Class 8 Science NCERT Notes – Chapter 8: Nature of Matter: Elements, Compounds and Mixtures (PDF, MindMap, Q&A, Quizzes)

Study Notes

This guide provides a comprehensive overview of “Chapter 8: Nature of Matter: Elements, Compounds and Mixtures” of Class 8 NCERT Science (Curiosity) Book.

1. Understanding Matter

Matter is the substance that makes up everything in the world that has mass and takes up space. Examples include everyday objects like staircases, air, water, food, clothes, books, trees, and even living beings. All matter is composed of tiny particles. However, concepts like light, heat, electricity, thoughts, and emotions are not considered matter.

2. Mixtures: Combining Substances

A mixture is formed when two or more substances are combined in such a way that each substance retains its own individual properties. The individual substances within a mixture are called components, and they do not react chemically with each other.

2.1 Types of Mixtures

Mixtures are categorized based on the visibility and distribution of their components:

• Non-Uniform Mixtures: In these mixtures, the different components are generally visible to the naked eye or with a magnifying device. The components are not evenly distributed. Examples include poha, sprout salad (containing green gram, chickpeas, onion, tomato), carbon particles in air, sand and water, and oil and water.

• Uniform Mixtures: In these mixtures, the components are evenly distributed and cannot be distinguished from one another, even with a microscope. The entire mixture appears the same throughout. Examples include sugar dissolved in water, air, seawater, and alloys.

2.2 Air: A Uniform Mixture

Air is a uniform mixture of several gases. Its primary components are:

• Nitrogen (~78%)
• Oxygen
• Argon
• Carbon dioxide
• Water vapour

Air also contains suspended pollutants like dust and soot (particulate matter) and gases such as carbon monoxide, ozone, nitrogen dioxide, and sulfur dioxide. The Air Quality Index (AQI) is a tool used to describe air quality.

Demonstrating Components of Air:

Carbon Dioxide 

The presence of CO₂ in the air can be confirmed using lime water (a solution of calcium hydroxide). When exposed to air, the CO₂ reacts with the calcium hydroxide to form insoluble white particles of calcium carbonate, turning the lime water milky.

• Chemical Reaction: Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

Dust Particles 

Tiny dust particles suspended in the air can be observed by placing a clean black sheet of paper near an open window for a few hours. Particles will settle on its surface.

2.3 Alloys: Special Mixtures

An alloy is a uniform mixture containing two or more metals, or a metal and a small amount of non-metal, resulting in a substance with properties distinct from its constituent metals.

• Stainless Steel: An alloy of iron, nickel, chromium, and a small amount of carbon.

• Brass: An alloy of copper and zinc.

• Bronze (Kamsya): An alloy of copper (4 parts) and tin (1 part). Ancient Indian texts mention its use for medicinal purposes.

3. Pure Substances: The Scientific Definition

In common usage, “pure” might mean unadulterated. In science, a pure substance is one that consists of only one type of particle and cannot be separated into other kinds of matter by any physical process. For a scientist, items like milk or ghee are considered mixtures, not pure substances.

Pure substances are classified into two categories: elements and compounds.

4. Elements: The Building Blocks of Matter

An element is a pure substance that cannot be broken down into simpler substances. Elements are the fundamental building blocks of all matter.

• Each element is made up of identical particles called atoms.
• The atoms of one element are different from the atoms of any other element.
• Two or more atoms can combine to form a stable particle of that element called a molecule (e.g., a molecule of hydrogen is formed from two hydrogen atoms).
• There are 118 known elements.

Classification and States of Elements:

• Metals: Examples include gold, silver, iron, aluminium, mercury.
• Non-metals: Examples include carbon, sulfur, hydrogen, oxygen, bromine.
• Metalloids: Elements with properties intermediate between metals and non-metals, such as silicon and boron.

States at Room Temperature:

• Solid: Most elements.
• Gas: Eleven elements (all are non-metals, e.g., oxygen, helium, nitrogen).
• Liquid: Two elements—mercury (a metal) and bromine (a non-metal). Gallium and caesium become liquid around 30 °C.

5. Compounds: Chemically Combined Elements

A compound is a pure substance formed when two or more different elements combine chemically in a fixed ratio.

• The properties of a compound are entirely new and different from those of its constituent elements.
• The constituent elements of a compound cannot be separated by physical methods.

Key Examples and Experiments:

Water (H₂O):

• Formed from hydrogen (a fuel) and oxygen (supports combustion), but water itself extinguishes fire.
• The ratio of hydrogen atoms to oxygen atoms is fixed at 2:1.
• Electrolysis of Water (Activity 8.3): Passing an electric current through water breaks it down into hydrogen gas (identified by a ‘pop’ sound with a burning candle) and oxygen gas (causes a burning candle’s flame to glow brighter). This demonstrates that water is a compound, not an element.

Sodium Chloride (NaCl):

• Formed from sodium (a soft metal) and chlorine (a hazardous gas) in a 1:1 ratio to create a harmless, essential substance (common salt).

Sugar (C₁₂H₂₂O₁₁):

• Heating Sugar (Activity 8.4): Gently heating sugar causes it to decompose. It turns brown, then black (charring), leaving behind charcoal (carbon) and releasing water droplets. This shows that sugar is a compound made of carbon, hydrogen, and oxygen.

Iron Sulfide (FeS):

• Mixture vs. Compound (Activity 8.5):
  • Sample A (Mixture): A simple mix of iron filings and sulfur powder. The components retain their properties. Iron can be separated using a magnet. Adding dilute hydrochloric acid produces hydrogen gas from the iron.
  • Sample B (Compound): Created by heating the iron-sulfur mixture. A new black substance, iron sulfide, is formed. It is not attracted by a magnet. It reacts with dilute hydrochloric acid to produce hydrogen sulfide gas, which has a rotten egg smell. This demonstrates the formation of a compound with new properties.

6. Minerals and Applications

• Minerals: Natural, solid substances found on Earth with a fixed chemical composition. Most rocks are mixtures of minerals.
  • Native Minerals: Pure elements found in nature, such as gold, silver, copper, sulfur, and carbon.
  • Compound Minerals: Most minerals are compounds, such as quartz, calcite, mica, and talc.
• Applications:
  • Innovation: Understanding elements and compounds is key to creating medicines, fertilizers, and advanced materials.
  • Materials Science: Engineers design alloys (like stainless steel) and use mixtures (wood, concrete) for construction. Graphene aerogel, a material made from carbon, is the lightest known material and has high absorption capacity, making it useful for cleaning oil spills.
  • Art: The Indian Dhokra art uses alloys like brass or bronze to create figures.

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Quiz

Answer each question in 2-3 sentences based on the information provided in Chapter 8 of the NCERT Class 8 Science Curiosity book.

1. What is the scientific definition of a pure substance, and how does it differ from the common use of the word “pure”?

2. Explain the key difference between a uniform mixture and a non-uniform mixture, providing one example of each.

3. Describe the experiment that demonstrates that air contains carbon dioxide. What is the observable result?

4. What is an element, and what are its fundamental particles called?

5. Water is a compound. How do its properties compare to the properties of its constituent elements, hydrogen and oxygen?

6. What is an alloy? Provide two examples mentioned in the text.

7. What happens when sugar is heated in a test tube, and what does this experiment prove about the composition of sugar?

8. In the experiment comparing an iron-sulfur mixture (Sample A) and heated iron-sulfur (Sample B), how does their reaction to a magnet differ and why?

9. What are minerals, and what are the two main types of minerals based on their composition?

10. Besides physical objects, what are some examples of things that are not considered matter?

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Answer Key

1. In science, a pure substance consists of only one type of particle and cannot be separated by physical means. In common usage, “pure” means unadulterated, but scientifically, products like milk are considered mixtures because they contain more than one substance.

2. A uniform mixture has components that are evenly distributed and cannot be seen separately, like air or salt water. A non-uniform mixture has components that are not evenly distributed and are often visible, such as a sprout salad or sand in water.

3. The experiment involves exposing lime water (calcium hydroxide solution) to the air. The carbon dioxide in the air reacts with the calcium hydroxide to form calcium carbonate, which causes the clear lime water to turn milky.

4. An element is a pure substance that cannot be broken down into simpler substances. Its fundamental particles are called atoms, and all atoms of a specific element are identical.

5. Water’s properties are completely different from its elements. Hydrogen is a fuel and oxygen supports combustion, whereas water is a compound used to extinguish fire.

6. An alloy is a uniform mixture of two or more metals, or a metal and non-metal, with properties distinct from its components. Examples include bronze (copper and tin) and stainless steel (iron, chromium, nickel, carbon).

7. When sugar is heated, it turns brown and then black, leaving a residue of carbon and releasing water droplets. This proves that sugar is not an element but a compound composed of carbon, hydrogen, and oxygen.

8. A magnet attracts the iron filings in the iron-sulfur mixture (Sample A), allowing for physical separation. The magnet has no effect on Sample B because the iron and sulfur have chemically reacted to form a new compound, iron sulfide, which is not magnetic.

9. Minerals are natural, solid substances with a fixed chemical composition found on Earth. The two types are native minerals, which are pure elements like gold or sulfur, and compound minerals, which are made of more than one element like quartz or calcite.

10. Things that are not considered matter do not have mass or take up space. Examples given include light, heat, electricity, thoughts, and emotions.

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Essay Questions

1. Using the experiments involving water (Activity 8.3) and the iron-sulfur mixture (Activity 8.5), explain in detail the fundamental differences between a compound and a mixture in terms of formation, properties, and separation of components.

2. Discuss the classification of matter. Begin with the primary division into pure substances and mixtures, and then elaborate on the sub-categories (elements, compounds, uniform mixtures, non-uniform mixtures), providing specific examples for each category from the text.

3. Describe the composition of air as a uniform mixture. Explain the methods used to demonstrate the presence of two of its components (carbon dioxide and dust particles) and discuss the role of other components like oxygen and nitrogen.

4. Explain how an understanding of elements, compounds, and mixtures is crucial for innovation and daily life, citing examples such as medicine, materials science (graphene aerogel, alloys), and art (Dhokra art).

5. Define elements and describe how they are classified. Include information on the number of known elements, their states at room temperature, and the distinction between metals, non-metals, and metalloids.

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Glossary

Term	Definition
Adulteration	An illegal process of adding cheaper or poor-quality substances to a product to increase quantity or reduce manufacturing cost, which deteriorates quality.
Air Quality Index (AQI)	A tool used to describe the quality of the air.
Alloy	A uniform mixture of two or more metals, or a metal and a non-metal, with properties distinct from its constituent metals. Examples: stainless steel, brass, bronze.
Atom	The identical, tiny particles that make up an element.
Compound	A pure substance composed of two or more elements chemically combined in a fixed ratio, having properties different from its constituent elements.
Components	The individual substances that make up a mixture.
Element	A pure substance that cannot be broken down into simpler substances. They are the building blocks of all matter.
Graphene Aerogel	A ‘wonder’ material made from carbon, said to be the lightest material on earth, with a high absorbing capacity.
Lime Water	A solution of calcium hydroxide in water, used to test for the presence of carbon dioxide.
Matter	Everything that has mass and takes up space. It is made up of tiny particles.
Metalloids	Elements that have intermediate properties between those of metals and non-metals. Examples: silicon, boron.
Minerals	Natural, solid substances found on the Earth with a fixed chemical composition. Most are compounds, but some are pure elements.
Mixture	A combination of two or more substances where each substance retains its individual properties and does not react chemically.
Molecule	A stable particle of an element formed when two or more atoms of that element combine.
Non-Uniform Mixture	A mixture where the different components are generally visible and are not evenly distributed.
Pollutants	Substances present in the air that are not an integral part of it, such as dust, soot, carbon monoxide, and sulfur dioxide.
Pure Substance	In science, a substance that consists of only one type of particle and cannot be separated into other kinds of matter by any physical process.
Uniform Mixture	A mixture where the components are evenly distributed and cannot be distinguished from one another, even with a microscope.