Exploration Class 9 Science Chapter 5: Exploring Mixtures and their Separation — NCERT Solutions

Explanation: Seawater is a solution of salt in water. It is collected in shallow pits or pans where the heat of the Sun evaporates the water slowly. As water is lost, the solution becomes more and more concentrated until it is saturated, that is, it cannot hold any more dissolved salt at that temperature. On further evaporation the excess salt separates out as solid crystals (crystallisation), which are collected, and the process is repeated with fresh seawater.

Explanation: Salt in water is a true solution, chalk powder in water is a suspension, and milk in water is a colloid. Solution particles are smaller than 1 nm, so they are invisible and cannot scatter light; the beam passes straight through beaker A leaving only a spot on the wall. Suspension particles are larger than 100 nm, so they can be seen with the naked eye, scatter the laser strongly making its path visible in B, and settle at the bottom under gravity on standing. Colloidal particles (between 1 nm and 100 nm) are too small to be seen and too small to settle, yet large enough to scatter light, so the beam path is visible in C (Tyndall effect) and the mixture stays uniform on standing.

Explanation: (i) At 40 °C the solubility of potassium nitrate is 62 g per 100 g of water, so for 50 g of water the mass needed is 62 x 50/100 = 31 g. (ii) Solubility of potassium chloride decreases as temperature falls; a solution saturated at 80 °C holds 54 g per 100 g of water, but at 25 °C water can hold only about 35-36 g, so the extra KCl can no longer stay dissolved and crystallises out, leaving a solution that is saturated at the lower temperature. (iii) Reading each row of Table 5.4 from 10 °C to 80 °C shows that solubility increases with temperature for every salt, but the extent differs greatly: potassium nitrate shows the steepest rise, ammonium chloride a moderate rise, potassium chloride only a small rise, and sodium chloride remains almost unaffected by temperature.

Explanation: Naphthalene sublimes, so the mixture is first heated under an inverted funnel (step 1); naphthalene vapours rise and condense on the cooler walls of the funnel, leaving sand and salt behind. Water is then added to the remaining mixture: salt dissolves while sand does not, so on filtration (step 3) the sand stays on the filter paper and the salt solution passes through as the filtrate. Finally, the filtrate is heated in an evaporating dish (step 2); the water evaporates and solid common salt is left behind. Each step works because it uses a property unique to one component - sublimation of naphthalene, insolubility of sand, and non-volatility of salt.

Explanation: The method for each mixture is chosen from the property in which the two components differ. Mud particles are insoluble and larger than the pores of filter paper, so filtration retains them. Blood cells are denser than plasma, so rapid spinning in a centrifuge packs them down, leaving plasma on top. Naphthalene changes directly from solid to vapour on heating while sand does not; chalk is insoluble in water while salt dissolves; and salt, being non-volatile, is left behind when the water is evaporated or the water is recovered by distillation. Oil and water are immiscible and have different densities, so they form separate layers in a separating funnel, while flower pigments differ in solubility in the solvent, so they rise to different heights on chromatography paper. Every mixture in the table can therefore be separated by a common method.

Explanation: Fill the table from the defining property of each mixture - particle size - and let the other rows follow from it. Below 1 nm the particles are individual ions or molecules, so a solution is homogeneous, never settles, passes through filter paper and shows no Tyndall effect. Between 1 nm and 1000 nm the particles are big enough to scatter a light beam (Tyndall) but still too small to settle or be filtered out, which is a colloid. Above 1000 nm the particles are heavy enough to settle under gravity and large enough to be trapped by filter paper, which is a suspension. Use the 1 nm and 1000 nm boundaries this book states; an older convention put the colloid range at 1-100 nm.

Explanation: In the set-up, the mixture is boiled in flask A, the vapour of the lower-boiling component passes into condenser B where it is cooled back to liquid, and the liquid collects in receiver C - this is distillation. Simple distillation works for a miscible liquid pair whose boiling points differ by more than about 25 °C, or for a liquid containing a dissolved non-volatile solid. Water (100 °C) and acetone (56 °C) differ by 44 °C, so they separate well, and in salt water only the water vaporises, leaving salt behind in the flask. Acetone and alcohol (56 vs 78 °C), alcohol and chloroform (78 vs 61 °C) and alcohol and benzene (78 vs 80 °C) have boiling point gaps smaller than 25 °C, so simple distillation cannot separate them and fractional distillation would be required. Sand and salt are both non-volatile solids, so distillation does not apply to them at all.

Explanation: Sort every term in the box by particle size, because size decides all the other behaviour. Solutions have the smallest particles, so they are transparent, stay evenly mixed and slip through filter paper - and note that brass belongs here, a solid solution of zinc in copper, which shows a solution need not be a liquid. Suspensions have the largest particles, so they look heterogeneous, settle out on standing and can be filtered. Colloids sit in between: the particles are too small to settle or be filtered but large enough to scatter light, which is why milk, smoke and butter all show the Tyndall effect while staying evenly distributed.

Explanation: In the solid state, one metal cannot spread appreciably into another, but on heating the metals melt and their atoms mix freely, just like a solute dissolving in a liquid solvent. When this uniform molten mixture cools, it solidifies with one metal evenly distributed throughout the other, forming an alloy. Alloys are therefore regarded as solid solutions in which the metal present in the larger amount acts as the solvent. Brass (zinc in copper) and bronze (tin in copper) are everyday examples, showing that dissolving one metal into another is feasible.

Explanation: The usefulness of a solution depends on its concentration, that is, the amount of solute dissolved in a given amount of solvent. Too little solute makes the solution ineffective, while too much can be harmful: an ORS with the wrong salt concentration can worsen dehydration instead of curing it, and an over-concentrated medicine dose can be toxic. Antiseptics disinfect safely only at the recommended dilution, saline given in a drip must match the salt concentration of body fluids, and excess chlorine makes drinking water unsafe. Hence, in each of these daily-life uses, the solute and solvent must be taken in the correct proportion for the solution to serve its purpose.

Explanation: In a separating funnel, immiscible liquids separate into layers because the denser liquid sinks to the bottom while the lighter one floats on top. Layer formation thus depends on a difference in density, not merely on immiscibility. If the two liquids have equal densities, there is no tendency for one to settle below the other, so instead of two clear layers the mixture stays as a turbid dispersion of droplets of one liquid spread through the other. Such a pair therefore cannot be separated using a separating funnel.

Explanation: Each part turns on particle size or on where a change happens. (i) Settling is decided by size: mud particles exceed 1000 nm, so gravity pulls them down and the water clears, whereas milk's fat and protein particles lie between 1 nm and 1000 nm, small enough that they stay evenly dispersed indefinitely. (ii) Evaporation only involves the faster molecules escaping from the surface, so it is slow and needs no particular temperature, while boiling forms bubbles of vapour throughout the liquid and only once the boiling point is reached. (iii) A light beam is invisible unless something scatters it into your eye; the colloid-sized dust and droplets in air do exactly that, which is why the path of sunlight shows up through a gap.

Explanation: A fair test must change only one variable, the rate of cooling, so the same saturated solution is split into two equal portions kept in identical containers under otherwise identical conditions. On rapid cooling in ice-cold water, the solubility of copper sulfate drops suddenly and the excess solute is forced out of solution very quickly, giving its particles little time to arrange themselves in an orderly pattern, so many tiny, poorly formed crystals appear. On slow cooling at room temperature, crystallisation proceeds gradually and the particles get enough time to build large, well-defined crystals. Observing both sets of crystals with a hand lens or microscope and comparing their size and shape shows whether the results match the hypothesis; repeating the experiment makes the conclusion more reliable.

Explanation: A true solution does not show the Tyndall effect, so the assertion is correct. The reason, however, is wrong: the particles of a solution are extremely small, less than 1 nm in diameter, not larger than 100 nm. It is precisely because they are so small that they cannot scatter a beam of light passing through the solution. Particles larger than 100 nm occur in suspensions, and suspensions do scatter light and make the beam path visible.

Explanation: Muddy water (suspension), milk (colloid), and blood (colloid) are all heterogeneous. Brass (alloy) is a homogeneous mixture. Option (iv) correctly classifies all of them.

Explanation: The Tyndall effect is shown by colloids and suspensions. Air with dust particles (aerosol) and starch in water are colloids, while copper sulfate solution and acetone in water are true solutions. Therefore, (a) and (c) will show the effect.