Master Chapter 5 Class 7 - Changes Around Us: Physical and Chemical (Curiosit with comprehensive NCERT Solutions, Practice Questions, MCQs, Sample Papers, Case Based Questions, and Video lessons.
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Welcome to Chapter 5. Our journey begins with a simple observation. In fact, it begins with several, all happening at the same time:
An ice cube, left on a table, is now a puddle of water.
A rose bud that was closed yesterday has bloomed into a beautiful flower.
A bottle of cold water is not cold anymore .
A banana that was yellow yesterday now has more brown spots and a strong smell.
Our world is in a constant state of change. Nothing stays the same. The ice melts, the flower blooms, the food ripens, and the seasons turn. We see these changes every moment of every day. We can observe them with our senses—we see the ice melt, we smell the banana, we feel the water warm up.
But as scientists, our job is not just to observe change, but to understand it. We must ask the big question: "Can we arrange these changes into categories?".
Are all these changes the same? Is a melting ice cube the same kind of change as a ripening banana? This chapter is our investigation to find the answer. We are about to become detectives, learning to sort every change we see into two of the most important categories in all of science: Physical Changes and Chemical Changes.
We will start our investigation with some simple, hands-on activities.
What happens when you take a piece of paper and fold it into an airplane?
What happens when you inflate a balloon?
What happens when you crush a piece of chalk into powder?
In all of these cases, you have made a change. The paper has a new shape. The balloon has a new size. The chalk is in a new form. But a key question remains: is it still the same "stuff"?
Of course! If you unfold the paper airplane, it is still a piece of paper. If you let the air out of the balloon, it is still a balloon. And that powder is still, chemically, chalk.
This is the key to our first category. These are called physical changes.
A physical change is one where the properties of a substance change—like its shape, size, or state—but the substance itself does not become something new. No new substance is formed.
The most famous physical change is one we see every day: water.
When solid ice melts, it becomes liquid water.
When liquid water boils, it becomes gaseous steam.
When that steam condenses, it becomes liquid water again.
Through all of this, its appearance changes dramatically, but its chemical identity never does. It is always, and only, water .
This is where the real transformation happens. These are changes where the substance does not remain the same.
Our first clue comes from a fascinating experiment. We will take two glasses, one with tap water and one with lime water (chuna). Then, using a straw, we will simply exhale and blow bubbles into each.
In the tap water, nothing happens. You just get bubbles.
But in the lime water, a bizarre change occurs: the clear liquid will instantly turn milky.
What just happened? This is not a physical change. You cannot "un-milk" the water. We have just witnessed a chemical change.
A chemical change is a process in which one or more new substances are formed. The "old" substances are used up, and completely new ones, with brand-new properties, are created. This process is called a chemical reaction.
Why did the lime water turn milky? The carbon dioxide in your breath (which is invisible) reacted with the lime water (calcium hydroxide) to create a brand new, white substance (calcium carbonate) that you could see .
What about the ripening banana? That is also a chemical change. The original substances in the banana are changing into new ones, which is why the colour, smell, and taste all change.
This new world of chemical versus physical changes can be tricky. Is a burning candle a physical change (because the wax melts) or a chemical change (because the wick burns)? Is dissolving salt in water a physical change (because you can get the salt back) or a chemical one? The lines can seem blurry.
This is where Teachoo comes in. Our entire job is to help you master these core concepts. We have designed clear, step-by-step explanations and comparisons to help you build a solid framework in your mind. We will walk you through every example, from the melting ice to the burning candle, and give you the tools to confidently identify and classify any change you see, just like a real scientist.
Throughout this chapter, we will explore many different kinds of chemical changes. We will learn to spot the clues that a chemical reaction is happening.
Making a New Gas: We will mix vinegar and baking soda . You will immediately hear a fizzing, bubbling sound. This is the sign that a new gas (carbon dioxide) is being created.
Rusting: We will investigate the rusting of iron. That flaky brown rust is not just "wet iron"; it is a new substance (iron oxide) formed from a chemical reaction . We will even set up an experiment to prove that rusting needs both air (oxygen) and water to happen.
Combustion (Burning): Burning is the ultimate chemical change.
When you burn a magnesium ribbon, it explodes in a dazzling white light, leaving behind a white, powdery new substance (magnesium oxide) .
Combustion is a chemical reaction where a substance reacts with oxygen and produces heat and/or light.
We will learn about the "Fire Triangle" : to have a fire, you must have three things: Fuel, Oxygen, and Heat (enough to reach the ignition temperature) . If you take away any one of these, the fire goes out. This is why wrapping a person in a blanket helps put out a fire—it cuts off the oxygen.
Finally, once we have mastered the difference between physical and chemical changes, we will learn a few other ways to classify them.
Reversible vs. Irreversible: Can the change be undone?
Reversible: Melting ice is reversible; you can just freeze it again.
Irreversible: Making popcorn from corn is irreversible; you can never get the kernel back.
Desirable vs. Undesirable: Do we want the change to happen?
Desirable: The ripening of fruit or the cooking of food.
Undesirable: The rusting of your bicycle or the spoiling of food.
We will also see that context matters: the decomposition of food is undesirable in your kitchen, but desirable when you are making compost for a garden .
This chapter will change the way you see the world. You will no longer just see "things happening." You will see a constant, dynamic interplay of physical and chemical changes, and you will have the scientific language to describe, categorise, and understand all of them.
To get started on this exciting journey, click on any topic link to begin your exploration.