Where Mindful Learning Begins
Where Mindful Learning Begins
Class 8 Science Curiosity NCERT Solutions Chapter 8 — every Probe and Ponder prompt, all 5 Activities, all 12 "Keep the Curiosity Alive" exercise questions, and all 4 "Discover, Design, and Debate" project prompts, solved and explained on one page.
This chapter sorts the matter all around us into mixtures (like poha, air, and seawater), and pure substances — which are further split into elements (like hydrogen, oxygen, iron, and gold) and compounds (like water and common salt). Along the way, it shows how to actually test for these using lime water, electrolysis, heating sugar, and mixing iron with sulfur — and how it all connects to minerals and everyday materials.
Chapter 8 starts by looking at everyday dishes like poha and sprout salad, using them to introduce mixtures — substances mixed together without reacting chemically, where each component keeps its own properties. It then separates non-uniform mixtures (whose components are visible) from uniform mixtures (whose components blend so completely they can't be told apart), before turning to what scientists mean by "pure." From there, the chapter builds up elements (like hydrogen and oxygen, obtained by passing electricity through water) and compounds (like water itself, common salt, and iron sulfide), showing exactly how compounds differ from both elements and mixtures, and rounds off with how minerals fit into this same picture. Every Activity and exercise is solved here exactly as the textbook presents it.
Two or more substances combined without a chemical reaction — each keeps its own properties, whether uniform or non-uniform.
A substance with no other substance present in it — made up of only one type of particle throughout.
Elements can't be broken down further; compounds are two or more elements chemically joined in a fixed ratio, with entirely new properties.
Mixtures
When two or more substances are mixed without reacting chemically, each substance keeps its own properties — this is called a mixture, and the individual substances in it are its components.
Table 8.1 (completed) — types of mixtures by physical state:
| Mixture type | Examples | Uniform or non-uniform |
|---|---|---|
| Gas and gas | Air | Uniform |
| Gas and liquid | Aerated water (soda water) | Uniform |
| Gas and liquid | Oxygen dissolved in water | Uniform |
| Solid and gas | Carbon particles in air | Non-uniform |
| Liquid and liquid | Acetic acid in water (vinegar) | Uniform |
| Liquid and liquid | Oil and water | Non-uniform |
| Solid and liquid | Sand and water | Non-uniform |
| Solid and liquid | Seawater | Uniform |
| Solid and solid | Baking powder (baking soda + tartaric acid) | Uniform |
| Solid and solid | Alloys | Uniform |
Alloys — like stainless steel (iron, nickel, chromium, and a little carbon), brass (copper and zinc), and bronze (copper and tin) — are uniform mixtures of metals, mixed so thoroughly that individual components cannot be seen.
Pure substances
In common usage, "pure" means unadulterated (not mixed with cheaper or lower-quality substances). In science, a pure substance is one with no other substance present in it — it cannot be separated into other kinds of matter by any physical process, and consists of only one type of particle throughout. Pure substances are of two kinds: elements and compounds.
Classifying milk, packed fruit juice, baking soda, sugar, and soil:
| Item | Mixture or pure substance |
|---|---|
| Milk | Mixture (water, fats, proteins, sugars, and minerals combined) |
| Packed fruit juice | Mixture (water, fruit extract, sugar, and other added substances) |
| Baking soda | Pure substance (a compound — sodium bicarbonate) |
| Sugar | Pure substance (a compound — sucrose) |
| Soil | Mixture (minerals, organic matter, water, and air combined) |
Elements
An element is a pure substance made of identical particles called atoms, and cannot be broken down into any simpler substance — elements are the building blocks of all matter (e.g. hydrogen, oxygen, gold, silver, sulfur, carbon). Atoms of most elements cannot exist independently and combine to form a molecule — for example, two hydrogen atoms form one molecule of hydrogen, and two oxygen atoms form one molecule of oxygen.
| Category | Quick facts |
|---|---|
| Metals | Gold, silver, magnesium, iron, aluminium |
| Non-metals | Carbon, sulfur, hydrogen, oxygen |
| Metalloids | Silicon, boron — intermediate properties between metals and non-metals |
| Total known elements | 118, most of them solids at room temperature |
| Gaseous elements | 11 elements are gases at room temperature, all non-metals (e.g. oxygen, helium, nitrogen) |
| Liquid elements | Only 2 — mercury (a metal) and bromine (a non-metal) |
| Near-liquid solids | Gallium and caesium are solids that turn liquid at around 30 °C (303 K) |
Compounds
A compound forms when different elements combine chemically in a fixed ratio to make something entirely new, with properties completely different from its constituent elements. The elements in a compound cannot be separated by any physical method.
| Compound | Elements & ratio |
|---|---|
| Water | Hydrogen : Oxygen = 2 : 1 |
| Common salt (sodium chloride) | Sodium : Chlorine = 1 : 1 |
Minerals
A mineral is a natural, solid substance with a fixed chemical composition, found in rocks. Native minerals are pure elements (metals like gold, silver, copper; non-metals like sulfur, carbon). Most minerals, though, are compounds of more than one element — common examples include quartz, calcite, mica, pyroxene, olivine, and talc. Cement is made from calcite, quartz, alumina, and iron oxide.
Sample answer: in a scene like a school courtyard, entities such as the students, their clothes, backpacks, books, the football, the trees, flowers, and the building all consist of matter, since they all have mass and take up space. However, things like sunlight falling across the courtyard, the shadows cast by the trees and building, and the sounds of children playing do not consist of matter — light, shadow (an absence of light), and sound are all real and important parts of the world, but none of them are made of particles the way matter is.
Elements combine to form a compound when their atoms join together chemically in a fixed, definite ratio — for example, hydrogen and oxygen atoms combining in a 2:1 ratio to form water. This chemical joining creates an entirely new substance with its own distinct properties, different from either starting element, and the elements can no longer be separated from each other by ordinary physical means.
A compound that could efficiently absorb carbon dioxide directly from the air could help tackle climate change by actively pulling out one of the main greenhouse gases responsible for global warming, potentially being used in large-scale air-capture technology, industrial exhaust filters, or building materials that passively soak up CO₂ from the atmosphere — helping to slow the rate at which greenhouse gases build up.
This is an open reflection prompt meant to set up the chapter's central ideas — mixtures, pure substances, elements, and compounds — all explained in full through the Activities below.
Observation: adding calcium oxide (quick lime) to water causes a vigorous reaction that releases heat and forms calcium hydroxide, which, once stirred, filtered, and dissolved, gives a colourless solution called lime water. After being left exposed to air for a few hours (with occasional stirring), the colourless lime water turns milky.
Explanation: lime water turns milky because the carbon dioxide present in air reacts with the calcium hydroxide in it, forming calcium carbonate — an insoluble substance made of tiny white particles that give the solution its milky appearance — along with water.
Word equation: Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water.
Conclusion: since lime water only turns milky when exposed to air, this activity confirms that carbon dioxide is indeed present in the air.
Observation: after a few hours, tiny particles are visibly settled on the surface of the black sheet of paper, which can be examined more closely using a magnifying glass.
Answer: this shows that dust particles are suspended in the air around us — these particles are not an integral part of the air itself, but are considered pollutants. The nature and amount of dust in the air can vary from place to place and from time to time (for example, the tiny shining specks sometimes seen moving in a sunbeam entering a dark room are exactly these kinds of suspended dust particles).
Observation: as electricity passes through the acidified water, gas bubbles form at both terminals inside the water-filled test tubes. After 10–15 minutes, the volume of gas collected is different in the two test tubes — one test tube collects roughly double the volume of gas compared to the other.
Testing the gases: bringing a burning candle near the mouth of each test tube gives two different results — one test tube's gas makes a distinctive "pop" sound, confirming it is hydrogen gas; the other test tube's gas makes the candle's flame burn brighter and more vigorously, confirming it is oxygen gas. (These gases cannot be water vapour, since water vapour would simply condense back into water rather than support combustion or produce a pop sound.)
Answer: since passing electricity through water produces both hydrogen and oxygen gas, this shows that water is composed of these two constituents: Water → Hydrogen + Oxygen. Since hydrogen gas is produced in roughly double the volume of oxygen gas, this matches the fact that water's hydrogen and oxygen atoms combine in a 2:1 ratio.
Going further — is this a physical or chemical change? Breaking water down into hydrogen and oxygen using electricity (electrolysis) is a chemical change, not a physical one — water is an entirely new compound made by chemically combining hydrogen and oxygen, and splitting it back into these two elements involves breaking that chemical bond, producing substances (hydrogen gas and oxygen gas) with completely different properties from water itself, which cannot be reversed by a simple physical process.
Going further — atoms and molecules: hydrogen and oxygen are both pure substances called elements, made of identical particles called atoms. Since atoms of most elements cannot exist independently, two atoms of hydrogen combine to form one molecule of hydrogen, and two atoms of oxygen combine to form one molecule of oxygen.
Observation: as the sugar is heated, it first turns brown, and then chars further, turning blackish. Small droplets of water can be seen forming inside the boiling tube near its open end, and a black solid (charcoal) is left behind, which can be scooped out onto a watch glass.
Where did the water come from? Since the tube itself is what's being heated (not the surrounding air), the water droplets must have come from within the dry sugar itself, not from condensation of water vapour already present in the air.
Answer: heating decomposes the sugar into carbon (left behind as charcoal, which can be tested to see that it burns like coal) and water. Since water itself is known to be made up of hydrogen and oxygen, this shows that sugar cannot be an element — it must be a chemical compound made of the elements carbon, hydrogen, and oxygen, since heating it breaks it down into these simpler constituents.
Table 8.2 (completed) — comparison of Sample A and Sample B:
| Experiment | Sample A (iron + sulfur mixture) | Sample B (heated black mass — iron sulfide) |
|---|---|---|
| Appearance (colour, texture) | Speckled — separate black (iron) and yellow (sulfur) particles clearly visible | Uniform black colour and texture throughout, no separate particles visible |
| Magnet test | Iron filings are attracted to the magnet and get pulled out, separating from the sulfur | Not attracted to the magnet at all — no separation occurs |
| Gas test — odour | Gas is colourless and has no smell | Gas is colourless but has a strong rotten-egg-like odour |
| Gas test — burning | Burns with a distinctive "pop" sound (hydrogen gas) | Does not give a pop sound like hydrogen; identified mainly by its rotten-egg smell (hydrogen sulfide gas) |
Sample A reaction (only the iron reacts): Iron + Dilute hydrochloric acid → Iron chloride + Hydrogen gas. The sulfur is left behind unreacted as a yellow solid, showing sulfur does not react with hydrochloric acid.
Sample B reaction: Iron sulfide + Dilute hydrochloric acid → Iron chloride + Hydrogen sulfide.
Answering the discussion points:
Mishraloha was the name given in ancient India to a mixture of two or more metals whose properties were distinct from any of its individual constituent metals — in other words, an early recognition of what we today call an alloy. Ancient Indian medical and alchemical texts, including the Charaka Samhita, Susruta Samhita, Rasaratna Samucchaya, and Rasa Jala Nidhi, describe the deliberate use of such alloys for medicinal purposes.
One striking example is bronze, known in these texts as Kamsya — an alloy made from copper (Tamra, 4 parts) and tin (Vanga, 1 part) — which was historically used to improve digestion and boost immunity. This shows that the practical idea of combining metals to deliberately create a new substance with different, useful properties was understood and applied in India many centuries ago.
Dhokra art is an old metal-casting craft from Bihar and Odisha that turns molten alloys into beautifully detailed figures, often depicting animals, people, and scenes from nature. The process begins by shaping a design in beeswax, which is then covered in clay to form a mould. Once the clay hardens, the wax is melted out, leaving behind a hollow cavity in the exact shape of the original design.
This hollow space is then filled with molten brass or bronze — both alloys — which sets to give the finished piece its characteristic strength and shiny golden colour. Dhokra art remains a living example of tribal creativity and tradition, and a real-world showcase of how mixtures of metals (alloys) are put to lasting artistic use.
Stainless steel is a uniform mixture (alloy) of iron, nickel, chromium, and a small amount of carbon, mixed so completely that no individual substance can be seen. Brass (copper and zinc) and bronze (copper and tin) are other common alloys, all mixed so thoroughly they look and behave like a single substance.
The major pollutants present in air include particulate matter (dust and soot) as well as gases like carbon monoxide, ozone, nitrogen dioxide, and sulfur dioxide. The Air Quality Index (AQI) is the tool used to describe how polluted or clean the air is at a given time and place.
Out of the 118 known elements, most are solids, but 11 (all non-metals, like oxygen, helium, and nitrogen) are gases at room temperature. Only two elements are liquid at room temperature — mercury (a metal) and bromine (a non-metal) — while gallium and caesium are solids that turn liquid at just around 30 °C.
More than 45 different elements — including aluminium, copper, silicon, cobalt, lithium, gold, and silver — go into manufacturing a single mobile phone, from its screen and battery to its many internal components.
Graphene aerogel, made from carbon, is said to be the lightest material on Earth — so light that even a blade of grass can support it. Its extremely porous structure gives it a very high absorbing capacity, making it useful for cleaning up oil spills at sea or on land, and for fabricating energy-saving devices and special building coatings.
Answer: (iv) A and B are elements, C is a compound, and has a fixed composition.
Reason: since A and B cannot be broken down into simpler substances at all, they must be elements by definition. When two elements A and B combine to form a new substance C, that new substance is a compound — and compounds are always formed when elements combine in a fixed ratio, giving C a fixed composition.
Answer: (i) Both Assertion and Reason are true, and Reason is the correct explanation for Assertion.
Reason: air genuinely is a mixture (mainly of nitrogen, oxygen, argon, carbon dioxide, and water vapour), and it qualifies as a mixture precisely because these gases are combined without reacting chemically with each other — each gas keeps its own individual properties, exactly matching the given Reason, which correctly explains why the Assertion is true.
Answer: hydrogen is a highly flammable gas, commonly used as a fuel, while oxygen is a gas that actively supports and helps sustain combustion. If water simply retained the properties of its constituent elements (the way a mixture would), it should logically behave in a way related to burning or supporting fire.
Instead, water — formed when hydrogen and oxygen combine chemically in a fixed 2:1 ratio — is a liquid at room temperature that is commonly used to extinguish fires, the complete opposite of what either of its constituent elements would suggest. This dramatic contrast (a flammable fuel gas and a combustion-supporting gas combining to form a fire-extinguishing liquid) clearly justifies that compounds have properties entirely different from those of the elements that form them.
Answer: (iii) Pure substances — carbon dioxide, iron, oxygen, sugar.
Reason: all four of these are genuinely pure substances — iron and oxygen are elements, while carbon dioxide and sugar are compounds — so option (iii) is entirely correct.
Why the others are wrong: (i) is incorrect because water is a compound (not an element) and air is a mixture (not an element). (ii) is incorrect because minerals are not, by themselves, mixtures — they are typically pure elements or compounds. (iv) is incorrect because air and brass are both uniform mixtures, not non-uniform ones.
| Substance | Classification | Justification |
|---|---|---|
| Iron | Element | Cannot be broken down into any simpler substance |
| Moist air | Mixture | Air and water vapour combined without reacting chemically; both keep their own properties |
| Iron oxide (rust) | Compound | Formed when iron and oxygen combine chemically in a fixed ratio; has different properties from either iron or oxygen |
| Magnesium | Element | Cannot be broken down into any simpler substance |
| Oxygen | Element | Cannot be broken down into any simpler substance |
| Magnesium oxide | Compound | Formed when magnesium and oxygen combine chemically in a fixed ratio, giving a new substance |
| Elements | Compounds | Mixtures |
|---|---|---|
| Aluminium | Carbon dioxide | Sand |
| Gold | Magnesium oxide | Seawater |
| Oxygen | Rust (iron oxide) | Muddy water |
| Nitrogen | Iron sulfide | Air |
| Sulfur | Glucose | Fruit juice |
| Hydrogen | Water | |
| Sodium chloride | ||
| Baking soda |
Pure substances (all elements + all compounds): aluminium, gold, oxygen, nitrogen, sulfur, hydrogen, carbon dioxide, magnesium oxide, rust, iron sulfide, glucose, water, sodium chloride, and baking soda.
New substance formed: iron sulfide, a compound. Word equation: Iron + Sulfur → Iron sulfide.
How it differs from the original mixture: the original mixture had visibly separate black (iron) and yellow (sulfur) particles, the iron in it was attracted to a magnet, and its components could be physically separated. Iron sulfide, by contrast, is a uniform black solid throughout with no visible separate particles, is not attracted to a magnet at all, and its iron and sulfur can no longer be separated by any physical method — showing that an entirely new compound, with completely different properties, has been formed through a chemical reaction rather than simple physical mixing.
Answer: no, a substance cannot be classified as both an element and a compound at the same time.
Reason: an element is made of only one type of atom and, by definition, cannot be broken down into any simpler substance at all — not even by chemical means. A compound, on the other hand, is made of two or more different elements chemically combined in a fixed ratio, and it can be broken down (through chemical processes) into its constituent elements. Since a substance either can or cannot be broken down further into simpler substances, it must fall into exactly one of these two categories — never both — though together, elements and compounds make up all pure substances.
Answer: if water were simply a mixture of hydrogen gas and oxygen gas rather than a chemically bonded compound, it would likely behave in a highly dangerous way — since hydrogen is a flammable fuel and oxygen actively supports combustion, this "water" would probably be a highly flammable, even explosive, mixture rather than the safe, stable liquid we rely on.
It would no longer have the properties we depend on every day — being a stable liquid at room temperature, being safe to drink, or being usable to put out fires — and in fact, exposing it to any flame or spark could be extremely hazardous. This scenario helps justify why compounds have entirely new properties of their own, unlike mixtures, where each component simply keeps its original properties side by side.
Gas A is hydrogen gas.
Word equation: Iron + Dilute hydrochloric acid → Iron chloride + Hydrogen gas.
Water (H₂O) — made from hydrogen and oxygen, both non-metals. Uses: (1) essential for drinking and sustaining all life; (2) used as a solvent and coolant in countless industrial and household processes.
Carbon dioxide (CO₂) — made from carbon and oxygen, both non-metals. Uses: (1) used by green plants during photosynthesis to make their food; (2) used to carbonate fizzy drinks and in certain types of fire extinguishers.
Answer: gold qualifies as a metal because it is an element with typical metallic properties — it is lustrous, malleable, ductile, and conducts electricity well.
Gold also qualifies as a mineral because it occurs naturally in the Earth's crust in a solid, pure elemental form — this makes it what the chapter calls a native mineral, a mineral that is a pure element rather than a compound. Since gold satisfies the definitions of both categories at once (a naturally occurring pure element found in rocks, with metallic properties), it can correctly be classified as both a mineral and a metal.
These four prompts are creative, research-based, and discussion projects rather than fixed-answer questions. Here's guidance on how to approach each one.
Guidance: pick one clear, relatable real-life example for each category and build a short 3–4 panel comic strip around it. For an element, you could feature a gold ring, showing how it's made of only gold atoms and can't be broken down further. For a compound, you could feature table salt or water, showing two very different-looking elements (like sodium metal and chlorine gas) combining to form a completely different, safe substance. For a mixture, you could feature a fruit salad or seawater, showing separate ingredients or substances sitting together while keeping their own individual properties. Adding simple particle diagrams (dots for atoms, grouped differently for elements, compounds, and mixtures) alongside the comic panels makes the differences visually clear.
Starting points for your research (verify exact dates and details from a reliable science-history source before presenting):
Present each discovery with the element/compound/mixture's key properties and its main modern-day uses, alongside the historical story.
Guidance: check the ingredients list on the packaging of a detergent or packaged snack. Any named chemical compound listed (such as sodium bicarbonate, citric acid, or monosodium glutamate) is a compound in its own right. The final packaged product itself (like the detergent powder or the snack as a whole) is almost always a mixture, since it's made by deliberately combining several different compounds (and sometimes elements) together — each of which keeps its own properties within the mixture, letting manufacturers combine cleaning power, fragrance, colour, or flavour and preservation all in one product.
Case for elements: elements are the fundamental building blocks of absolutely everything — without elements like oxygen, carbon, and hydrogen, neither compounds nor mixtures could exist at all, since every compound and mixture is ultimately made up of elements.
Case for compounds: compounds create entirely new substances with properties essential to life and technology that no single element could provide on its own — water (essential for all life), carbon dioxide (essential for photosynthesis), and medicines are all compounds with unique, life-sustaining properties.
Case for mixtures: mixtures make up most of the material world we actually interact with daily — the air we breathe, the food we eat, seawater, and the alloys used in construction and technology are virtually all mixtures, letting different substances combine flexibly for countless practical purposes.
A balanced conclusion for the debate: rather than any one category being "most important" in isolation, each depends entirely on the other two — elements are the necessary starting ingredients, compounds are what let those elements create entirely new, essential substances, and mixtures are how compounds and elements combine flexibly to make up the vast majority of materials we use every day.
Now that mixtures, elements, and compounds are covered, move on to how solutes and solvents combine to form the amazing world of solutions, revisit Chapter 7, or book a free demo class for personalised coaching.
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