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Class 8 Science: Elements, Compounds, and Mixtures, 8 practice questions with answers

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These are 8 practice questions on the eighth chapter of the new NCERT Class 8 Science textbook, Curiosity, called Nature of Matter: Elements, Compounds, and Mixtures. Each has a short answer and the textbook page it comes from. They cover uniform and non-uniform mixtures, what "pure" means in science, the fixed atom ratio in a compound, sorting substances, the fixed mass ratio of a compound, telling a mixture from a compound, which elements are liquid, and minerals. For a walk-through of how the book teaches science, see our Class 8 Science Chapter 1 explainer. Try each question before you read the answer.

These questions are our own, drafted with AI help. Every fact was checked against the page it cites by a test script, but no practising teacher has reviewed them yet. If you find a mistake, please tell us.

What the chapter says you need

Facts from the NCERT textbook (Curiosity, Grade 8, Chapter 8, printed pages 116 to 133):

Mixtures and pure substances

Question 1. Sort these into uniform and non-uniform mixtures: (a) jaggery stirred into water until it has all dissolved; (b) chalk powder stirred into water; (c) cornflakes in milk; (d) lemonade in which the sugar and salt have fully dissolved. (e) Which of the four would separate by itself if left to stand for an hour?

Answer: uniform: (a) and (d); non-uniform: (b) and (c); (e) the chalk in water. In a uniform mixture the components are spread so evenly that the mixture looks the same throughout and you cannot see the parts separately. Dissolved jaggery and the fully dissolved lemonade are like this. In a non-uniform mixture you can see the different components: the white chalk specks in the water and the cornflakes floating in the milk. (e) The chalk powder does not dissolve, so it slowly settles at the bottom, leaving clearer water above; dissolved jaggery, sugar and salt do not settle out. (Uniform and non-uniform mixtures, pages 117, 118 and 120.)

Question 2. A bottle of mineral water says "100% pure". A scientist says the water in it is a mixture. (a) Who is right, and why can both be right? (b) What would a scientist call pure water?

Answer: (a) both, because they use "pure" in different ways; (b) water made of only water particles, with nothing dissolved in it. (a) On a label, "pure" means the product is not adulterated: nothing cheaper or harmful has been added. In science, a pure substance is made of only one kind of particle. Mineral water has minerals dissolved in it (that is what gives it its taste), so it is made of more than one substance: a uniform mixture. The label is fair in everyday language, and the scientist is right in science. (b) A scientist's pure water contains only water particles, with no dissolved minerals or gases. (Pure substances, pages 120 and 121.)

Question 3. In ammonia, a gas with a sharp smell, every particle has 3 hydrogen atoms joined to 1 nitrogen atom. (a) How many hydrogen atoms go with 40 nitrogen atoms? (b) How many atoms are there in all in 10 particles of ammonia? (c) Ammonia is a compound. Can it be separated into nitrogen and hydrogen by a physical method, such as boiling or filtering? Why?

Answer: (a) 120; (b) 40; (c) no. (a) The ratio of hydrogen atoms to nitrogen atoms is fixed at 3 : 1, so 40 nitrogen atoms go with 3 × 40 = 120 hydrogen atoms. (b) Each particle has 3 + 1 = 4 atoms, so 10 particles have 40 atoms. (c) No. In a compound the elements are combined chemically in a fixed ratio, forming a new substance with its own properties, so physical methods cannot pull them apart. Water is the same: its hydrogen and oxygen, in the ratio 2 : 1, cannot be separated by boiling. (Compounds and fixed ratios, page 124.)

Question 4. Sort these into elements, compounds and mixtures: brass, helium, sugar, bronze, bromine, calcium carbonate, lime water.

Answer: elements: helium and bromine; compounds: sugar and calcium carbonate; mixtures: brass, bronze and lime water. Helium is one of the eleven gaseous elements, and bromine is one of only two elements that are liquid at room temperature. Sugar is a compound of carbon, hydrogen and oxygen (heating it leaves carbon and gives off water), and calcium carbonate is the white compound that forms when carbon dioxide turns lime water milky. Brass and bronze are alloys, which are mixtures of metals, and lime water is calcium hydroxide dissolved in water. (Pages 118, 119, 123 and 125.)

Compounds

Question 5. When magnesium burns in oxygen it forms magnesium oxide, and 24 g of magnesium always combines with 16 g of oxygen. (a) What is the ratio of magnesium to oxygen by mass, in lowest terms? (b) How much oxygen combines with 6 g of magnesium? (c) If 6 g of magnesium burns in a jar holding 10 g of oxygen, what happens to the extra oxygen?

Answer: (a) 3 : 2; (b) 4 g; (c) about 6 g of oxygen is left unused. (a) 24 : 16, dividing both by 8, is 3 : 2. (b) 6 g is a quarter of 24 g, so it needs a quarter of 16 g: 4 g of oxygen. (c) A compound always forms in a fixed ratio. 6 g of magnesium can take only 4 g of oxygen, so the other 10 − 4 = 6 g stays as oxygen: extra oxygen does not make more magnesium oxide or change what forms. (Compounds form in a fixed ratio, page 124.)

Question 6. Sample P is copper powder mixed with sulfur powder. Sample Q is copper sulfide, a black solid made when the two are heated together. (a) How would a hand lens help you tell which is the mixture? (b) Why would a magnet not help here, even though a magnet separates iron filings from sulfur? (c) One sample can be made with more or less copper; the other always has copper and sulfur in the same ratio. Which is which?

Answer: (a) in P you can see reddish copper grains and yellow sulfur grains; Q is black all through; (b) copper is not attracted by a magnet; (c) P can vary, Q is fixed. (a) In a mixture each component keeps its own properties, so under a hand lens Sample P shows separate reddish-brown copper grains and yellow sulfur grains. Sample Q has the same colour and texture throughout: a new substance. (b) A magnet separates a mixture only when one component is magnetic. Iron is, but copper and sulfur are not, so the magnet pulls neither out of P or Q. A different property has to be used. (c) Sample P, the mixture, can be made with any amount of each powder. Sample Q, the compound, always has its elements combined in a fixed ratio. (Mixtures and compounds, pages 124 to 128.)

Elements and minerals

Question 7. A teacher shows five elements, each sealed in its own glass tube: mercury, bromine, gallium, iron and sulfur. (a) On a cool 20 °C morning, which of them are liquid? (b) On a hot 35 °C afternoon in the same room, which are liquid? (c) Which are metals and which are non-metals?

Answer: (a) mercury and bromine; (b) mercury, bromine and gallium; (c) metals: mercury, gallium and iron; non-metals: bromine and sulfur. (a) At room temperature just two elements, mercury and bromine, are liquids. Gallium, iron and sulfur are solids. (b) Gallium is a solid that turns into a liquid at around 30 °C, so at 35 °C gallium has melted too. Iron and sulfur stay solid. (c) Mercury, gallium and iron are metals; bromine and sulfur are non-metals. Mercury is the metal that is liquid at room temperature, and bromine the non-metal. (The tubes stay sealed: mercury and bromine are poisonous.) (Elements, metals and non-metals, page 123.)

Question 8. (a) What is a native mineral? Give two examples. (b) Cement is made from which minerals? (c) Name three things around us that are not matter.

Answer: (a) a mineral that is a pure element, such as gold or sulfur; (b) calcite, quartz, alumina and iron oxide; (c) light, heat and electricity. (a) Most minerals are compounds, but native minerals are pure elements, either metals such as gold, silver and copper or non-metals such as sulfur and carbon. (b) The ingredients of cement are calcite, quartz, alumina and iron oxide, all minerals or obtained from minerals. (c) Light, heat and electricity (and thoughts and feelings) are part of our world but are not made of matter. (Minerals and what is not matter, pages 129 and 130.)

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Sources

All opened and read on 2 November 2026. Page numbers are the printed page numbers of the textbook; printed page = PDF page + 115 for this chapter.

The facts are from the textbook pages above. The questions, answers and checks are our own. This guide is general information; follow your teacher and the textbook.

Practise with the worksheet: Elements, Compounds and Mixtures, Class 8. Printable, in English, Hindi and Marathi, with an answer key.