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Milk looks uniform, yet it scatters a torch beam. Sea water looks clear, yet it leaves salt when it dries. Questions in this chapter test whether you can explain such things using particle size and physical properties, and whether you can calculate a concentration without slipping on the denominator. This post covers what changed in the syllabus, the classification of mixtures, three concentration calculations, how to choose a separation method, and a practice routine.
What changed in the syllabus?
The 2025–26 CBSE syllabus covered this topic as "Is Matter Around Us Pure", with separation methods specifically excluded ("excluding separating the components of a mixture"). In 2026–27 the new NCERT book *Exploration* has Chapter 5, Exploring Mixtures and their Separation, and the CBSE Class IX syllabus for 2026–27 now includes concentration of solutions, solubility curves and separation by crystallisation, distillation, paper chromatography, sublimation, centrifugation and coagulation, with practicals on several of these. Check the syllabus for your exam year.
How do you tell a solution, a colloid and a suspension apart?
NCERT sorts them by the size of the dispersed particles:
Solution: particles smaller than 1 nm. Homogeneous, does not settle, passes through filter paper, does not scatter light. Example: salt in water.
Colloid: particles 1 to 1000 nm. Looks uniform and does not settle, but scatters light. Examples: milk, blood, ice cream.
Suspension: particles larger than 1000 nm, visible to the eye. Heterogeneous, settles on standing, can be filtered, scatters light. Example: chalk powder in water.
The scattering of a light beam by particles is the Tyndall effect. It is the quickest test: shine a laser pointer (never into anyone's eyes) through the mixture. If you see the beam's path, it is a colloid or a suspension.
How do you calculate concentration?
The chapter uses three percentages:
% m/m = (mass of solute ÷ mass of solution) × 100
% m/v = (mass of solute in g ÷ volume of solution in mL) × 100
% v/v = (volume of solute ÷ volume of solution) × 100
Example 1: % m/m. 12 g of sugar is dissolved in 188 g of water.
Mass of solution = 12 + 188 = 200 g.
% m/m = 12/200 × 100 = 6%.
The common wrong answer is 12/188 × 100 ≈ 6.4%, which divides by the solvent instead of the solution.
Example 2: % m/v. A hospital saline drip is 0.9% m/v sodium chloride, as NCERT notes. How much salt is in a 500 mL bag? 0.9 g per 100 mL, so 0.9 × 5 = 4.5 g.
Example 3: % v/v. 25 mL of a liquid concentrate is made up to 250 mL with water. % v/v = 25/250 × 100 = 10%.
What does a solubility curve tell you?
Solubility is the most solute that dissolves in 100 g of solvent at a given temperature. For most solids it rises with temperature; for gases it falls. NCERT's solubility curve for a compound "B" shows 287 g per 100 g of water at 60 °C and 241 g at 40 °C. Cool a saturated solution from 60 °C to 40 °C and 287 − 241 = 46 g of B comes out as crystals for every 100 g of water. That is crystallisation, and the slower the cooling, the larger the crystals.
Students' own ideas about dissolving can mix everyday experience with what they are taught. Blanco and Prieto (1997) asked students aged 12 to 18 to draw and explain how stirring and heating affect dissolving, and identified four different explanatory patterns in their answers. Draw the particles yourself when you explain why hot water dissolves sugar faster; it exposes gaps quickly.
How do you choose a separation method?
Ask which physical property differs between the components, then pick the method that uses it:
Solubility changes with temperature: crystallisation (copper sulfate from its solution).
Boiling points at least about 25 °C apart: distillation. NCERT's example is acetone (about 56 °C) and water (100 °C). Closer boiling points need fractional distillation, as in a petroleum refinery.
How fast components travel with a solvent on paper: paper chromatography (dyes in black ink).
Two liquids that do not mix: separating funnel (mustard oil and water; the denser water is drained from the bottom).
One solid sublimes: sublimation (camphor from sand).
Tiny suspended particles of different density: centrifugation (blood components) or coagulation with alum (muddy water), followed by settling and filtering.
Practice question. A mixture contains salt, sand and water. Filter out the sand, then evaporate or distil the filtrate to get the salt. If you also want the water back, choose distillation, because evaporation loses it.
Which mistakes come up most?
Dividing by the mass of solvent in % m/m.
Calling milk a solution because it looks uniform. It shows the Tyndall effect, so it is a colloid.
Using simple distillation for liquids whose boiling points are close.
Mixing up evaporation and crystallisation. Crystallisation cools a saturated solution to get pure crystals; evaporation simply removes the solvent.
Forgetting the units: % m/v needs grams per 100 mL.
When a method's principle is unclear, the Fermi Concept Library covers every concept in chemistry, each with its own lesson, so you can revise solubility or the Tyndall effect separately. The concentration ideas here lead into molarity in Class 11; our guide to molarity, molality and concentration shows what comes next.
A practice routine
Complete NCERT's Table 5.1 (solution, suspension, colloid) from memory, then check.
Do the three Pause and Ponder concentration questions near the start of the chapter, writing the formula each time.
For six mixtures from your kitchen, name the separation method and the property it uses.
Try the paper chromatography activity with a black sketch pen and a strip of filter paper.
Today, shine a torch through a glass of milk-and-water and a glass of salt water in a dark room and note which one shows the beam.
