NCERT Class 8 Science Curiosity Chapter 08 Nature of Matter Elements Compounds and Mixtures PDF Download

Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures

Probe and ponder

• Which of the entities in the picture above consist of matter, and which of them do not?
• How can elements be combined to form a compound?
• How could the discovery of a compound that absorbs carbon dioxide from the air contribute to solving environmental challenges?
• Share your questions _____________________________ ?

Have you ever wondered what the world around you is made of? Look around! The staircase you use, the air you breathe, the water in your bottle, the food in your lunch box, the clothes and shoes you wear, the book you read, the trees outside, the ball you play with and even the stick you carry - all of these are examples of matter, which you have learnt in earlier grades.

You have also learnt that all these things are made up of tiny particles. Most of the things around us are not made of just one substance; rather they are made up of two or more substances mixed together. Let us now understand how different substances come together to form mixtures.

8.1 What Are Mixtures?

Have you ever wondered what makes your poha (Fig. 8.1) so delicious or how to make the perfect sprout salad? While these dishes may seem very different, they share something in common - they are both made by mixing several ingredients. We observe the mixing of substances in everyday life. Sugar dissolved in water is also a mixture, and so are soups and lemonade.

When two or more substances are mixed, where each substance retains its properties, it is called a mixture. The individual substances that make up a mixture are called its components. The components of a mixture do not react chemically with each other. In some mixtures, the components - like green gram, chickpeas, onion, and tomato in a sprout salad (Fig. 8.2), are easy to see. Such mixtures, where the different components are generally visible with the naked eye or with a magnifying device, are non-uniform in nature. Can you identify a few more examples of non-uniform mixtures around you?

On the other hand, some mixtures have components that cannot be seen separately even with the help of a microscope. For example, sugar and water particles cannot be seen separately in their mixture. Such types of mixtures, where the components are evenly distributed and cannot be distinguished, are uniform in nature (Fig. 8.3). Can you list a few uniform mixtures?

[Figure 8.1: Poha, See in your textbook]

[Figure 8.2: Sprout salad, See in your textbook]

[Figure 8.3: Uniform mixture of sugar and water, See in your textbook]

Teacher's Note

In exams, do not confuse uniform and non-uniform mixtures. A non-uniform mixture has components you can see as separate parts (like vegetables in a salad), while a uniform mixture looks the same throughout with no visible separate parts (like sugar dissolved in water). If you can identify individual substances with your eyes or a magnifying glass, it is non-uniform.

A step further

Do you know that stainless steel is also a mixture? Stainless steel contains iron, nickel, chromium, and a small amount of carbon. They are mixed so uniformly that the entire mixture appears the same throughout and one cannot see the individual substances. Such mixtures are known as alloys. Brass, a mixture of copper and zinc, and bronze, a mixture of copper and tin, are some other examples of alloys (Fig. 8.4).

[Figure 8.4: Utensils made of stainless steel, brass, and bronze, See in your textbook]

Our scientific heritage

Mishraloha was the name given to the mixture of two or more metals that had properties distinct from its constituent metals. Ancient Indian texts, such as the Charaka Samhita, Susruta Samhita, Rasaratna Samucchaya, Rasa Jala Nidhi, etc., mention the use of alloys for medicinal purposes. For example, Bronze, also known as Kamsya, is an alloy made up of Copper (Tamra, 4 parts) and Tin (Vanga, 1 part), was used to improve digestion and boost immunity.

8.1.1 Is air a mixture?

In Curiosity, Grade 6, you learnt about air and its composition in the chapter 'Nature's Treasures'. Is air a mixture? What kind of mixture is it?

You have learnt that air is a uniform mixture of mainly nitrogen, oxygen, argon, carbon dioxide, and water vapour. Out of these, oxygen is required by most of the living beings to stay alive. It also helps in combustion. Nitrogen, which constitutes about 78% of the air, does not take part in combustion. We also learnt that air has water vapour in it. When warm air touches a cool surface, the water vapour turns into liquid water, forming tiny droplets. Recall the experiment where you tested the presence of carbon dioxide in the air that we exhale. Let us confirm the presence of carbon dioxide in the air.

Activity 8.1: Let us experiment

• Take a glass tumbler and fill it half with water.
• Add a small amount of calcium oxide (quick lime) slowly to it.
• What do you observe?
• Calcium oxide reacts vigorously with water to form calcium hydroxide and releases heat.

Safety first

Perform this step carefully.

Teacher's Note

When the question asks "Is air a mixture?", remember to answer with two points: first, what type of mixture it is (uniform), and second, name the main components it contains. This is how examiners expect a complete answer to this question.

Key Points

• A mixture is formed when two or more substances combine while keeping their individual properties, and they do not react chemically with each other.
• Non-uniform mixtures have visible separate components that you can see with your eyes or a magnifying glass, like a sprout salad; uniform mixtures look the same throughout and their components cannot be seen separately, like sugar dissolved in water.
• Air is a uniform mixture mainly containing nitrogen, oxygen, argon, carbon dioxide, and water vapour.
• Alloys are uniform mixtures of two or more metals, such as stainless steel, brass, and bronze.

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