Nature of Matter Class 8: Elements, Compounds, Mixtures, Pure Substances and Minerals

What Nature of Matter: Elements, Compounds, and Mixtures is about

Key concepts

Mixtures: uniform and non-uniform

NCERT section 8.1 What Are Mixtures?

A mixture forms when two or more substances are mixed and each keeps its own properties. The substances in it are its components, and they do not react chemically with each other.

• Non-uniform mixtures: components can be seen separately with the eye or a magnifier: sprout salad, poha, sand in water, oil and water.
• Uniform mixtures: components are evenly spread and cannot be told apart even under a microscope: sugar in water, air, vinegar, soda water, alloys.

Alloys are uniform mixtures of metals (sometimes with a little non-metal): stainless steel (iron, nickel, chromium and a little carbon), brass (copper and zinc), bronze (copper and tin). Ancient Indian texts such as the *Charaka Samhita* and *Susruta Samhita* describe *mishraloha*; bronze (*kamsya*) was made from copper (*tamra*, 4 parts) and tin (*vanga*, 1 part).

In everyday language a mixture's components may themselves be mixtures (onion, tomato), but in science the components of a mixture must be pure substances.

Worked example

Classify as uniform or non-uniform mixtures: sea water, muddy water, brass, a sprout salad, vinegar, sand and iron filings.

Try it first, then show the solutionSolution

1. Uniform (components cannot be seen separately): sea water, brass, vinegar.
2. Non-uniform (components visible): muddy water, sprout salad, sand and iron filings.

Answer: Uniform: sea water, brass, vinegar. Non-uniform: muddy water, sprout salad, sand with iron filings.

Air is a mixture

NCERT section 8.1.1 Is air a mixture?

Air is a uniform mixture of mainly nitrogen (about 78%), oxygen, argon, carbon dioxide and water vapour.

• Oxygen is needed by most living things and supports burning; nitrogen does not take part in burning.
• Water vapour shows up as droplets when warm air touches a cold surface.

Activity 8.1, carbon dioxide in air: calcium oxide (quicklime) added to water reacts vigorously, giving calcium hydroxide and heat; stirred and filtered, this gives clear lime water. Left in a petri dish in air for a few hours (stirring now and then), it turns milky: carbon dioxide from the air forms tiny white particles of calcium carbonate.

Word equation: calcium hydroxide + carbon dioxide → calcium carbonate + water.

Activity 8.2, dust: a clean black sheet left near an open window collects tiny particles: dust is suspended in air, but it is a pollutant, not a true part of air. Other pollutants: soot, carbon monoxide, ozone, nitrogen dioxide, sulphur dioxide. The Air Quality Index (AQI) describes air quality.

Worked example

Give three pieces of evidence from the chapter that air is a mixture and not a single substance.

Try it first, then show the solutionSolution

1. Lime water turns milky in air, showing carbon dioxide is present along with other gases.
2. Water droplets form on a cold surface, showing water vapour is present.
3. Oxygen supports burning while nitrogen does not, so air contains different gases that keep their own properties; their amounts can vary (more dust or vapour in some places).

Answer: Carbon dioxide test, water vapour on cold surfaces, components with their own separate properties.

Types of mixtures by state

NCERT section 8.1.2 Types of mixtures

Mixtures can be grouped by the states of their components:

• gas in gas: air (uniform);
• gas in liquid: soda water, oxygen dissolved in water (uniform);
• solid in gas: carbon (soot) particles in air (non-uniform);
• liquid in liquid: vinegar, acetic acid in water (uniform); oil and water (non-uniform);
• solid in liquid: sea water (uniform); sand and water (non-uniform);
• solid in solid: alloys (uniform); baking powder, a mix of baking soda and tartaric acid (non-uniform).

Why separate mixtures? At home we separate to keep the useful part (removing stones from rice). In science we separate mixtures to obtain pure substances.

Worked example

State the type (by states of components) and whether uniform or non-uniform: (a) soda water, (b) smoke in air, (c) oil and water, (d) bronze.

Try it first, then show the solutionSolution

1. (a) Gas in liquid; uniform.
2. (b) Solid in gas; non-uniform.
3. (c) Liquid in liquid; non-uniform (two layers).
4. (d) Solid in solid (alloy); uniform.

Answer: (a) gas–liquid, uniform (b) solid–gas, non-uniform (c) liquid–liquid, non-uniform (d) solid–solid, uniform.

Pure substances: everyday versus scientific meaning

NCERT section 8.2 What Are Pure Substances?

On packets of milk, ghee or spices, 'pure' means unadulterated. Adulteration is the illegal adding of cheaper or poorer substances to increase quantity or cut costs; it spoils quality and can harm health.

In science, a pure substance contains only one kind of particle and cannot be separated into other kinds of matter by any physical process. So milk, fruit juice, soil and air are mixtures, even if they are unadulterated, while sugar, baking soda, water, oxygen and gold are pure substances.

Pure substances are of two types: elements and compounds.

Worked example

A milk packet says '100% pure'. Is milk a pure substance in science? Explain the two meanings.

Try it first, then show the solutionSolution

1. On the packet, 'pure' means nothing cheaper or harmful has been added (not adulterated).
2. In science, pure means made of one kind of particle only.
3. Milk contains water, fats, proteins, sugar and more, so it is a mixture, not a pure substance.

Answer: No; it is unadulterated, but scientifically it is a mixture.

Elements and the electrolysis of water

NCERT section 8.3.1 Elements

Water can change between ice, liquid and vapour and back, keeping the same particles. But Activity 8.3 (demonstration) passes electricity through water: two water-filled test tubes are placed over the terminals of a 9 V battery in water with a few drops of dilute sulphuric acid. Bubbles collect in both tubes, with twice as much gas in one tube as in the other.

• A burning candle near one tube gives a pop sound: hydrogen.
• Near the other, the flame glows brighter: oxygen.
• These gases are not water vapour, or they would condense back to water.

So water is made of hydrogen and oxygen, and breaking it this way is a chemical change.

Elements are pure substances that cannot be broken down into simpler substances. Each is made of identical atoms, different from the atoms of any other element; they are the building blocks of all matter (hydrogen, oxygen, gold, silver, sulphur, carbon).

• Atoms of many elements join into molecules: two hydrogen atoms make a hydrogen molecule; two oxygen atoms make an oxygen molecule.
• Elements are metals (gold, silver, magnesium, iron, aluminium), non-metals (carbon, sulphur, hydrogen, oxygen) or metalloids (silicon, boron).
• 118 elements are known; most are solid; 11 are gases (all non-metals); only mercury and bromine are liquids at room temperature; gallium and caesium melt at about 30 °C.
• More than 45 elements are used to make a mobile phone.

Worked example

In Activity 8.3, 30 mL of gas in total is collected, with one tube holding twice as much as the other. How much gas is in each tube, and how would you identify each gas?

Try it first, then show the solutionSolution

1. Split 30 mL in the ratio 2 : 1: 20 mL and 10 mL.
2. The 20 mL tube holds hydrogen: a burning candle brought near gives a pop sound.
3. The 10 mL tube holds oxygen: the candle flame glows brighter.

Answer: 20 mL hydrogen (pop test) and 10 mL oxygen (brighter flame).

Compounds: fixed ratio, new properties

NCERT section 8.3.2 Compounds

In water, hydrogen and oxygen are held so tightly that physical methods cannot separate them. A compound forms when different elements combine chemically in a fixed ratio to make a new substance with different properties from its elements.

• Water: hydrogen and oxygen atoms in the ratio 2 : 1. Hydrogen is a fuel and oxygen supports burning, yet water puts out fires.
• Sodium chloride (common salt): soft metal sodium and poisonous gas chlorine, combined 1 : 1, give a safe, essential seasoning. Salt can be separated from water by evaporation (a mixture), but salt itself cannot be split into sodium and chlorine by physical means.
• Sugar (Activity 8.4): heated in a boiling tube, sugar turns brown, then black; water droplets appear near the mouth and charcoal (carbon) remains. So sugar is a compound of carbon, hydrogen and oxygen.

Other examples: carbon dioxide, iron oxide (rust), magnesium oxide, baking soda, glucose, iron sulphide.

Worked example

When sugar is heated, water droplets form near the mouth of the tube and black charcoal remains. Why can't the water have come from the air, and what does the experiment prove?

Try it first, then show the solutionSolution

1. The tube is being heated, so water vapour from the air would not condense inside the hot tube: the water must come from the sugar.
2. Charcoal is carbon, and water contains hydrogen and oxygen.
3. So sugar breaks down into simpler substances: it is a compound of carbon, hydrogen and oxygen, not an element.

Answer: The water comes from the sugar; sugar is a compound of carbon, hydrogen and oxygen.

Mixture versus compound: iron and sulphur

NCERT section 8.3.2 Compounds

Activity 8.5 (demonstration): 5.6 g of iron filings and 3.2 g of sulphur powder are mixed (Sample A); half is heated in a china dish until a black mass forms, which is ground (Sample B).

Test	Sample A (mixture)	Sample B (compound)
Appearance	grey iron and yellow sulphur particles visible	uniform black colour and texture
Magnet	iron filings are attracted, so iron can be separated	not attracted
Dilute hydrochloric acid	hydrogen: colourless, odourless, burns with a pop; sulphur left as yellow solid	hydrogen sulphide: colourless, smells of rotten eggs

Word equations:

• iron + dilute hydrochloric acid → iron chloride + hydrogen
• iron + sulphur → iron sulphide (on heating)
• iron sulphide + dilute hydrochloric acid → iron chloride + hydrogen sulphide

Mixture: components keep their properties, any proportion, separable by physical means. Compound: new substance, fixed ratio, new properties, not separable by physical means. (Do not smell gases directly: waft them towards your nose.)

Worked example

In Activity 8.5, why is Sample B not attracted by a magnet while Sample A is?

Try it first, then show the solutionSolution

1. In Sample A, iron is just mixed with sulphur and keeps its own properties, including magnetism.
2. In Sample B, iron has combined chemically with sulphur to form iron sulphide, a new substance.
3. Iron sulphide has different properties and is not attracted by the magnet.

Answer: Sample B is a new compound (iron sulphide) without iron's magnetic property.

Uses of elements, compounds and mixtures; minerals

NCERT section 8.5 What Are Minerals?

Uses:

• Air (mixture) for breathing; water (compound) for life; iron and aluminium (elements) for bridges, buildings and vehicles.
• Chemists combine elements into compounds to make medicines, vaccines and fertilisers.
• Engineers design alloys such as stainless steel (stronger and more durable than pure iron); wood, steel and concrete are mixtures.
• Graphene aerogel, made from carbon, is extremely light and porous and could help clean up oil spills.
• Dhokra art (Bihar and Odisha): a wax model is covered in clay, the wax melted out, and molten brass or bronze poured in.

Minerals (8.5): most rocks are mixtures of minerals. Minerals are natural solids with a fixed chemical composition.

• Native minerals are pure elements: metals (gold, silver, copper) or non-metals (sulphur, carbon).
• Most minerals are compounds: quartz, calcite, mica, pyroxene, olivine, talc.
• Cement is made from calcite, quartz, alumina and iron oxide; talcum powder from talc.

Not everything is matter: light, heat, electricity, thoughts and emotions have no mass and take up no space.

Worked example

Native copper and the mineral calcite are both found in rocks. Classify each as an element or a compound, and explain why both are still called minerals.

Try it first, then show the solutionSolution

1. Native copper is made of one kind of atom only, so it is an element (a metal); it is a native mineral.
2. Calcite is made of more than one element combined in a fixed ratio, so it is a compound mineral.
3. Both are natural solids with a fixed chemical composition, which is what makes something a mineral.

Answer: Native copper: element (native mineral); calcite: compound mineral; both are natural solids of fixed composition.

Science in real life: Nature of Matter: Elements, Compounds, and Mixtures

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Topics in this chapter

• Elements, Compounds and Minerals
• Mixtures, Air, Alloys and Pure Substances

Nature of Matter: Elements, Compounds, and Mixtures: formulas and key facts

Classification of matter

$\text{matter}\to \text{mixtures (uniform, non-uniform)}\mid \text{pure substances (elements, compounds)}$

Water

$\text{H : O atoms}=2:1,{\text{H}}_2\text{O}$

Common salt

$\text{Na : Cl}=1:1,\text{sodium chloride}$

Iron and sulphur (word equation)

$\text{iron}+\text{sulfur}\stackrel{\text{heat}}{\to }\text{iron sulfide}$

Lime water test (word equation)

$\text{calcium hydroxide}+\text{carbon dioxide}\to \text{calcium carbonate}+\text{water}$

Gas tests

$\text{hydrogen: pop sound};\text{oxygen: flame glows brighter};{\text{CO}}_2\text{: lime water milky}$

Bronze (ancient recipe)

$\text{copper : tin}=4:1$

Elements in numbers

$118\text{ known};11\text{ gases};2\text{ liquids (mercury, bromine)}$

Nature of Matter: Elements, Compounds, and Mixtures: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Nature of Matter: Elements, Compounds, and Mixtures

The question patterns to expect, and the method that works for each.

Tricks to remember Nature of Matter: Elements, Compounds, and Mixtures

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Mix keeps, compound changes

  In a mixture each part keeps its character; in a compound the elements change into something new.

  Use it when: Mixture versus compound.

• Pop for hydrogen, glow for oxygen

  Hydrogen pops, oxygen makes the flame glow, carbon dioxide turns lime water milky.

  Use it when: Gas tests.

• Water is 2 : 1

  Two H for every O, and twice as much hydrogen gas in electrolysis.

  Use it when: Water's composition.

• Eggs smell sulphide

  Rotten-egg smell means hydrogen sulphide from iron sulphide.

  Use it when: Iron sulphide test.

• Two liquid elements: Mercury and Bromine

  'Many Birds' sit liquid at room temperature: Mercury and Bromine.

  Use it when: Element facts.

Nature of Matter: Elements, Compounds, and Mixtures: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section E style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 4-mark question split into 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Nature of Matter: Elements, Compounds, and Mixtures

The slips examiners see most often, and what to write instead.

• ✗Mistake: Calling air a compound

  It looks uniform.

  Wrong working

  Air is a compound of nitrogen and oxygen.

  Right working

  Air is a uniform mixture; its gases keep their properties and their amounts vary.

  ✓Do this: Uniform does not mean compound.

  Usually costs: 1 mark.

• ✗Mistake: Calling brass or steel an element

  Alloys look like single metals.

  Wrong working

  Stainless steel is a pure metal element.

  Right working

  Stainless steel is an alloy, a uniform mixture of iron, nickel, chromium and a little carbon.

  ✓Do this: Alloys are mixtures.

  Usually costs: 1 mark.

• ✗Mistake: Saying the gases from water are water vapour

  Bubbles from water are assumed to be steam.

  Wrong working

  The gases in the tubes are water vapour.

  Right working

  They are hydrogen and oxygen; vapour would condense back to water, and the pop and glow tests identify them.

  ✓Do this: Use the gas tests to identify gases.

  Usually costs: 1 mark.

• ✗Mistake: Mixing up the gases from iron and iron sulphide with acid

  Both reactions give 'a gas'.

  Wrong working

  Both samples give hydrogen sulphide.

  Right working

  Iron (mixture) gives hydrogen (pop, no smell); iron sulphide gives hydrogen sulphide (rotten-egg smell).

  ✓Do this: Link each sample to its own gas and test.

  Usually costs: 1-2 marks.

• ✗Mistake: Using 'pure' in the everyday sense in science answers

  Packet labels are trusted.

  Wrong working

  Pure ghee is a pure substance.

  Right working

  Unadulterated ghee is still a mixture of several substances.

  ✓Do this: Scientific pure = one kind of particle.

  Usually costs: 1 mark.

NCERT exercise guide: Nature of Matter: Elements, Compounds, and Mixtures

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Practice questions on Nature of Matter: Elements, Compounds, and Mixtures

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   Which of these is an element?
   (a) Sugar
   (b) Sulphur
   (c) Salt
   (d) Steel

   Hint

   Cannot be broken down.

   Answer

   (b) Sulphur

2. Q2 · 1 mark · MCQExam level

   Which gas gives a pop sound with a burning candle?
   (a) Oxygen
   (b) Nitrogen
   (c) Hydrogen
   (d) Carbon dioxide

   Hint

   Electrolysis of water.

   Answer

   (c) Hydrogen

3. Q3 · 1 mark · Very short answerExam level

   Name the two elements that are liquids at room temperature.

   Hint

   One metal, one non-metal.

   Answer

   Mercury (metal) and bromine (non-metal).

4. Q4 · 2 marks · Very short answerExam level

   What is adulteration? Why is it harmful?

   Hint

   Cheaper substances.

   Answer

   Illegally adding cheaper or poor-quality substances to a product; it lowers quality and can harm health.

5. Q5 · 2 marks · Very short answerHarder exam level

   Explain why the black sheet of paper in Activity 8.2 collects particles, and why dust is not considered a part of air.

   Hint

   Pollutant.

   Answer

   Dust particles suspended in air settle on the sheet. Dust varies from place to place and time to time and is a pollutant, not a true component of air.

6. Q6 · 2 marks · Very short answerHarder exam level

   Sodium is a soft metal and chlorine a poisonous gas, yet sodium chloride is eaten daily. Explain what this shows about compounds.

   Hint

   New properties.

   Answer

   A compound has properties different from those of its elements; sodium and chlorine combine in a 1 : 1 ratio to form a new, safe substance.

7. Q7 · 3 marks · Short answerHarder exam level

   Complete: gas in liquid (example and uniform or not); liquid in liquid that is non-uniform; solid in solid that is uniform.

   Hint

   Table 8.1.

   Answer

   Gas in liquid: soda water or oxygen in water, uniform. Liquid in liquid, non-uniform: oil and water. Solid in solid, uniform: an alloy such as brass.

8. Q8 · 3 marks · Short answerHarder exam level

   Why can salt be separated from salt water by evaporation, but sodium cannot be separated from salt by evaporation?

   Hint

   Mixture versus compound.

   Answer

   Salt water is a mixture, so a physical process like evaporation separates salt from water. Salt itself is a compound of sodium and chlorine joined chemically; physical processes cannot split it.

9. Q9 · 2 marks · Very short answerHarder exam level

   Give one compound that contains a metal and one that is made only of non-metals, from the chapter. Name the elements in each.

   Hint

   Salt; water or carbon dioxide.

   Answer

   Sodium chloride (sodium, a metal, and chlorine, a non-metal); carbon dioxide (carbon and oxygen, both non-metals). Other examples: iron sulphide; water, glucose.

10. Q10 · 5 marks · Long answerHarder exam level

    Describe the electrolysis of water (Activity 8.3): set-up, observations, gas tests and conclusion.

    Hint

    9 V battery.

    Answer

    Set-up: beaker of water with a few drops of dilute sulphuric acid, a 9 V battery inside, two water-filled test tubes inverted over the terminals. Observations: bubbles at both terminals; one tube collects about twice as much gas. Tests: a burning candle near the larger volume gives a pop (hydrogen); near the smaller, the flame glows brighter (oxygen). Conclusion: water is a compound made of hydrogen and oxygen.

11. Q11 · 2 marks · Very short answerHOTS / competency

    Can two samples of a compound, such as pure water from a river and pure water from a lab, have different ratios of hydrogen to oxygen? Explain.

    Hint

    Fixed ratio.

    Answer

    No. A compound always has a fixed ratio of its elements; every water molecule has 2 hydrogen atoms to 1 oxygen atom, whatever its source.

12. Q12 · 3 marks · Short answerHOTS / competency

    Graphene aerogel is made from carbon and is highly porous. Is it an element, a compound or a mixture as described? Why could it help with oil spills?

    Hint

    Made from carbon.

    Answer

    It is a form of the element carbon (made only of carbon). Being extremely light and highly porous, it can soak up large amounts of oil, so it could help clean oil spills at sea and on land.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.What are the components of a mixture?Show answer

   The individual substances that make it up.

2. 2.What is an alloy? Give an example.Show answer

   A uniform mixture of metals (sometimes with a non-metal), such as brass or stainless steel.

3. 3.Which gas makes up about 78% of air?Show answer

   Nitrogen.

4. 4.What does lime water turning milky in air show?Show answer

   Air contains carbon dioxide.

5. 5.What is a pure substance in science?Show answer

   A substance made of only one kind of particle, which cannot be separated into other kinds of matter physically.

6. 6.What are the two gases obtained by passing electricity through water?Show answer

   Hydrogen and oxygen.

7. 7.Define an element.Show answer

   A pure substance that cannot be broken down into simpler substances; made of one kind of atom.

8. 8.Define a compound.Show answer

   A substance formed when elements combine chemically in a fixed ratio, with properties different from its elements.

9. 9.Which two elements are liquids at room temperature?Show answer

   Mercury and bromine.

10. 10.What is a native mineral?Show answer

    A mineral that is a pure element, such as gold, silver, copper, sulphur or carbon.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you define mixture, component, element and compound?
• ☐Can you sort ten substances into elements, compounds and mixtures?
• ☐Can you explain why air is a mixture?
• ☐Can you describe the lime water test for carbon dioxide?
• ☐Can you explain the two meanings of 'pure'?
• ☐Can you describe and explain the electrolysis of water with gas tests?
• ☐Can you explain why sugar is a compound?
• ☐Can you compare Samples A and B in the iron-sulphur activity?
• ☐Can you write the word equations from Activity 8.5?
• ☐Can you name native and compound minerals?

Nature of Matter: Elements, Compounds, and Mixtures: frequently asked questions