Class 8 Science Curiosity Chapter 8 Solutions (NCERT 2026–27) – Nature of Matter: Elements, Compounds, and Mixtures

These Class 8 Science Curiosity Chapter 8 solutions cover Nature of Matter: Elements, Compounds, and Mixtures from the new NCF-2023 textbook (2026–27), with every textbook question answered step by step.

Class: 8 Subject: Science Book: Curiosity Chapter: 8 Exercise: Keep the curiosity alive (12 Qs) Session: 2026–27
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Class 8 Science Curiosity Chapter 8 Solutions – Overview

Chapter 8 of Curiosity, Nature of Matter: Elements, Compounds, and Mixtures, explains how the matter around us is organised. It begins with mixtures (uniform and non-uniform) and their components, distinguishes the everyday meaning of “pure” from the scientific one, and defines a pure substance. It then separates pure substances into elements (the simplest substances, made of atoms; classified into metals and non-metals) and compounds (elements combined chemically in a fixed ratio, with new properties). Through activities such as the electrolysis of water and heating an iron–sulfur mixture, the chapter shows the difference between mixtures and compounds, and ends with minerals and the everyday uses of elements, compounds, and mixtures. These Class 8 Science Curiosity Chapter 8 solutions answer every textbook question step by step.

Key Concepts & Definitions

Mixture: two or more substances mixed together where each substance retains its own properties and the components do not react chemically. The individual substances are its components.

Uniform (homogeneous) mixture: components are evenly distributed and cannot be seen separately (e.g. sugar in water, air). Non-uniform (heterogeneous) mixture: components are visible with the naked eye or a magnifier (e.g. sprout salad, sand in water).

Pure substance: matter made of the same type of particles that cannot be separated into other kinds of matter by any physical process. It can be an element or a compound.

Element: the simplest substance, made of one kind of atom, that cannot be broken down into simpler substances by chemical reactions (e.g. hydrogen, oxygen, gold, iron, sulfur). Elements are classified into metals and non-metals (with a few metalloids such as silicon and boron).

Compound: formed when different elements combine chemically in a fixed ratio to give a new substance with properties different from its constituent elements (e.g. water = hydrogen + oxygen in 2:1 atom ratio; sodium chloride = sodium + chlorine in 1:1 ratio).

Alloy: a metallic material containing two or more elements, at least one a metal (e.g. brass = copper + zinc; bronze = copper + tin; steel includes iron and carbon). Common alloys are treated as uniform mixtures in this chapter, though real alloys can contain multiple microscopic phases. Stainless steels contain chromium, and many grades also contain nickel. Mineral: a natural solid substance with a fixed chemical composition; most are compounds, but a few (native minerals) are pure elements.

In-text & Activity Questions — Answers

Probe and ponder

Which of the entities in the picture above consist of matter, and which of them do not?

ANSWERThe students, trees, ball, bushes, staircase, railing, water bottle, books, school bag, clothes and the air around them all consist of matter, because they have mass and take up space. Things like the sunlight, heat, sound and the children’s thoughts or emotions are not matter, as they have no mass and occupy no space.

How can elements be combined to form a compound?

ANSWERTwo or more different elements combine chemically in a fixed ratio to form a compound. For example, hydrogen and oxygen combine in a 2:1 atom ratio to form water, and sodium and chlorine combine in a 1:1 ratio to form sodium chloride. The compound formed has entirely new properties, different from those of the elements that made it.

How could the discovery of a compound that absorbs carbon dioxide from the air contribute to solving environmental challenges?

ANSWERA compound that captures carbon dioxide could be used to reduce the amount of CO₂ in the air, helping to slow down global warming and the greenhouse effect. It could be fitted in factory chimneys and vehicles to trap CO₂ before it escapes, improving air quality and supporting a cleaner, healthier environment.

Activity 8.1 — Testing carbon dioxide in air

On adding calcium oxide (quick lime) to water and then leaving the colourless lime water in a petri dish — what do you observe? Does it turn milky? Can you explain why the solution has turned milky?

ANSWER Calcium oxide reacts vigorously with water and releases heat, forming calcium hydroxide; filtering gives the colourless solution called lime water. On standing in air, the lime water turns milky. This happens because carbon dioxide present in the air reacts with calcium hydroxide to form insoluble white particles of calcium carbonate and water. The milkiness therefore proves that carbon dioxide is present in the air. Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

Activity 8.2 — Dust particles in air

After placing a clean black sheet of paper near an open window for a few hours — what do you observe?

ANSWERTiny particles settle on the black sheet and can be seen clearly with a magnifying glass. This shows that dust particles are suspended in the air. They are not an integral part of air; they are pollutants whose nature and number vary from place to place and time to time.

Table 8.1 — Complete the third column (Uniform or non-uniform)

ANSWER
S.No.Mixture-typeExamplesUniform / non-uniform
1.Gas and gasAirUniform
2.Gas and liquidAerated water (soda water); Oxygen dissolved in waterUniform; Uniform
3.Solid and gasCarbon particles in airNon-uniform
4.Liquid and liquidAcetic acid in water (vinegar); Oil and waterUniform; Non-uniform
5.Solid and liquidSand and water; SeawaterNon-uniform; Uniform
6.Solid and solidBaking powder (baking soda and tartaric acid); AlloysNon-uniform; Uniform

Activity 8.3 — Passing electricity through water

Is the volume of the gas collected the same in both the test tubes? On testing with a burning candle, what happens in each case and which gas is present in each test tube? Can these collected gases be water vapour?

ANSWERNo. Under the same temperature and pressure, the volume of hydrogen at the negative electrode (cathode) is about twice that of oxygen at the positive electrode (anode). The volume ratio is 2:1.In a teacher-led demonstration with suitable apparatus, a small separate hydrogen sample gives a pop with a flame; oxygen relights a glowing splint. These characteristic tests identify new substances, not water vapour. Water vapour can exist at room temperature, so failure to condense by itself is not a reliable identification test.Word equation: Water → Hydrogen + Oxygen (electrical energy supplied). Do not mix or ignite the collected gases; perform gas tests only under a trained teacher’s supervision.
Electrolysis produces two volumes of hydrogen at the negative cathode for one volume of oxygen at the positive anode.
Gas-volume ratio at the same temperature and pressure. Concept diagram, not assembly instructions. Tap to view at full size.

Activity 8.4 — Heating sugar

On heating a teaspoon of sugar in a boiling tube — what do you observe? Can you predict what is left behind?

ANSWEROn heating, the sugar first turns brown and then chars to a black mass, and small droplets of water collect near the open end of the tube. The black solid left behind is a carbon-rich char, not necessarily pure carbon. Since sugar decomposes on heating to give carbon and water (and water itself is hydrogen + oxygen), sugar cannot be an element — it is a compound made of carbon, hydrogen and oxygen.

Activity 8.5 — Iron and sulfur (Samples A and B)

Compare Sample A (the mixture of iron filings and sulfur) and Sample B (the black mass formed on heating). Record appearance, magnet test and gas test; and categorise the substances as mixtures, compounds or elements.

ANSWER
ExperimentSample A (mixture)Sample B (compound)
Appearance (colour, texture)Black and yellow particles seen separately; non-uniformUniform black mass; same colour and texture throughout
Magnet testIron filings are attracted to the magnet (can be separated)Not attracted by the magnet
Gas test with dilute HClColourless, odourless gas that burns with a ‘pop’ (hydrogen); sulfur is left as a yellow solidHydrogen sulfide, a toxic colourless gas; its known rotten-egg odour must never be tested by sniffing
Sample A is a mixture of two elements, iron and sulfur — the components keep their own properties and can be separated by a magnet. Sample B is a compound, iron sulfide, formed by heating iron and sulfur. It has new properties, is not magnetic, and its elements can no longer be separated by physical means. Iron + Dilute Hydrochloric acid → Iron chloride + Hydrogen gas Iron sulfide + Dilute Hydrochloric acid → Iron chloride + Hydrogen sulfide Classification of substances used: Elements → iron, sulfur; Compounds → iron sulfide, hydrogen chloride, iron(II) chloride, hydrogen sulfide; Mixtures → Sample A (iron + sulfur) and dilute hydrochloric acid (hydrogen chloride dissolved in water).

Laboratory safety: heating sulfur mixtures, handling acids and generating gases require a trained teacher and appropriate laboratory controls. Do not make or sniff hydrogen sulfide at home. Its smell is not a reliable warning of exposure (CDC/NIOSH chemical guidance).

A step further — classifying everyday materials

According to science, how would you classify milk, packed fruit juice, baking soda, sugar, and soil — as mixtures or pure substances?

ANSWERMixtures: milk, packed fruit juice and soil (each is made of more than one substance). Pure substances: baking soda (sodium hydrogen carbonate) and sugar are compounds, so scientifically they are pure substances.

Class 8 Science Curiosity Chapter 8 Solutions — Keep the Curiosity Alive

1. Consider the following reaction where two substances, A and B, combine to form a product C: A + B → C. Assume that A and B cannot be broken down into simpler substances by chemical reactions. Based on this information, which of the following statements is correct? (i) A, B, and C are all compounds and only C has a fixed composition. (ii) C is a compound, and A and B have a fixed composition. (iii) A and B are compounds, and C has a fixed composition. (iv) A and B are elements, C is a compound, and has a fixed composition.

ANSWER (iv) — A and B are elements, C is a compound, and it has a fixed composition. Since A and B cannot be broken down into simpler substances, they must be elements. When they combine chemically, they form a new substance C, which is a compound with a fixed ratio (fixed composition) of A and B.
Wording note: option (iv) is the most complete intended answer. Option (ii) is also scientifically true as written, since elements A and B have definite identities/composition. The options overlap; it is misleading to call (ii) wholly false.

2. Assertion: Air is a mixture. Reason: A mixture is formed when two or more substances are mixed, without undergoing any chemical change. (i) Both Assertion and Reason are true and Reason is the correct explanation for Assertion. (ii) Both Assertion and Reason are true, but Reason is not the correct explanation for Assertion. (iii) Assertion is true, but Reason is false. (iv) Assertion is false, but Reason is true.

ANSWER (i) — Both Assertion and Reason are true and the Reason is the correct explanation. Air is indeed a mixture of gases (nitrogen, oxygen, argon, carbon dioxide, water vapour) that are simply mixed together. Because these gases are mixed without any chemical change and keep their own properties, air is a mixture — exactly as the Reason states.

3. Water, a compound, has different properties compared to those of the elements oxygen and hydrogen from which it is formed. Justify this statement.

ANSWER When hydrogen and oxygen combine chemically in a fixed 2:1 ratio, they form a completely new substance, water, whose properties are quite different from those of its constituent elements. Hydrogen is a highly flammable fuel and oxygen supports combustion (helps things burn). But water is not a flammable fuel; its properties differ from those of the two gases. Water is a liquid at room temperature, whereas both hydrogen and oxygen are gases. This shows that a compound has its own characteristic properties, different from the elements that make it.

4. In which of the following cases are all the examples correctly matched? Give reasons in support of your answers. (i) Elements — water, nitrogen, iron, air. (ii) Uniform mixtures — minerals, seawater, bronze, air. (iii) Pure substances — carbon dioxide, iron, oxygen, sugar. (iv) Non-uniform mixtures — air, sand, brass, muddy water.

ANSWER (iii) Pure substances — carbon dioxide, iron, oxygen, sugar is correctly matched. Iron and oxygen are elements, while carbon dioxide and sugar are compounds; all of them consist of a single type of substance, so all are pure substances. (i) is wrong because water is a compound and air is a mixture — not elements. (ii) is wrong because minerals are mostly compounds (not all uniform mixtures). (iv) is wrong because air is a uniform mixture and brass is a uniform mixture (alloy), not non-uniform mixtures.

5. Iron reacts with moist air to form iron oxide, and magnesium burns in oxygen to form magnesium oxide. Classify all the substances involved in the above reactions as elements, compounds or mixtures, with justification.

ANSWER Elements: iron, oxygen and magnesium — each is made of a single kind of atom and cannot be broken down into simpler substances. Compounds: iron oxide and magnesium oxide — each is formed when two elements combine chemically in a fixed ratio, giving a new substance. Mixture: moist air — it is a mixture of gases (mainly nitrogen, oxygen, carbon dioxide) along with water vapour.

6. Classify the following as elements, compounds, or mixtures in Table 8.3: Carbon dioxide, sand, seawater, magnesium oxide, muddy water, aluminium, gold, oxygen, rust, iron sulfide, glucose, air, water, fruit juice, nitrogen, sodium chloride, sulfur, hydrogen, baking soda. Identify the pure substances among these.

ANSWER
ElementsCompoundsMixtures
Aluminium, gold, oxygen, nitrogen, sulfur, hydrogenCarbon dioxide, magnesium oxide, rust, iron sulfide, glucose, water, sodium chloride, baking sodaSand, seawater, muddy water, air, fruit juice
Pure substances (elements + compounds): aluminium, gold, oxygen, nitrogen, sulfur, hydrogen, carbon dioxide, magnesium oxide, rust, iron sulfide, glucose, water, sodium chloride, baking soda.
Classification note: this is the chapter-level classification of rust as hydrated iron oxide. Actual rust can contain several iron oxides/oxyhydroxides and variable water, so real rust is not generally a single pure compound. Natural sand is a mixture; chemically pure silicon dioxide would be a compound. Seawater here means its uniform dissolved-salt component, without suspended sediment.

7. What new substance is formed when a mixture of iron filings and sulfur powder is heated, and how is it different from the original mixture? Also, write the word equation for the reaction.

ANSWER On heating, the new substance formed is iron sulfide, a compound. Differences from the original mixture: the mixture is non-uniform and the iron in it is attracted by a magnet and can be separated, while iron sulfide is a uniform black mass that is not magnetic and cannot be separated into iron and sulfur by physical means. The compound also has completely new properties. Word equation: Iron + Sulfur → Iron sulfide

8. Is it possible for a substance to be classified as both an element and a compound? Explain why or why not.

ANSWER No, a substance cannot be both an element and a compound. An element is made of only one type of atom and cannot be broken down into simpler substances. A compound is made of two or more different elements joined chemically and can be broken down into those elements. Since these definitions are opposite, a single substance must be one or the other — it cannot be both at the same time.
Particle models show one kind of atom in an element, identical bonded groups of different atoms in a compound, and uncombined different particles in a mixture.
Original comparison: colours stand for different elements. Touching paired circles indicate chemical bonding; positions are schematic. Tap to view at full size.

9. How would our daily lives be changed if water were not a compound but a mixture of hydrogen and oxygen?

ANSWER If water were only a mixture of hydrogen and oxygen, it would behave like its components instead of being a stable liquid. Hydrogen is highly flammable and oxygen supports burning, so such a “mixture” would be explosive and extremely dangerous rather than safe to drink. We would have no liquid water to drink, cook, bathe, or grow crops; it could not be used to put out fires; and the hydrogen and oxygen could separate easily, so it would not stay together. Life as we know it would be impossible without water as a compound.

10. Analyse Fig. 8.24. Identify Gas A. Also, write the word equation of the chemical reaction.

ANSWER In Fig. 8.24, dilute hydrochloric acid is added to iron filings, so Gas A is hydrogen. It is a colourless, odourless gas that burns with a ‘pop’ sound. Word equation: Iron + Dilute Hydrochloric acid → Iron chloride + Hydrogen gas

11. Write the names of any two compounds made only from non-metals, and also mention two uses of each of them.

ANSWER Water (H₂O) — made of hydrogen and oxygen (both non-metals). Uses: drinking and cooking; and for irrigation/growing crops (also used to put out fires). Carbon dioxide (CO₂) — made of carbon and oxygen (both non-metals). Uses: used by green plants for photosynthesis; and in fire extinguishers and fizzy (aerated) drinks.

12. How can gold be classified as both a mineral and a metal?

ANSWER Gold is a metal because it is an element with metallic properties — it is shiny (lustrous), can be beaten into thin sheets and drawn into wires, and conducts heat and electricity. Gold is also a mineral because it occurs naturally in the Earth as a solid with a fixed composition. It is a native mineral — a mineral that exists as a pure element rather than as a compound. So gold is both a metal and a (native) mineral.

Common Misconceptions to Avoid

Watch out for these

  • Thinking the everyday meaning of “pure” (unadulterated) is the same as the scientific one — scientifically, pure milk or pure ghee is still a mixture.
  • Confusing a mixture with a compound — in a mixture components keep their properties and can be separated physically; in a compound the elements join chemically in a fixed ratio and gain new properties.
  • Assuming all mixtures are non-uniform — air, seawater and alloys (brass, bronze, stainless steel) are uniform mixtures.
  • Believing water can “burn” because it contains hydrogen and oxygen — water is a compound with its own properties and is not a flammable fuel; this does not make it suitable for every type of fire.
  • Calling every element a metal — many elements (oxygen, hydrogen, carbon, sulfur) are non-metals, and a few (silicon, boron) are metalloids.
  • Thinking minerals are always pure elements — most minerals are compounds; only a few native minerals (gold, silver, copper, sulfur) are elements.

Extra Practice Questions

These questions and MCQs are ClearStudy practice, separate from the NCERT exercise.

Very Short Answer Type Questions

Q1. In what ratio (by number of atoms) are hydrogen and oxygen present in water?

ANSWER2:1 (two atoms of hydrogen to one atom of oxygen).

Q2. Name one liquid metal and one liquid non-metal at room temperature.

ANSWERMercury is a liquid metal; bromine is a liquid non-metal.

Q3. What is an alloy? Give one example.

ANSWERAn alloy is a metallic material containing two or more elements, at least one of which is a metal. Example: brass contains copper and zinc. Steel includes the non-metal carbon as well as iron. The chapter treats common alloys as uniform mixtures.

Short Answer Type Questions

Q1. Differentiate between a uniform and a non-uniform mixture with one example each.

ANSWERIn a uniform mixture the components are evenly distributed and cannot be seen separately, e.g. sugar dissolved in water. In a non-uniform mixture the components are not evenly spread and can be seen with the naked eye or a magnifier, e.g. sand mixed with water.

Q2. How would you show experimentally that air contains carbon dioxide?

ANSWERLeave colourless lime water (calcium hydroxide solution) exposed to air for some time. It slowly turns milky because the carbon dioxide in air reacts with calcium hydroxide to form insoluble calcium carbonate. This milkiness proves carbon dioxide is present in air.

Long Answer Type Question

Q1. Compare an element, a compound and a mixture on the basis of composition, separation and properties, giving one example of each.

ANSWER Element: made of only one kind of atom; cannot be broken into simpler substances by chemical reactions; has its own fixed properties. Example: oxygen, iron. Compound: two or more elements combined chemically in a fixed ratio; can be broken into its elements only by chemical means; has new properties different from its elements. Example: water, sodium chloride. Mixture: two or more substances mixed in any proportion; components keep their own properties and can be separated by physical methods; no new substance is formed. Example: air, sand and water.

MCQs & Assertion–Reason

1. Which of the following is a uniform mixture?

(a) sand and water    (b) sprout salad    (c) air    (d) oil and water

2. A pure substance that cannot be broken down into simpler substances is called a/an:

(a) mixture    (b) element    (c) compound    (d) solution

3. The ratio of hydrogen atoms to oxygen atoms in water is:

(a) 1:1    (b) 1:2    (c) 2:1    (d) 3:1

4. Brass is an alloy of:

(a) copper and tin    (b) copper and zinc    (c) iron and carbon    (d) iron and nickel

5. When iron filings and sulfur powder are strongly heated, the substance formed is:

(a) iron oxide    (b) iron chloride    (c) iron sulfide    (d) iron sulfate

6. Which gas turns lime water milky?

(a) oxygen    (b) hydrogen    (c) nitrogen    (d) carbon dioxide

7. Which of the following is a compound?

(a) gold    (b) air    (c) sodium chloride    (d) brass

8. The two elements that are liquid at room temperature are:

(a) mercury and bromine    (b) gallium and caesium    (c) sodium and chlorine    (d) silicon and boron

9. On passing electricity through acidified water, the gas collected in larger volume is:

(a) oxygen    (b) hydrogen    (c) nitrogen    (d) carbon dioxide

10. A mineral that occurs as a pure element rather than a compound is called a:

(a) native mineral    (b) compound mineral    (c) metalloid    (d) alloy

Answer key: 1-(c), 2-(b), 3-(c), 4-(b), 5-(c), 6-(d), 7-(c), 8-(a), 9-(b), 10-(a).

For each Assertion–Reason question, choose: (A) Both true and the Reason correctly explains the Assertion; (B) Both true but the Reason is not the correct explanation; (C) Assertion true, Reason false; (D) Assertion false, Reason true.

A-R 1. Assertion: Water is a compound.

Reason: Hydrogen and oxygen are combined chemically in a fixed ratio in water and cannot be separated by physical means.

A-R 2. Assertion: The components of a mixture retain their own properties.

Reason: The components of a mixture do not react chemically with each other.

A-R 3. Assertion: Iron sulfide is attracted by a magnet.

Reason: Iron sulfide is a compound with properties different from iron and sulfur.

A-R 4. Assertion: All minerals are pure elements.

Reason: Minerals are natural solid substances with a fixed chemical composition.

A-R 5. Assertion: An element cannot be broken down into simpler substances by chemical reactions.

Reason: An element contains atoms with the same number of protons; chemical reactions do not change that identity.

Answer key: 1-(A), 2-(A), 3-(D), 4-(D), 5-(A).

Quick Revision Summary

  • A mixture has two or more substances mixed together; the components keep their own properties and do not react chemically.
  • Mixtures are uniform (evenly distributed, e.g. air, seawater) or non-uniform (components visible, e.g. sprout salad, sand in water).
  • A pure substance is made of the same type of particles and can be an element or a compound.
  • Elements are the simplest substances, made of atoms of the same element; they are classified into metals and non-metals (118 known; only mercury and bromine are liquid at room temperature).
  • Compounds form when elements combine chemically in a fixed ratio (water = H:O 2:1; sodium chloride = Na:Cl 1:1) and have new properties.
  • Alloys (brass, bronze, stainless steel) contain a metal and one or more other elements and are treated as uniform mixtures in this chapter; minerals are natural solids, mostly compounds but sometimes native elements like gold.

Real-life Applications

The idea of elements, compounds and mixtures is everywhere around us. The air we breathe is a mixture of gases, and water is a compound essential for life. Elements like iron and aluminium build bridges, buildings and vehicles, while chemists combine elements to make life-saving medicines, vaccines and fertilisers. Engineers design useful materials such as stainless steel, which is stronger and more rust-resistant than pure iron, and wonder materials like graphene aerogel that can soak up oil spills. Even traditional crafts such as Dhokra art use alloys of brass and bronze, and everyday products like cement and talcum powder come from minerals.

How to write a clear answer

Be able to tell a mixture from a compound at a glance: mixtures can be separated physically and keep their properties; compounds need chemical change and have new properties. Memorise key examples — water (H:O = 2:1) and sodium chloride (Na:Cl = 1:1) as compounds, air and seawater as uniform mixtures, and alloys as mixtures of metals. For activity questions, always name the gas tests (pop sound = hydrogen, glowing splint = oxygen, lime water turns milky = carbon dioxide, hydrogen sulfide is toxic and must not be identified by sniffing) and write the word equations neatly.

Frequently Asked Questions

What is Class 8 Science Curiosity Chapter 8 about?

Chapter 8, Nature of Matter: Elements, Compounds, and Mixtures, explains mixtures (uniform and non-uniform), pure substances, elements (metals and non-metals), compounds, alloys and minerals, and how to tell them apart through simple activities.

What is the difference between a compound and a mixture?

In a mixture, substances are simply mixed, keep their own properties and can be separated by physical methods. In a compound, elements are combined chemically in a fixed ratio, form a new substance with different properties, and can be separated only by chemical means.

How many questions are in the “Keep the curiosity alive” exercise of Chapter 8?

There are 12 exercise questions, all solved on this page along with the activity and in-text questions.

Are these Class 8 Science Curiosity Chapter 8 solutions free?

Yes. All ClearStudy NCERT Solutions for Class 8 Science Curiosity are free and follow the official NCERT textbook for 2026–27.

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