ڤنرڠن
Everyday Separation of Mixtures
A mixture consists of two or more substances physically combined, meaning each substance retains its own chemical properties and can be separated using physical methods. In everyday life and basic laboratory work, simple techniques exploit differences in physical properties such as particle size, density, and magnetic susceptibility. Hand picking, threshing, winnowing, and sieving are foundational techniques used across households and agriculture to isolate components. Hand picking involves manually removing undesirable substances or large impurities from a mixture based on size, colour, or shape. Sieving allows finer solid particles to pass through the mesh of a sieve while larger particles are retained, which is common in flour milling and construction.
Sedimentation and decantation rely on differences in density for solid-liquid or liquid-liquid mixtures. When a suspension is left to stand, heavier insoluble particles settle to the bottom under the influence of gravity, a process called sedimentation. The clear liquid above the sediment, known as the supernatant, can then be carefully poured off without disturbing the settled solid, a process called decantation. Loading can be used to speed up sedimentation by adding substances like alum, which causes suspended fine clay particles to clump together and settle faster. These physical techniques form the baseline of separation science before moving to more specialised laboratory procedures.
Separating Insoluble Solids by Filtration
When a solid does not dissolve in a liquid, it forms a suspension. Filtration is the standard laboratory method used to separate an insoluble solid from a liquid. The mixture is poured through a porous barrier, typically filter paper housed inside a glass funnel. The pores of the filter paper are large enough to let the liquid molecules and any dissolved particles pass through, forming the filtrate, but too small to let the larger insoluble solid particles through. These trapped solid particles form the residue.
During a filtration procedure, the filter paper is folded into a cone shape and moistened with a little distilled water so it adheres firmly to the walls of the glass funnel. The stem of the funnel should touch the inner wall of the receiving beaker to prevent splashing and ensure a steady flow. Filtration is a critical purification step in chemical preparations, particularly when isolating an insoluble reactant or removing unreacted solid impurities from a newly formed reaction mixture.
Recovering Soluble Solids via Evaporation and Crystallisation
When a solid is dissolved in a liquid, it forms a solution where the solute particles are uniformly dispersed among the solvent molecules. Filtration cannot separate a dissolved solute from its solvent because the solute particles are small enough to pass through filter paper. Instead, thermal methods such as evaporation and crystallisation are employed.
Evaporation involves heating a solution until the liquid solvent turns into gas and escapes into the atmosphere, leaving the solid solute behind. This method is effective when the solid is thermally stable and does not decompose upon heating, such as sodium chloride. However, many hydrated salts decompose or lose their water of crystallisation if heated to dryness directly with a Bunsen burner. For these substances, crystallisation is preferred.
In crystallisation, the solution is heated gently to evaporate only a portion of the solvent, creating a saturated solution. A saturated solution is one that holds the maximum amount of dissolved solute at that temperature. The concentrated solution is then left to cool slowly at room temperature. As the solution cools, solubility decreases, and excess solute particles join together in a regular geometric pattern to form pure crystals. The remaining liquid, called the mother liquor, is filtered off, and the crystals are dried by patting them between sheets of filter paper.
Preparing and Purifying Salts
Soluble salts can be prepared in the school laboratory through neutralization reactions involving acids and metals, or acids and metal carbonates. The choice of method depends on the reactivity of the metal and whether the metal oxide, carbonate, or unreacted metal is used. For example, reacting an excess of an insoluble metal carbonate or an excess of a reactive metal with a dilute acid produces a salt solution, water, and carbon dioxide gas if a carbonate is used.
To ensure the final salt is pure and neutral, an excess of the solid metal or metal carbonate is added to the acid until no more gas is given off and no more solid dissolves. This guarantees that all the acid has reacted, preventing contamination of the salt product with excess acid. The excess unreacted solid is then removed from the mixture using filtration, leaving behind a clean filtrate of the aqueous salt solution.
The filtrate is subsequently subjected to crystallisation. The solution is heated to evaporate excess water and form a saturated solution, then allowed to cool so that pure, dry crystals of the salt precipitate out. This integrated sequence of reaction, filtration, evaporation, and crystallisation is the standard protocol for preparing soluble salts.
Paper Chromatography
Paper chromatography is a powerful analytical technique used to separate and identify mixtures of soluble substances, such as food colourings, plant pigments, or inks. The method relies on the differing solubilities of the components in a moving solvent and their varying levels of attraction to a stationary phase.
A baseline is drawn near the bottom of a strip of chromatography paper using a pencil, never ink, because ink contains soluble dyes that would interfere with the chromatogram. Spots of the test samples are placed along the baseline. The paper is then suspended in a sealed container containing a shallow layer of solvent, ensuring the solvent level sits below the pencil baseline so the spots do not dissolve directly into the solvent pool.
As the solvent travels up the paper by capillary action, it carries the dissolved sample components with it. Components that are highly soluble in the solvent and have a low affinity for the paper travel further up the stationary phase. Components that are less soluble or bind strongly to the paper lag behind. Pure substances produce a single spot on the chromatogram, whereas mixtures separate into multiple distinct spots.
To identify unknown substances, scientists calculate the retardation factor, or Rf value. The Rf value is a constant for a given substance under specific conditions and is calculated by dividing the distance travelled by the substance spot from the baseline by the distance travelled by the solvent front from the baseline. Comparing the Rf values of spots from an unknown mixture with known reference values allows for definitive identification.
