Question Opening prompt 1
Which things in the opening picture are matter and which are not?
Steps only
Matter has mass and takes up space; light and heat are forms of energy.
Answer
Classify each pictured item using the mass-and-space test.
CLASS 8 · SCIENCE · CHAPTER 8
Nature of Matter: Elements, Compounds, and Mixtures
Simple answers to the opening prompts, all five chapter investigations, Activities 8.1–8.5, end questions, reflection prompts, and projects.
29 worked answersComplete chapter answers
29 worked answers · short steps first, explanations when you need them
Probe and Ponder
Textbook pages 116
Question Opening prompt 2
How can elements be combined to form a compound?
Steps only
Elements react chemically and combine in a fixed ratio.
Answer
They form a new substance with different properties.
Question Opening prompt 3
How could a compound that absorbs carbon dioxide from the air help solve environmental challenges?
Steps only
Capture and safely store or use carbon dioxide so less enters the air.
Answer
It could help lower emissions, alongside other climate actions.
Question Opening prompt 4
Share one question you have about elements, compounds, or mixtures.
Steps only
Ask a clear question about the substances around you.
Answer
Example: How can we tell a mixture from a compound?
Chapter Investigations
Textbook pages 118–127
Question Activity 8.1
What happens to lime water after it is left exposed to air for a few hours, and what does this show?
Steps only
What happens to lime water after it is left exposed to air for a few hours, and what does this show?
Answer
The clear lime water turns milky because carbon dioxide in the air reacts with calcium hydroxide to form tiny solid calcium carbonate particles and water.
Question Activity 8.2
What may collect on a clean black sheet left undisturbed near an open window, and what does this show?
Steps only
What may collect on a clean black sheet left undisturbed near an open window, and what does this show?
Answer
Tiny dust particles may settle on the black sheet and can be seen more clearly with a magnifying glass.
Question Table 8.1
Classify the examples in Table 8.1 as uniform or non-uniform mixtures: air; soda water and oxygen in water; carbon particles in air; vinegar and oil with water; sand and water and seawater; baking powder and alloys.
Steps only
Decide whether each mixture is evenly mixed throughout.
Answer
Uniform: air, soda water, oxygen in water, vinegar-water, seawater, blended baking powder, alloys. Non-uniform: smoke, oil-water, sand-water.
Question Pure-substance prompt
Classify milk, packed fruit juice, baking soda, sugar, and soil as mixtures or pure substances in science.
Steps only
Check whether each sample contains one substance or several.
Answer
Mixtures: milk, fruit juice, soil. Pure compounds: baking soda, sugar.
Question Activity 8.3
When electricity is passed through acidified water, what gases form, how can they be identified, and what does this show?
Steps only
Electrolysis forms hydrogen and oxygen from water.
Answer
Water is a compound of hydrogen and oxygen; this demonstration needs teacher supervision.
Question Activity 8.4
What do you observe when sugar is gently heated, and what does the result show?
Steps only
Heating sugar produces carbon and water, showing sugar contains different elements.
Answer
Sugar is a compound; this experiment must be teacher-supervised.
Question Activity 8.5
How does a mixture of iron and sulfur (Sample A) differ from the black material made by heating it (Sample B)?
Steps only
A keeps iron and sulfur properties; B is a new compound with different properties.
Answer
A: separable mixture. B: iron sulfide compound.
Keep the Curiosity Alive
Textbook pages 131–133
Question 1
A and B combine to form C. A and B cannot be broken down into simpler substances by chemical reactions. Which statement is correct? (i) A, B, C are compounds; (ii) C is a compound and A and B have fixed composition; (iii) A and B are compounds; (iv) A and B are elements and C is a compound with fixed composition.
Steps only
A and B are elements; their product C is a compound.
Answer
(iv)
Question 2
Assertion: Air is a mixture. Reason: A mixture forms when two or more substances are mixed without a chemical change. Choose the correct assertion-reason option.
Steps only
Air's gases are mixed without becoming a new compound.
Answer
(i) Both are true, and the Reason explains the Assertion.
Question 3
Water has different properties from the hydrogen and oxygen elements from which it forms. Justify this statement.
Steps only
Chemical combination makes a new substance with new properties.
Answer
Water's properties differ from hydrogen and oxygen.
Question 4
Which group is correctly matched? (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.
Steps only
Find the list in which every item is classified correctly.
Answer
(iii) Carbon dioxide, iron, oxygen, and sugar.
Question 5
Iron reacts with moist air to form iron oxide, and magnesium burns in oxygen to form magnesium oxide. Classify each substance as an element, compound, or mixture.
Steps only
Classify the starting materials and products by their composition.
Answer
Elements: iron, magnesium, oxygen. Mixture: moist air. Compounds: iron oxide and magnesium oxide.
Question Question-making activity
Prepare questions using prompts such as why, when, how long, where, and why not, based on the chapter.
Steps only
Ask clear questions about matter, substances, and mixtures.
Answer
Answers vary; questions should connect to chapter ideas.
Question 6
Classify these as elements, compounds, or mixtures, then list the pure substances: carbon dioxide, sand, seawater, magnesium oxide, muddy water, aluminium, gold, oxygen, rust, iron sulfide, glucose, air, water, fruit juice, nitrogen, sodium chloride, sulfur, hydrogen, and baking soda.
Steps only
Elements and compounds are pure substances; the listed mixed samples are mixtures.
Answer
Elements: Al, gold, O₂, N₂, S, H₂. Compounds: CO₂, MgO, rust, FeS, glucose, water, NaCl, baking soda. Mixtures: sand, seawater, muddy water, air, fruit juice.
Question 7
What forms when iron filings and sulfur are heated? How is it different from their mixture? Write the word equation.
Steps only
Iron and sulfur chemically combine when heated.
Answer
Iron + sulfur → iron sulfide; the product is a new compound.
Question 8
Can a substance be both an element and a compound? Explain.
Steps only
An element has one type of atom; a compound has different elements combined.
Answer
No, one substance cannot be both an element and a compound.
Question 9
How would daily life change if water were a mixture of hydrogen and oxygen instead of a compound?
Steps only
A mixture would keep hydrogen's and oxygen's properties, not water's.
Answer
It would not behave like safe liquid water, so daily life would be very different.
Question 10
In Fig. 8.24, iron filings react with dilute hydrochloric acid. Identify Gas A and write the word equation.
Steps only
Iron reacting with dilute hydrochloric acid releases hydrogen.
Answer
Gas A: hydrogen. Iron + hydrochloric acid → iron(II) chloride + hydrogen.
Question 11
Name two compounds made only from non-metals and give two uses of each.
Steps only
Choose two compounds made only of non-metals and list familiar uses.
Answer
Water: drinking, washing. Carbon dioxide: photosynthesis, some fire extinguishers.
Question 12
How can gold be classified as both a mineral and a metal?
Steps only
Mineral describes natural occurrence; metal describes gold's properties.
Answer
Naturally occurring gold is a native mineral and the element gold is a metal.
Question Reflection activity
Reflect on the questions framed by your friends and try to answer them.
Steps only
Use the chapter's definitions and examples to answer classmates.
Answer
Answers vary; support each answer with a reason.
Discover, Design, and Debate
Textbook pages 133
Question Project 1
Create a comic strip from real-life examples that distinguishes elements, compounds, and mixtures, with diagrams, properties, and uses.
Steps only
Show one element, compound, and mixture with a simple diagram and use.
Answer
Example characters: iron, water, and air.
Question Project 2
Research discoveries of elements such as phosphorus or sodium, a compound such as penicillin, and mixtures such as brass, bronze, or stainless steel. Present your findings.
Steps only
Research one element, one compound, and one mixture; record origin and use.
Answer
Examples: phosphorus, penicillin, and bronze. Credit reliable sources.
Question Project 3
Read labels on detergents or snacks and list the mixtures and compounds they contain.
Steps only
Copy real ingredient lists, then classify the whole product and named substances.
Answer
Packaged foods and detergents are mixtures; some ingredients are compounds.
Question Project 4
Role-play as an element, compound, or mixture and debate which category is most important.
Steps only
Compare what elements, compounds, and mixtures each contribute.
Answer
All three matter; their importance depends on the use.