Nature of Matter: Elements, Compounds, and Mixtures – Class 8 Science Chapter 8 Notes & NCERT PDF

Class 8ScienceChapter 8NCERT book: Curiosity - Science Class 8

Notes and a simple summary of Chapter 8 of the NCERT Class 8 Science book, with the official chapter PDF, flashcards, an MCQ quiz and an AI tutor for your doubts.

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

Probe and Ponder

Which things around us are matter, and which are not?

How can elements combine to form a compound?

How could a compound that absorbs carbon dioxide from the air help the environment?

Introduction

The staircase, the air, the water in your bottle, your lunch and clothes are all matter, made of tiny particles. Most of these are not one single substance; they are two or more substances mixed together. This chapter sorts all matter into mixtures and pure substances (elements and compounds).

Key Concepts

8.1 What are mixtures?

When two or more substances are mixed, and each substance retains its properties, it is called a mixture. The individual substances are its components.

Components of a mixture do not react chemically with each other.

Non-uniform mixtures: components are visible to the naked eye or with a magnifying device. Examples: poha, sprout salad (green gram, chickpeas, onion, tomato).

Uniform mixtures: components are evenly spread and cannot be told apart, even under a microscope. Example: sugar in water.

Alloys are uniform mixtures of metals:

Stainless steel: iron, nickel, chromium and a little carbon

Brass: copper and zinc

Bronze: copper and tin

Our scientific heritage: Ancient texts (Charaka Samhita, Susruta Samhita, Rasaratna Samucchaya, Rasa Jala Nidhi) mention Mishraloha, alloys used in medicine. Bronze (Kamsya) was made of copper (Tamra, 4 parts) and tin (Vanga, 1 part).

#### Is air a mixture?

Air is a uniform mixture mainly of nitrogen, oxygen, argon, carbon dioxide and water vapour.

Nitrogen makes up about 78% of air and does not take part in combustion; oxygen supports combustion and life.

Activity 8.1: Quick lime (calcium oxide) + water gives lime water (calcium hydroxide). Left in air, it turns milky, which proves carbon dioxide is present in air.

Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

Activity 8.2: A black sheet left near a window collects dust. Dust is suspended in air but is a pollutant, not an integral part of air.

Major air pollutants: particulate matter (dust, soot), carbon monoxide, ozone, nitrogen dioxide, sulfur dioxide. The Air Quality Index (AQI) describes air quality.

#### Types of mixtures (by state)

Gas + gas: air

Gas + liquid: soda water, oxygen dissolved in water

Solid + gas: carbon particles in air

Liquid + liquid: vinegar (acetic acid in water), oil and water

Solid + liquid: sand and water, seawater

Solid + solid: baking powder (baking soda + tartaric acid), alloys

In science, the components of a mixture must themselves be pure substances.

8.2 What are pure substances?

In everyday use, "pure" means unadulterated. Adulteration is the illegal adding of cheaper or poorer-quality substances, which can make a product hazardous.

In science:

A pure substance cannot be separated into other kinds of matter by any physical process; it consists of the same type of particles.

8.3 Types of pure substances

Activity 8.3 (demonstration): A 9 V battery is placed in water with a few drops of dilute sulfuric acid, with two water-filled test tubes over its terminals.

Gas bubbles collect; the volumes in the two tubes are not equal.

A burning candle gives a pop sound in one tube: hydrogen.

In the other, the flame glows brighter: oxygen.

Water → Hydrogen + Oxygen

#### Elements

Elements cannot be broken down into simpler substances. Each is made of identical particles called atoms.

Elements are the building blocks of all matter: gold, silver, sulfur, carbon, and so on.

Most atoms combine to form molecules: 2 hydrogen atoms make 1 hydrogen molecule; 2 oxygen atoms make 1 oxygen molecule.

Elements are metals (gold, silver, magnesium, iron, aluminium), non-metals (carbon, sulfur, hydrogen, oxygen) or metalloids (silicon, boron).

Facts from the book:

118 elements are known; most are solids.

11 are gases at room temperature, all non-metals.

Only 2 are liquids: mercury (a metal) and bromine (a non-metal).

Gallium and caesium become liquid around 30 °C (303 K).

More than 45 elements are used to make a mobile phone.

#### Compounds

Compounds form when different elements combine chemically in a fixed ratio. Their properties differ from those of their elements, and they cannot be separated by physical methods.

Water: hydrogen (a fuel) and oxygen (supports combustion) form water, which puts out fire. The ratio of hydrogen to oxygen atoms is 2:1.

Sodium chloride: sodium (a soft metal) and chlorine (a hazardous gas) combine in a 1:1 ratio to make harmless common salt.

Activity 8.4 (sugar): Heating sugar turns it brown, then black; water droplets form and charcoal (carbon) is left. So sugar is a compound of carbon, hydrogen and oxygen.

#### Activity 8.5: mixture vs compound (iron and sulfur)

5.6 g iron filings + 3.2 g sulfur powder are mixed to give Sample A. Half of it is heated to a black mass, which is Sample B.

Appearance: A shows separate black and yellow particles; B is the same colour and texture throughout.

Magnet test: iron in A is attracted, so A can be separated; B is not attracted.

Acid test (dilute HCl):

- A gives hydrogen: colourless, no smell, burns with a pop. Sulfur is left as a yellow solid. Iron + Dilute hydrochloric acid → Iron chloride + Hydrogen gas

- B (iron sulfide) gives hydrogen sulfide: colourless, with a rotten-egg smell. Iron sulfide + Dilute hydrochloric acid → Iron chloride + Hydrogen sulfide

Conclusion: A is a mixture of two elements; B is a compound with completely new properties. Always waft a gas towards your nose; never smell directly.

8.4 Uses of elements, compounds and mixtures

Water (a compound) sustains life; iron and aluminium are used for bridges, buildings and vehicles.

Chemists combine elements to make medicines, vaccines and fertilisers.

Engineers design alloys such as stainless steel, which is stronger than pure iron. Wood, steel and concrete are mixtures.

Graphene aerogel, made from carbon, is said to be the lightest material on Earth. It can rest on grass and could help clean up oil spills.

8.5 What are minerals?

Most rocks are mixtures of minerals.

Native minerals are pure elements: gold, silver, copper, sulfur, carbon.

Most minerals are compounds: quartz, calcite, mica, pyroxene, olivine.

Cement comes from calcite, quartz, alumina and iron oxide; talcum powder comes from talc.

Our scientific heritage: Dhokra art (Bihar and Odisha). A beeswax model is covered in clay, the wax is melted out, and molten brass or bronze is poured in.

Not matter: light, heat, electricity, thoughts and emotions.

Key Terms

Mixture (मिश्रण): substances mixed without chemical change, each keeping its properties

Component (घटक): an individual substance in a mixture

Uniform mixture (समांगी मिश्रण): components evenly spread and indistinguishable

Non-uniform mixture (विषमांगी मिश्रण): components visible separately

Alloy (मिश्रधातु): a uniform mixture of metals (e.g. brass, bronze)

Pure substance (शुद्ध पदार्थ): made of one type of particle

Element (तत्व): cannot be broken into simpler substances

Atom (परमाणु): the identical particle making up an element

Molecule (अणु): stable particle of two or more atoms combined

Compound (यौगिक): elements chemically combined in a fixed ratio

Metalloid (उपधातु): element with properties between metals and non-metals

Mineral (खनिज): natural solid with fixed chemical composition

Adulteration (मिलावट): illegally adding cheaper or poor-quality substances

Real-life Connections

Checking food labels (detergents, snacks) for mixtures and compounds

Lime water turning milky proves the CO₂ we breathe out and live among

AQI reports tell us how polluted the air is

Brass and bronze utensils at home are alloys

Common Misconceptions

❌ "Air is an element." ✅ Air is a uniform mixture of gases.

❌ "A mixture of iron and sulfur is iron sulfide." ✅ Only heating forms the compound iron sulfide.

❌ "'Pure' on a ghee pack means pure in science." ✅ It means unadulterated; scientifically, ghee is a mixture.

❌ "Compounds have the properties of their elements." ✅ Water puts out fire, although hydrogen burns and oxygen supports burning.

❌ "The gases from electrolysis of water are water vapour." ✅ They would condense back if they were; they are hydrogen and oxygen.

Keep the Curiosity Alive (hints)

Q1: A and B are elements; C is a compound with a fixed composition, option (iv).

Q2: Both the assertion and the reason are true, and the reason explains the assertion.

Q4: Option (iii) is correct: carbon dioxide, iron, oxygen and sugar are all pure substances.

Q7: Heating gives iron sulfide, which is not magnetic and releases hydrogen sulfide with acid. Iron + Sulfur → Iron sulfide

Q10: Iron filings + dilute HCl gives hydrogen (Gas A). Iron + Dilute hydrochloric acid → Iron chloride + Hydrogen

Q12: Gold occurs naturally as a native mineral and is a metallic element.

💡 Key Learning Points

✓A mixture contains two or more substances that keep their properties and do not react; mixtures can be uniform or non-uniform.
✓A pure substance is made of one type of particle and cannot be separated by physical processes; it is either an element or a compound.
✓Elements cannot be broken down further; compounds are elements chemically combined in a fixed ratio (water 2:1, sodium chloride 1:1).
✓Heating iron and sulfur forms iron sulfide, a compound with new properties: non-magnetic, and giving hydrogen sulfide with dilute HCl.
✓Minerals are natural solids with fixed composition; a few are native elements (gold, sulfur) but most are compounds (quartz, calcite, mica).

👨‍🏫 Teaching Tips

→Start with poha, sprout salad and ORS to sort everyday foods into uniform and non-uniform mixtures before introducing definitions.
→Demonstrate Activity 8.3 (electrolysis with a 9 V battery) and let students hear the pop and see the brighter flame.
→Do the iron-sulfur comparison as a side-by-side table on the board (appearance, magnet, acid, smell) while students record observations.
→Model the safe wafting technique for smelling gases before any gas test.
→Bring brass, bronze and stainless-steel objects and link them to Mishraloha and Kamsya from Indian heritage.
→Set a sorting game with cards for the 19 substances in Question 6 (elements, compounds, mixtures).
→Discuss adulteration and food labels to show why the scientific meaning of 'pure' matters.

📋 Assessment Questions

Can the student classify given substances as elements, compounds or mixtures, with reasons?
Can the student list at least three differences between Sample A (mixture) and Sample B (compound) in Activity 8.5?
Can the student explain why water is a compound and not a mixture of hydrogen and oxygen?
Can the student describe the lime-water test for carbon dioxide and write its word equation?
Can the student give examples of native minerals and of compound minerals?

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