NCERT Solutions Class 8 Science Chapter 8 2026-27 PDF

Expert route. The statement gives two clues. First, A and B cannot be split chemically, so they are elements. Second, they combine to make C. Chemical combination gives a compound, and compounds have fixed composition. The option that says A and B are elements and C is a compound is therefore the only correct one.

1. Start from the classification rule, not from the object name alone.
2. A and B cannot be broken down into simpler substances by chemical reactions.
3. This means A and B fit the definition of elements.
4. A and B combine chemically to form a new product C.
5. A product formed by chemical combination of elements is a compound.
6. A compound has a fixed composition because its elements combine in a fixed ratio.
7. Therefore A and B are elements, and C is a compound with fixed composition.

Expert route. Both parts are true. Air has many components, and the components remain present as gases with their own properties. The reason is not a separate fact only. It is exactly the principle used to classify air as a mixture.

1. Start from the classification rule, not from the object name alone.
2. The assertion says that air is a mixture.
3. Air is made of several gases, mainly nitrogen and oxygen, along with other gases.
4. These gases are only mixed physically in air.
5. They do not combine chemically to make a single compound.
6. The reason correctly defines a mixture.
7. The reason also explains why air is a mixture.

Expert route. Do not treat water as a simple physical mixture of two gases. In water, hydrogen and oxygen are chemically joined in a fixed composition. The new substance is liquid water, with its own behaviour, taste, boiling point and uses. Since its properties are unlike hydrogen and oxygen, it is a compound.

1. Start from the classification rule, not from the object name alone.
2. Water is formed when hydrogen and oxygen combine chemically in a fixed ratio.
3. Hydrogen is a gas that burns easily.
4. Oxygen is a gas that supports burning.
5. Water is a liquid at room temperature and is commonly used to extinguish fire.
6. So the properties of water are not the same as the properties of hydrogen and oxygen.
7. This proves that a compound is a new substance with its own properties.

Expert route. This is an elimination question. Water and air immediately remove option (i). Minerals make option (ii) unsafe because minerals are natural substances with fixed composition in many cases. Air and brass remove option (iv). Option (iii) remains correct because elements and compounds are both pure substances.

1. Start from the classification rule, not from the object name alone.
2. Option (i) is wrong because water is a compound and air is a mixture, not elements.
3. Option (ii) is wrong because minerals are not always uniform mixtures. Many minerals are compounds or native elements.
4. Option (iii) is correct because carbon dioxide and sugar are compounds, while iron and oxygen are elements. All four are pure substances.
5. Option (iv) is wrong because air is a uniform mixture and brass is an alloy, a uniform solid mixture.
6. Sand and muddy water may be non-uniform mixtures, but the whole option must be correct.

Expert route. The reactants include elements and a mixture. Moist air is not a compound because its gases and water vapour are not chemically joined in one fixed ratio. The products are compounds because chemical reactions make new substances: iron oxide and magnesium oxide.

1. Start from the classification rule, not from the object name alone.
2. Iron is an element because it is made of one kind of atom.
3. Magnesium is an element for the same reason.
4. Oxygen is also an element.
5. Moist air is a mixture because it contains gases and water vapour mixed physically.
6. Iron oxide is a compound because iron and oxygen combine chemically.
7. Magnesium oxide is a compound because magnesium and oxygen combine chemically.

Expert route. The fastest route is to divide the list into single element names, known compounds and everyday mixtures. Air, seawater, muddy water, fruit juice and sand have variable composition. Carbon dioxide, magnesium oxide, glucose, water, sodium chloride, baking soda, rust and iron sulfide are compounds. The remaining single-element names go in the element column.

1. Start from the classification rule, not from the object name alone.
2. Elements: aluminium, gold, oxygen, nitrogen, sulfur and hydrogen.
3. Compounds: carbon dioxide, magnesium oxide, rust, iron sulfide, glucose, water, sodium chloride and baking soda.
4. Mixtures: sand, seawater, muddy water, air and fruit juice.
5. Pure substances are all the listed elements and compounds.
6. Mixtures are not pure substances in the scientific sense because their components can vary.

Expert route. The original sample is only a physical mixture, so iron and sulfur retain their properties. Heating provides conditions for chemical combination. The product is iron sulfide, a compound. Since a compound has fixed composition and new properties, it is different from the starting mixture.

1. Start from the classification rule, not from the object name alone.
2. Before heating, iron filings and sulfur powder form a mixture.
3. In this mixture, iron keeps its magnetic property and sulfur keeps its yellow colour.
4. When the mixture is heated strongly, iron and sulfur chemically combine.
5. The new substance formed is iron sulfide.
6. Iron sulfide does not behave like a simple mixture of iron and sulfur.
7. It has new properties and cannot be separated by a magnet like the original mixture.

Expert route. The classification depends on composition. If the substance contains only one type of atom, it is an element. If it contains atoms of different elements chemically combined in a fixed ratio, it is a compound. A single pure substance cannot meet both conditions together.

1. Start from the classification rule, not from the object name alone.
2. An element is the simplest pure substance.
3. It cannot be broken down into simpler substances by chemical reactions.
4. A compound is made when two or more elements combine chemically.
5. A compound can be broken down chemically into its elements.
6. So the same substance cannot satisfy both definitions at once.
7. Gold is an element, while water is a compound. They belong to different categories.

Expert route. This is a thought question about properties. In a mixture, the components keep their own properties. So a mixture of hydrogen and oxygen would be very different from water. It could support burning conditions and would not provide the stable liquid needed by living beings.

1. Start from the classification rule, not from the object name alone.
2. Real water is a compound and is stable under normal conditions.
3. Its properties are different from hydrogen and oxygen.
4. Hydrogen is highly combustible.
5. Oxygen supports combustion.
6. If water were only a mixture, hydrogen and oxygen would retain these properties.
7. Such a material would not behave like safe drinking water or a fire-extinguishing liquid.

Expert route. The apparatus is a gas collection setup. The reactants named in the figure are iron filings and dilute hydrochloric acid. A metal plus dilute acid commonly gives a salt plus hydrogen gas. Therefore Gas A is hydrogen, and the salt is iron chloride.

1. Start from the classification rule, not from the object name alone.
2. The figure shows iron filings in dilute hydrochloric acid.
3. When iron reacts with dilute hydrochloric acid, a gas is evolved.
4. This gas is hydrogen.
5. The salt formed is iron chloride.
6. The word equation is: Iron + Dilute hydrochloric acid → Iron chloride + Hydrogen.

Expert route. The question says any two, so choose familiar compounds. Carbon dioxide and water are easy because their elements are non-metals and their uses are common. Carbon dioxide has fire-control and beverage uses. Water has daily life and agriculture uses.

1. Start from the classification rule, not from the object name alone.
2. Carbon dioxide is made of carbon and oxygen, both non-metals.
3. Use 1: carbon dioxide is used in some fire extinguishers.
4. Use 2: carbon dioxide is used to make aerated drinks fizzy.
5. Water is made of hydrogen and oxygen, both non-metals.
6. Use 1: water is used for drinking.
7. Use 2: water is used for washing, cooking and irrigation.

Expert route. Gold does not stop being a metal because it occurs in nature. Its metallic nature comes from its physical and chemical properties. Its mineral status comes from natural occurrence with fixed composition. Hence gold can correctly be called a metal and a native mineral.

1. Start from the classification rule, not from the object name alone.
2. Gold shows metallic properties such as lustre, malleability and conductivity.
3. Therefore gold is classified as a metal.
4. Gold is also found naturally in the Earth's crust.
5. When it occurs naturally as a solid native element, it fits the definition of a mineral.
6. So the two classifications refer to different features.
7. Metal describes its properties, while mineral describes its natural occurrence.