fuel).
2. A supply of Oxygen (usually from the air).
3. Sufficient Heat to raise the fuel to its ignition temperature.

Can Physical And Chemical Changes Occur In The Same Process?

Some processes involve both types of changes happening simultaneously.

Activity 5.7: Think, Pair, And Share

Consider what happens when a candle burns. As the candle burns, the solid wax near the flame melts (a physical change). This melted wax is drawn up the wick (capillary action) and evaporates (a physical change). The wax vapour then burns in the flame, reacting with oxygen in the air to produce new substances like carbon dioxide, water vapour, soot, heat, and light (a chemical change).

So, the melting and vaporisation of wax are physical changes, while the burning of the wax vapour is a chemical change. The burning candle is an example where both physical and chemical changes take place as part of the same process.

KNOW A SCIENTIST: The burning candle was famously used by scientist Michael Faraday in his series of lectures titled "The Chemical History of a Candle" to explain various physical and chemical processes.

Are Changes Permanent?

Another way to categorize changes is by whether they can be reversed or not.

Activity 5.8: Let Us Think

Looking back at the changes discussed, some allow us to get the original substance or object back, while others do not.

S.No.	Change	The original state can be brought back (Yes/No)
1.	Melting ice cubes	Yes (by freezing water)
2.	Chopping vegetables	No
3.	Boiling water	Yes (water vapour can be condensed back to liquid water)
4.	Making popcorn from corn	No
5.	Cutting a piece of paper	No (pieces cannot be seamlessly rejoined)
6.	Adding beetroot extract to water	No (difficult to separate)
7.	Burning wood	No (wood is gone, ash/smoke formed)
8.	Drying wet clothes	Yes (clothes can get wet again)
9.	Making small balls of dough	Yes (balls can be flattened/re-shaped)
10.	Rolling small balls of dough into chapatis	Yes (chapati shape can be altered before cooking)
11.	Rusting of iron	No (rust is a new substance)
12.	Curdling of milk	No

Changes that can be undone, allowing the original substance or object to be recovered, are called reversible changes (e.g., melting, freezing, evaporation, condensation). Changes that cannot be easily undone, where the original substance is permanently altered or new substances are formed, are called irreversible changes (e.g., burning, rusting, cooking, chopping).

Are All Changes Desirable?

Changes can also be classified based on whether they are beneficial or harmful to us or the environment in a particular context.

• Desirable changes are those that are useful or beneficial, such as milk turning into curd, fruits ripening, food being cooked, or clothes drying.
• Undesirable changes are those that are harmful or unwanted, like food spoilage (decay) or the rusting of valuable iron objects.

However, whether a change is desirable can depend on the situation. For example, food decay is undesirable for eating, but the decomposition of food waste is desirable for creating compost, a valuable resource for plants.

Some changes, particularly those caused by human activities over time, can have long-term negative environmental impacts, such as increased air pollution from burning fuels or evaporating paints.

Some Slow Natural Changes

Many significant changes in nature happen very slowly over vast periods.

Weathering Of Rocks

Mountains and cliffs are constantly changing due to the breakdown of rocks. The accumulation of sand, soil, and stones (sediments) at the base of mountains is a result of a slow process called weathering.

Weathering involves both physical and chemical changes:

• Physical Weathering: Large rocks are broken into smaller pieces by forces like temperature fluctuations (heating and cooling causing expansion and contraction), the growth of plant roots in cracks, or the freezing of water inside rock fissures.
• Chemical Weathering: Water and dissolved chemicals react with the minerals in rocks, changing their composition. For example, rocks containing iron can react with oxygen and moisture (similar to rusting) to form iron oxides, often visible as reddish-brown layers on rock surfaces.

These combined physical and chemical processes of weathering eventually lead to the formation of soil over geological timescales.

Erosion

Once rocks are broken down by weathering, the resulting smaller particles (sand, soil, pebbles, sediments) can be moved from one place to another by natural agents like wind and flowing water. This transportation process is called erosion.

Erosion itself is primarily a physical process (movement of material). The constant rubbing of water and sediments can also make river rocks and pebbles smoother over time. When the wind or water slows down (e.g., as a river enters a lake or ocean), the eroded material is deposited. Over thousands or millions of years, these deposited sediments can harden and form new rocks. Many of these large-scale geological changes are irreversible from a human perspective.

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In a Nutshell:

• A physical change alters the physical properties (shape, size, state, etc.) of a substance but does not form a new substance.
• A chemical change results in the formation of one or more new substances through a chemical reaction, often involving changes in chemical composition.
• Combustion (burning), cooking, and rusting are examples of chemical changes.
• Combustion is a reaction with oxygen, producing heat and/or light. Substances that burn are combustible.
• The ignition temperature is the minimum temperature at which a substance catches fire.
• For combustion, you need fuel, oxygen, and heat to the ignition temperature.
• Some changes can be reversed (e.g., melting, boiling), while others are irreversible (e.g., burning, rusting).
• Changes can be desirable (useful) or undesirable (harmful), depending on the context.
• Natural processes like weathering (physical and chemical breakdown of rocks) and erosion (transport of broken material) are slow, long-term changes that shape the Earth's surface and form soil.

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