Class 7 Science Chapter 5: Physical and Chemical Changes Around Us – A Complete Guide 🔬

Class 7 Science Chapter 5 Physical and Chemical Changes Notes

Have you ever noticed how a bright green bud on a rose plant transforms into a beautiful flower overnight? Or perhaps you’ve seen a yellow banana develop brown spots and a strong smell after sitting on the counter for a few days. Even simple acts, like watching an ice cube disappear into a puddle of water, are signs that the world around us is in a constant state of flux.

Science helps us make sense of these transformations by categorizing them into two main types. By understanding the "why" behind these changes, we can better interact with our environment—from cooking the perfect meal to protecting the iron gates of our homes. Let’s dive into the fascinating world of physical and chemical changes!

Chapter Overview

In this guide, we will explore the following essential topics:

How we observe changes using our senses.
The defining differences between Physical and Chemical changes.
Deep dives into common processes like Rusting and Combustion.
The classification of changes: Reversible vs. Irreversible and Desirable vs. Undesirable.
Slow natural changes that shape our planet: Weathering and Erosion.

--------------------------------------------------------------------------------

Concept-Wise Explanation

Observing Everyday Changes

We notice changes in our surroundings by using our senses: sight 👀, smell 👃, touch, hearing 👂, and taste. These senses help us detect shifts in an object’s size, shape, color, smell, or state of matter (solid, liquid, or gas). For example, we hear the fizzing of a soft drink or see the color of a leaf change in autumn.

Section 1: Physical Changes

A Physical Change is a change in the physical properties of a substance—such as its shape, size, or state—where no new substance is formed. The material itself remains exactly the same.

Daily Life Examples:
- Folding paper to make airplanes, boats, or even complex origami animals.
- Inflating a balloon (the rubber stretches, but it is still rubber).
- Crushing a piece of solid chalk into a fine powder.
- Adding beetroot extract to water: The color of the water changes, but no new chemical substance is created.
- Melting ice into water or boiling water into steam (it is still H2O).
Key Feature: These changes are often reversible. You can often get the original substance back, like melting ice and then refreezing it.

Section 2: Chemical Changes

A Chemical Change occurs when one or more new substances are formed through chemical reactions. These changes are generally irreversible. 🧪 🫧

Detailed Example 1: Exhaling into Lime Water When you exhale into lime water (calcium hydroxide), the carbon dioxide in your breath reacts with it. This creates calcium carbonate, a white insoluble substance that makes the liquid look milky. This is the standard test for carbon dioxide; if a gas turns lime water milky, we know it is CO2!
Detailed Example 2: Vinegar and Baking Soda Mixing vinegar (an acid) with baking soda (sodium hydrogen carbonate) results in a fizzing sound and the formation of bubbles. These bubbles are carbon dioxide gas—a brand-new substance created by the reaction.

Section 3: Rusting and Combustion

Rusting: This is a chemical change where iron reacts with oxygen and moisture to form a new brown substance called iron oxide (rust). 🛠️
Combustion: This is a chemical reaction where a substance reacts with oxygen to produce heat and/or light. Substances that can burn, like wood, paper, cotton, and kerosene, are called combustible substances. 🕯️

Section 4: Combined Changes (The Candle Example)

A burning candle is a perfect example of how both types of changes can happen at once:

Physical Changes: The solid wax melts into a liquid, is drawn up the wick, and evaporates into a vapor. Solidifying wax is also a physical change.
Chemical Change: The wax vapor reacts with oxygen in the air (combustion) to produce a flame and new substances like carbon dioxide and water vapor.

--------------------------------------------------------------------------------

Important Laws, Principles & Key Points

For combustion to occur and stay active, three essential components must be present:

Fuel (A combustible substance).
Oxygen (The component of air that supports burning).
Heat (To reach the ignition temperature).

Teacher's Safety Tip: If a person's clothes catch fire, wrap them in a thick cotton blanket to cut off the oxygen. Never use a synthetic blanket, as it can melt and stick to the skin!

Ignition Temperature: The "minimum temperature at which a substance catches fire." A matchstick starts a fire because its flame is hotter than the paper's ignition temperature.
Bioluminescence: A fascinating chemical change where insects like fireflies produce "cold light" in their bodies without generating heat. 🪰✨

--------------------------------------------------------------------------------

Formula and Chemical Equations Section

In science, we use chemical equations to show how substances transform. Note the descriptions of the products:

Lime Water Test: Calcium hydroxide + Carbon dioxide -> Calcium carbonate + Water
Magnesium Burning: Magnesium + Oxygen -> Magnesium oxide (White powder) + Heat + Light
Vinegar Reaction: Vinegar + Baking soda (sodium hydrogen carbonate) -> Carbon dioxide + Other substances

--------------------------------------------------------------------------------

Classification of Changes

Reversible vs. Irreversible

Reversible: You can get the original substance back (e.g., melting ice, folding a mat, stretching a rubber band).
Irreversible: You cannot return to the original state.
- Examples: Stitching a shirt from cloth, making popcorn from corn, grinding wheat into flour, or chopping vegetables.

Desirable vs. Undesirable

Desirable: Useful changes like cooking food, ripening fruit, or making curd.
Undesirable: Harmful changes like rusting or food spoilage.
The Environmental Connection: Some changes are undesirable because of their impact. For example, the drying of paint on walls is a physical change (evaporation), but it is undesirable for the environment because it releases substances that cause atmospheric pollution.
Context Matters: Decomposition is undesirable in your fridge but highly desirable in a compost pit where it turns waste into nutrients. 🍎➡️♻️

--------------------------------------------------------------------------------

Natural Slow Changes: Weathering & Erosion

Nature reshapes the Earth over thousands of years through slow, irreversible processes:

Weathering: The breaking of large rocks into smaller pieces (sediments).
- Physical Weathering: Occurs via temperature changes or frost wedging (where water freezes in rock cracks, expands, and forces the rock to break).
- Chemical Weathering: Occurs when minerals in rocks react with water or air. For example, black basalt rocks containing iron can develop a red-coloured layer of iron oxide when exposed to moisture over a long time. 🏔️
Erosion: The movement of these sediments from one place to another by wind or flowing water. Over time, erosion makes river pebbles appear smooth. 💨

--------------------------------------------------------------------------------

“Did You Know?” / Fun Facts

Michael Faraday: This famous 19th-century scientist gave a legendary series of lectures called "The Chemical History of a Candle." He believed a candle was the perfect object for scientific study because it involves so many different processes.
Chameleons: These amazing creatures change color to blend in or express emotions. Let's think like scientists: Since the chameleon can change back and forth, is this a physical change or a chemical one? (Hint: Does it become a new substance?)

--------------------------------------------------------------------------------

FAQs (Exam-Oriented)

Q: Why does a candle go out when covered with a glass? A: Combustion requires oxygen. Once the glass covers the candle, the limited oxygen inside is used up, the oxygen supply is cut off, and the flame is extinguished.

Q: Is dissolving sugar in water a chemical change? A: No, it is a physical and reversible change. No new substance is formed, and you can recover the sugar by evaporating the water.

Q: How can we prevent an iron statue from rusting? A: Apply a protective coat of paint or wax. This creates a barrier that prevents the iron from coming into contact with air and moisture.

Q: Is the formation of curd from milk reversible? A: No, it is an irreversible chemical change. Bacteria change the milk into a new substance (curd) that cannot be turned back into milk.

--------------------------------------------------------------------------------

Conclusion & Exam Tips

This chapter shows us that the world is full of transformations, but they all follow specific scientific rules. By checking if a new substance is formed, you can distinguish a simple physical shift from a complex chemical reaction.

Pro-Tips for Your Exam:

The "White Powder" Rule: If a question asks about burning magnesium, remember the product is Magnesium Oxide, which appears as a white powder.
The Fire Triangle: To stop any fire, you must remove one of the three: Fuel, Oxygen, or Heat.
The Standard Test for CO2: If an experiment asks how to identify Carbon Dioxide, the answer is always the Lime Water Test (it turns milky/cloudy due to calcium carbonate).

Keep observing the world around you—science is happening in every breath you take and every flame you see! 🌟