Master Chapter 4 Class 7 - The World of Metals and Non-metals (Curiosity) with comprehensive NCERT Solutions, Practice Questions, MCQs, Sample Papers, Case Based Questions, and Video lessons.
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Welcome to Chapter 4. Our journey begins in a village in Rajasthan, where two students, Yashwant and Anandi, are on a school project to learn about local crafts. They decide to visit the workshop of an ironsmith, Sudarshan uncle, to see how he creates tools.
When they arrive, they see him take a solid block of iron, heat it in a furnace until it glows red-hot, and then... he starts beating it with a hammer. With each powerful blow, the hard, solid iron does not shatter or crack. Instead, it slowly flattens and changes its shape, right before their eyes, as Sudarshan skillfully shapes it into an axe.
Anandi is amazed by this. She has never seen anything like it. You cannot take a piece of wood and hammer it flat. You cannot take a lump of coal and beat it into a sheet. It would just break into pieces. This leads her to the central question of our entire chapter: "Wow, a piece of iron can be beaten into a flat shape! Can we do this with other metals as well?"
What is this "magic" property that iron has? And is it unique to iron, or do other materials share it?
This chapter is our investigation to answer Anandi's question. In the last chapter, we learned one way to classify materials: by seeing if they conduct electricity. But that is just one small piece of the puzzle. Now, we are going to become material scientists and explore all the hidden properties that separate the world's materials into two great families: the Metals and the Non-metals.
To find out what makes metals special, we are going to become detectives and test a whole collection of materials, just like in Activity 4.1. We will gather an iron nail, a piece of copper, some aluminum, a lump of coal, a piece of sulfur, and a block of wood.
We will put each one through a series of "challenges" to discover its true properties.
The Hammer Test (Malleability): This is Anandi's first question. What happens when we beat them with a hammer? We will discover that the iron, copper, and aluminum all flatten into thin sheets. This special property is called malleability. It is the reason we can have paper-thin aluminum foil for wrapping food or delicate silver foil on sweets. The coal and sulfur, on the other hand, will shatter into pieces. We will learn that they are brittle.
The Stretch Test (Ductility): Can we stretch the material? We will find that metals have a "sister" property to malleability: they can be pulled and drawn out into thin wires. This is called ductility. This is why the electrical wiring in our homes is made of copper or aluminum. It is also why a single gram of gold can be stretched into a wire two kilometres long!
The Shine Test (Lustre): What do they look like? We will notice that the metals, when freshly cut, are shiny. This property is called metallic lustre. Non-metals like coal, sulfur, and wood are dull or non-lustrous.
The Sound Test (Sonority): What sound do they make when dropped or struck? We will find that metals produce a clear, ringing sound. This is called sonority. It is the reason why school bells, temple bells, and ghungroos (dancing bells) are made of metal, while a piece of wood just makes a dull "thud."
The Touch Test (Hardness): How do they feel? We will find that most metals, like iron, are very hard. This is why we use them for building bridges and tools. (Though we will also learn about some strange exceptions, like the metal sodium, which is so soft it can be cut with a knife!).
The Heat Test (Conduction): What happens when we heat them? In Activity 4.3, we will put a metal spoon and a wooden spoon into a cup of hot water. We will quickly find that the handle of the metal spoon becomes hot, while the wooden spoon's handle stays cool. This is because metals are excellent conductors of heat. This is precisely why we make our cooking pots out of metal—to get the heat to our food. It is also why the handles are made of wood or plastic, which are poor conductors (or insulators).
The Electricity Test (Conduction): This is what we learned in the last chapter. We will use our tester circuit again and confirm that all the metals—iron, copper, aluminum—are good conductors of electricity. This is why they are used for wires. The non-metals are all poor conductors (insulators).
This is a long, detailed list of new scientific words: malleable, ductile, lustrous, sonorous, brittle, conductor, insulator. It can be a challenge to keep them all straight—to remember which one means "hammering" and which one means "wires." This is where having a clear guide is essential. Teachoo is built to help you with exactly this. Think of us as your personal science dictionary, a place that breaks down each of these new terms into simple, clear explanations and connects each property to a real-world example you already know. We help you build your scientific vocabulary, so you can speak and think like a scientist.
Once we have learned all the physical properties (how they look, feel, and act), our investigation will go even deeper. We will start asking about their chemical properties. What happens when these materials react with the world around them?
The Mystery of Rust
We have all seen it: iron objects left outside get covered in a flaky, brown substance. This is rust. But how does it happen? Does it just need air? Does it just need water? We will solve this mystery with a clever experiment (Activity 4.5).
We will place one iron nail in a bottle with dry air only.
We will place a second nail in a bottle with water only (boiled, with a layer of oil on top to keep air out).
We will place a third nail in a bottle with both air and water.
We will discover that only the third nail rusts! This proves that rusting is not a simple process; it is a chemical reaction that requires both air (oxygen) and water to happen. We will also learn that this is part of a larger family of reactions called corrosion, which is why copper turns green and silver turns black.
The Litmus Test Revisited: The Secret of Oxides
This is the most exciting part of our investigation. We will connect this chapter to what we learned in Chapter 2, "Acids, Bases, and Neutral." What happens when we burn metals and non-metals?
Metals: We will take a magnesium ribbon (a metal) and burn it. It will burn with a dazzling white flame, leaving a white, powdery ash. This ash is called magnesium oxide. When we mix this ash with water and test it with litmus paper, we will find that it turns red litmus blue. This proves that the oxide of a metal is basic in nature.
Non-metals: We will then take some sulfur powder (a non-metal) and burn it. It will produce a gas (sulfur dioxide). When we mix this gas with water, it forms a new solution. And when we test this solution, we will find that it turns blue litmus red. This proves that the oxide of a non-metal is acidic in nature.
This is a fundamental rule of chemistry!
Metals $\rightarrow$ Basic Oxides
Non-metals $\rightarrow$ Acidic Oxides
So, we now have a complete picture. We can finally define the two great families. Metals are the materials that are generally hard, lustrous, malleable, ductile, sonorous, and good conductors.
And what are non-metals? They are the exact opposite. They are generally soft (or brittle, like coal), dull in appearance, not malleable or ductile, not sonorous, and are poor conductors of both heat and electricity.
But this does not make them less important. In fact, non-metals are the building blocks of life itself. We will learn that oxygen, the gas we cannot live without, is a non-metal. Carbon, the element that makes up all food (proteins, fats, and carbohydrates) and all living things, is a non-metal. Nitrogen, which is essential for plants to grow, is a non-metal.
This chapter will change the way you see the world. You will no longer just see a "spoon" or a "wire." You will see a "malleable, conductive, sonorous metal." You will no longer just see a "lump of coal." You will see a "brittle, non-lustrous, non-metal." You will learn the secret code of properties that explains why things are the way they are.
To get started on this journey, click on any topic link to begin your exploration.