Extra Questions for Class 8 Science Chapter 9: The Amazing World of Solutes, Solvents and Solutions

Original extra practice questions for Class 8 Science Chapter 9 (The Amazing World of Solutes, Solvents and Solutions), going beyond the textbook exercise with fresh numbers and applied reasoning. Full explanations included. These Class 8 Science Chapter 9 important questions are handy for last-minute exam practice.

Q1 (Floating fraction). A block of wood has mass 540 g and volume 900 cm³. Will it float or sink in water? Find the fraction of its volume that stays submerged when floating.
Solution: Density = 540/900 = 0.6 g/cm³, which is less than water’s density (1 g/cm³), so it floats. Submerged fraction = object density ÷ water density = 0.6/1 = 0.6, i.e. 60% of its volume stays submerged.

Q2 (Solubility scaling). At 20°C, 36 g of salt dissolves fully in 100 g of water to make a saturated solution. If you have 250 g of water at the same temperature, what is the maximum mass of salt that can dissolve?
Solution: Solubility ratio = 36 g per 100 g water. For 250 g water: 36 × (250/100) = 90 g.

Q3 (Assertion–Reason). Assertion (A): A saturated solution can never dissolve any additional substance. Reason (R): Solubility is a fixed, temperature-independent property.
Solution: A saturated solution CAN dissolve more solute if the temperature is raised (solubility of most solids increases with heat), so Assertion is false. Solubility does change with temperature, so Reason is also false.

Q4 (Identify the metal). An unknown metal cube has side 2 cm and mass 63.2 g. Using standard densities (iron ≈7.9, aluminium ≈2.7, copper ≈8.9, lead ≈11.3 g/cm³), identify the metal.
Solution: Volume = 2³ = 8 cm³. Density = 63.2/8 = 7.9 g/cm³ — this matches iron.

Q5 (Displacement + density). A stone is lowered into a measuring cylinder containing 40 mL of water; the level rises to 65 mL. The stone’s mass is 87.5 g. Find its density and predict whether it would float in mercury (density ≈13.6 g/cm³).
Solution: Volume displaced = 65−40 = 25 cm³ = stone’s volume. Density = 87.5/25 = 3.5 g/cm³. Since 3.5 < 13.6, the stone would float in mercury.

Q6 (Spot the error). A student says: “Since a sponge floats in water, its density must be greater than water’s density.” Is this correct?
Solution: Incorrect. An object floats specifically because its (effective) density is LESS than water’s density, not greater — a sponge floats partly because trapped air pockets lower its overall density below 1 g/cm³.

Q7 (Applied reasoning). Using the concept of solubility and temperature, explain why a warm soft drink loses its fizz faster than a cold one.
Solution: Carbon dioxide gas is more soluble in cold liquid than in warm liquid. As a drink warms up, its dissolved CO₂ escapes more readily because gas solubility falls with rising temperature — so a warm drink holds less dissolved gas and goes flat faster than a cold one.

Q8 (Two-object displacement). Two stones together displace water from 50 mL to 78 mL in a cylinder. If Stone A alone displaces water from 50 mL to 62 mL, find Stone B’s volume.
Solution: Total combined volume = 78−50 = 28 cm³. Stone A’s volume = 62−50 = 12 cm³. Stone B’s volume = 28−12 = 16 cm³.

Q9 (Classification). Classify each as a solution or a non-uniform mixture (suspension): sugar syrup, muddy river water, seawater, chalk powder shaken in water.
Solution: Solutions (uniform): sugar syrup, seawater. Non-uniform mixtures (suspensions): muddy river water, chalk powder in water (both settle on standing).

Q10 (Identify by density). A rectangular metal block has dimensions 5 cm × 4 cm × 2 cm and mass 316 g. Find its density, and state whether it’s closest to aluminium (2.7), iron (7.9) or lead (11.3) g/cm³.
Solution: Volume = 5×4×2 = 40 cm³. Density = 316/40 = 7.9 g/cm³ — closest to iron.

More on This Chapter

See also: NCERT Solutions for Class 8 Science Chapter 9, Class 8 Science NCERT Book and the Class 8 Science Formulas/Concepts Handbook.

Revision Notes: Revision Notes for Class 8 Science Chapter 9

Written by Satish

NCERTBooks.org is an independent educational resource run by a small team focused on making official NCERT textbooks easy to find, read, and download for students, parents, and teachers across India. We are not affiliated with NCERT or the Ministry of Education -- we organise publicly available NCERT content by class and subject, verify links against official sources, and build tools (like our in-browser reader) that make studying more convenient. Every guide we publish is written and reviewed by our team based on the actual NCERT curriculum and syllabus.