NCERT Solutions Class 8 Science Chapter 08 Nature of Matter Elements, Compounds and Mixtures

Question 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 has a fixed composition.
In simple words: Since A and B cannot be broken down further, they must be elements. When two elements join together through a chemical reaction, they always form a compound with the same makeup every time.

Exam Tip: Remember that elements cannot be split into simpler parts, while compounds are made when two or more elements link chemically.

Question 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 Reason is the correct explanation for Assertion.
In simple words: Air is truly a mixture because it contains nitrogen, oxygen, and other gases all mixed together without any chemical reaction happening. The reason given correctly explains why air is a mixture.

Exam Tip: In assertion-reason questions, check both statements first, then decide if one truly explains the other - not just whether both are separately correct.

Question 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: Hydrogen is a gas that easily catches fire. Oxygen is a gas that helps things burn. Yet when these two elements join together in a fixed ratio of 2 to 1, they create water - a liquid that puts out fires. The properties of water are completely different from the properties of hydrogen and oxygen separately. This is how compounds behave - they show new and different qualities compared to the elements that make them up.
In simple words: When hydrogen and oxygen mix chemically, they make water which has totally new properties. Water stops burning, but both hydrogen and oxygen help things burn.

Exam Tip: Always compare the properties of elements with the properties of the compound formed - this contrast is what examiners want to see in your answer.

Question 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: (i) Incorrect - Water and air are not elements; they are mixtures or compounds.
(ii) Incorrect - Minerals are usually compounds, not uniform mixtures.
(iii) Correct - Carbon dioxide, iron, oxygen, and sugar are all pure substances.
(iv) Incorrect - Air is a uniform mixture, and brass is an alloy which is also uniform.

Only option (iii) is correctly matched.
In simple words: Pure substances like iron and oxygen stay the same throughout. Uniform mixtures like seawater are the same everywhere you sample them. Non-uniform mixtures like sand and dirt are not the same all over.

Exam Tip: To classify correctly, remember: elements and compounds are pure substances, while mixtures can be uniform or non-uniform based on whether they look the same everywhere.

Question 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:
Iron - Element (a pure substance made of only iron atoms)
Moist air - Mixture (contains air and water vapour blended together)
Iron oxide - Compound (formed when iron and oxygen chemically combine)
Magnesium - Element (a pure substance made of only magnesium atoms)
Oxygen - Element (a pure substance made of only oxygen atoms)
Magnesium oxide - Compound (formed when magnesium and oxygen chemically combine)
In simple words: Single metals like iron and magnesium are elements. When they join with oxygen through burning, they become compounds. Water vapour mixed with air stays a mixture because no chemical bond forms.

Exam Tip: Look for whether substances chemically combined (compounds) or just stayed mixed together (mixtures) - this decides the classification.

Question 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.
Answer:

Pure substances: All elements (Al, Au, O₂, N₂, S, H₂) and all compounds (CO₂, MgO, rust, FeS, glucose, H₂O, NaCl, NaHCO₃)
In simple words: Elements are single pure materials that cannot be broken down. Compounds form when elements chemically join. Mixtures happen when different things just get stirred together without chemical bonds.

Exam Tip: If something can be separated by filtering, evaporation, or a magnet, it is likely a mixture, not a pure substance.

Question 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: When iron filings and sulfur powder are heated together, they chemically combine to form iron sulfide (FeS), a new compound.

Differences:
Original mixture: The components (iron and sulfur) keep their own properties. You can separate them using a magnet because iron remains magnetic.

Product (Iron sulfide): This is a compound with completely new properties. It is no longer magnetic and cannot be separated by a magnet. The properties are totally different from the starting materials.

Word equation:
Iron + Sulfur - Iron sulfide
In simple words: When you heat iron and sulfur together, they stick to each other chemically and make something new. The new thing (iron sulfide) acts nothing like iron or sulfur did on their own.

Exam Tip: To check if a chemical reaction happened, look for new properties - if the result acts completely different and cannot be separated back, a compound has formed.

Question 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 at the same time. These are two separate and opposite groups. An element is made up of only one kind of atom (for example, O₂, H₂, or Fe). A compound is formed when two or more different elements join together through a chemical reaction (for example, H₂O or CO₂). Since the definitions are different, the classification is mutually exclusive - a substance must fall into one group or the other, but never both.
In simple words: Elements have just one type of atom. Compounds have two or more types of atoms stuck together. Something cannot be in both groups because they are opposite.

Exam Tip: Remember the core difference: elements cannot be broken down chemically, while compounds can be - this single fact keeps them mutually exclusive.

Question 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 simply a mixture of hydrogen and oxygen gases instead of a true compound, life as we know it could not exist. The two gases would not chemically bind to each other, so the mixture would remain explosive and unsafe for any use. Water would be impossible to drink or use in daily activities. Furthermore, water's remarkable abilities - such as being a solvent that dissolves many substances, regulating temperature in our bodies, and supporting all life - come directly from its nature as a compound (H₂O) with its own unique structure and properties. Without these special properties that only the compound has, water would not provide the life-sustaining support that all living things depend on.
In simple words: Water works for life because hydrogen and oxygen are chemically joined. If they were just mixed, water would be dangerous and useless. Life needs water's special compound powers.

Exam Tip: This question tests understanding of why compounds are different from mixtures - focus on how the chemical bond creates new properties that are essential for life.

Question 10. Analyse Fig. 8.24. Identify Gas A. Also, write the word equation of the chemical reaction.
Answer: The figure shows iron filings reacting with dilute hydrochloric acid in a test tube setup. Gas A is hydrogen gas (H₂), which is produced when iron reacts with the acid. Hydrogen gas bubbles out of the solution and can be collected in the test tube or identified by the characteristic "pop" sound when a burning splint is held near the mouth of the tube.

Word equation:
Iron + Hydrochloric acid - Iron chloride + Hydrogen
In simple words: When iron metal meets dilute hydrochloric acid, it reacts and releases hydrogen gas. The gas bubbles rise up through the liquid and escape from the tube.

Exam Tip: Always identify the gas by its properties - hydrogen makes a "pop" sound with a burning splint, which is a quick test in the lab.

Question 11. Write the names of any two compounds made only from non-metals, and also mention two uses of each of them.
Answer: 1. Carbon dioxide (CO₂):
- Used in fire extinguishers to smother flames and stop combustion.
- Used in making carbonated (fizzy) drinks to give them their bubbles and taste.

2. Sulfur dioxide (SO₂):
- Used in bleaching processes and as a preservative to keep foods fresh longer.
- Used in the industrial manufacture of sulfuric acid, which is a key chemical in many industries.
In simple words: Carbon dioxide puts out fires and makes soda fizzy. Sulfur dioxide keeps food fresh and helps make other important chemicals in factories.

Exam Tip: When listing uses, try to give one practical/everyday use and one industrial use - this shows wider knowledge of the compound's importance.

Question 12. How can gold be classified as both a mineral and a metal?
Answer: Gold can be classified in two different ways based on different scientific systems. As a mineral, gold occurs in the earth's crust in its natural, native (pure) form without needing any processing or chemical extraction. By the definition used in geology, anything that forms naturally in the earth qualifies as a mineral. At the same time, gold is also an element and a metal - it has all the typical metallic properties such as shine (lustre), the ability to be stretched into thin wires (ductility), and the ability to conduct heat and electricity (conductivity). These two classifications are not contradictory because they come from different branches of science - one focuses on where and how it naturally occurs (geology), while the other focuses on its physical and chemical nature (chemistry and metallurgy).
In simple words: Gold is a mineral because it grows naturally in rocks. Gold is a metal because it shines, bends easily, and conducts electricity. These are just two different ways of looking at the same material.

Exam Tip: Understand that a single substance can have multiple correct classifications depending on which scientific property or origin you are focusing on - this is not a contradiction.