Separating mixtures

Sand mixed into sugar, salt dissolved in sea water, muddy water fresh from a rainstorm, kerosene floating on water in a leaky can — mixtures are everywhere, and so is the practical problem of pulling their components apart again. Because a mixture is only a physical combination, never a chemical one, every component inside it keeps its own private set of physical properties: size, solubility, density, magnetism, boiling point, or whether it turns straight from solid to gas on heating. Separating mixtures is really the skill of spotting which one of those properties differs between the components, then choosing the technique — or short sequence of techniques — built to exploit exactly that difference.

The core insight

Every separation technique works by exploiting one specific physical property difference between the components of a mixture, so the first step in any separation problem is identifying which property (size, solubility, density, magnetism, or volatility on heating) actually distinguishes them.

The core idea

Mixtures keep their components' identities. Nothing chemical happens when substances are mixed — no new bonds form — so calcium carbonate powder in a salt mixture is still, chemically, calcium carbonate, and it still behaves exactly like pure calcium carbonate when treated with water, heat, or a magnet. This is the whole reason physical separation is even possible: a compound cannot be pulled apart this way, because its atoms are chemically bonded and only a chemical reaction can break them apart.

Match the property to the technique. Each classical technique below is built around exactly one property difference, and using the wrong one for a given mixture simply fails.

Technique	Property exploited	Typical use
Sieving	Difference in particle size	Stones from rice, flour from husk
Magnetic separation	One component is magnetic	Iron filings from sand or sulphur
Sublimation	One component sublimes on heating	Ammonium chloride or camphor from sand
Sedimentation & decantation	Insoluble solid settles under gravity	Mud from water
Filtration	Solid particles too large to pass filter paper	Chalk powder from water
Evaporation	Solute soluble and non-volatile	Salt from salt solution
Separating funnel	Liquids immiscible, different densities	Kerosene from water

Many mixtures need more than one step. A single technique only separates one pair of properties at a time, so a three-part mixture — say iron filings, sand and salt — needs a short sequence: magnetic separation pulls out the iron first, then water dissolves the salt away from the sand, then filtration and evaporation finish the job.

Track how much you actually got back. Since no chemical change occurs, the mass of a properly recovered component should be very close to the mass that was originally present in the mixture — real losses only come from careless handling (spillage, residue left on glassware), which chemists quantify as a percentage recovery:

\%\ \text{recovery} = \frac{m_{\text{recovered}}}{m_{\text{original}}} \times 100. \qquad (1)

where: $m_{\text{recovered}}$ is the mass of the pure component obtained after separation, and $m_{\text{original}}$ is the mass of that same component present in the starting mixture. A recovery close to $100\%$ tells you the technique and its execution were clean; a low recovery is a signal of spillage, incomplete filtration, or evaporating a solution too fast and losing spatters of solid.

Figure

Flowchart of separating calcium carbonate and salt from a solid mixture using water dissolve filtration and evaporation

Why it works

Physical separation is possible only because a mixture never involves chemical bonding between its components. Each substance in the CaCO3-NaCl mixture, or in kerosene-water, remains fully itself throughout — same melting point, same solubility, same density — right up until you deliberately act on it. That is the reasoning chain worth internalising, because it is what every board question is really testing. First, ask what stays different. Calcium carbonate does not dissolve in water; sodium chloride dissolves readily. That single fact is the entire justification for using water at all — without a solubility gap, dissolving would do nothing useful. Second, ask what a given apparatus actually does to that difference. Filter paper has pores small enough to hold back the solid particles suspended in a liquid but wide enough to let the liquid itself pass; it cannot, on its own, separate two things that are both dissolved, or two liquids that mix into one phase. Third, ask what happens to the filtrate, not just the residue — students often stop at "filtration separates them" and forget the salt is still dissolved in the water that passed through. Evaporation is the second, separate step, needed because filtration alone only removed the insoluble solid; it did nothing to the soluble one. Finally, for liquid-liquid mixtures like kerosene and water, filtration and evaporation are both useless, because neither liquid is a suspended solid nor a dissolved solute — the relevant property is immiscibility plus a density gap, which is exactly what a separating funnel is built to use, letting the denser layer drain out first while the lighter one is held back by the closed stopcock.

Worked example

A 10.0 g mixture of calcium carbonate ( $\text{CaCO}_3$ ) powder and common salt ( $\text{NaCl}$ ) is known to contain 4.0 g of CaCO3 and 6.0 g of NaCl. After separating them by dissolving, filtering, washing, drying and evaporating, a student recovers 3.8 g of dry CaCO3 and 5.7 g of dry NaCl. Describe the separation and find the percentage recovery of each solid.

Step 1. Identify the property and dissolve. CaCO3 is insoluble in water while NaCl is readily soluble, so adding water and stirring dissolves all 6.0 g of the salt, leaving the 4.0 g of CaCO3 undissolved as suspended solid particles.

Step 2. Filter. Pouring the mixture through filter paper holds back the CaCO3 as the residue, while the salt solution passes through as the filtrate. The residue is washed with a little distilled water to remove any clinging salt solution, then dried, giving 3.8 g of recovered CaCO3.

Step 3. Evaporate the filtrate. Heating the salt solution gently drives off the water, which is volatile, while the non-volatile NaCl is left behind as dry white crystals — 5.7 g recovered.

Step 4. Compute the percentage recovery using equation (1). For CaCO3: $\%\ \text{recovery} = (3.8/4.0)\times100 = 95\%$ . For NaCl: $\%\ \text{recovery} = (5.7/6.0)\times100 = 95\%$ . Both solids are recovered at 95% — the missing 5% in each case is realistic experimental loss from residue sticking to the filter paper, evaporating dish, or beaker walls, not from any chemical change.

Competitive stretch

Extend the same mixture with 2.0 g of iron filings stirred in, making a three-component solid mixture. A magnet is run through the dry mixture first, pulling out the iron filings (exploiting magnetism, a property neither CaCO3 nor NaCl has) before the dissolve-filter-evaporate sequence above is applied to the remaining CaCO3-NaCl mixture exactly as before.

Common traps

Choosing filtration for two liquids, or a separating funnel for a solid-liquid mixture — always match the technique to the actual property involved (miscibility and density for liquid-liquid; solubility for solid-liquid).
Forgetting evaporation only works when the solute is non-volatile; if both components are volatile liquids, evaporation cannot cleanly separate them.
Stopping after filtration and forgetting the filtrate still holds a dissolved second component — two steps are often needed, not one.
Confusing sedimentation and decantation (letting a solid settle, then pouring off the liquid) with filtration (passing the mixture through filter paper) — decantation cannot remove very fine suspended particles the way filtration does.

Ask the tutor: Got a mixture from your textbook or lab manual that you're not sure how to separate, or want to check your own percentage recovery calculation — ask the AI tutor to walk through it with you.

Why can't I just heat the whole CaCO3 and NaCl mixture directly instead of dissolving it first?

Heating the dry solids alone gives you no way to physically separate them — both are white powders mixed together, and heat alone does not exploit any property that tells them apart. Dissolving first turns the problem into a solubility difference: NaCl disappears into solution while CaCO3 stays solid, and that visible difference is what filtration can act on. Evaporation only becomes useful in the second step, once the two components are already in different physical states (solid residue versus dissolved filtrate).

What's the actual difference between decantation and filtration, since both seem to remove a solid from a liquid?

Decantation just lets gravity do the work: the solid settles to the bottom over time, and you carefully pour off the clear liquid on top, leaving the solid behind. It only works for solids that settle quickly and completely. Filtration instead forces the mixture through filter paper with tiny pores, physically trapping even fine suspended particles that would never settle out on their own. Muddy water is often decanted first to remove the bulk of the mud, then filtered to catch the fine particles that remain suspended.

If I get less than 100% recovery in a real separation, does that mean I did the chemistry wrong?

Not necessarily — no chemical change happens in these separations, so a low recovery almost always points to a physical, mechanical loss rather than an error in reasoning. Common culprits are solid sticking to the filter paper, a little solution splashing out while decanting, or crystals left clinging to the evaporating dish. As long as your identified property and chosen technique were correct, a recovery in the 90s of a percent is a normal, expected result of real lab work, not a sign the underlying method was wrong.