Lesson 1.3 · Unit 1 · HS-PS1-1, HS-PS1-2
Pure substances and mixtures, and separating one from the other
Almost nothing you meet outside a laboratory is a pure substance. Air, seawater, steel, milk and gasoline are all mixtures, and the practical question in chemistry is usually how to get one component out of a mixture cleanly. Every separation method works by exploiting a property the components do not share, so choosing a method means first naming the property that differs.
- A pure substance has a fixed composition and a set of properties that do not vary between samples. It is either an element or a compound.
- An element cannot be broken down chemically. A compound contains two or more elements chemically bonded in a fixed ratio, and its properties bear no resemblance to those of its elements.
- A mixture contains two or more substances that are not chemically bonded, in any proportion, each keeping its own properties.
- Homogeneous mixtures are uniform throughout and are also called solutions. Heterogeneous mixtures have visibly distinct regions. The distinction depends on the scale you look at.
- Every separation exploits a property difference: particle size for filtration, boiling point for distillation, solubility for extraction and crystallization, affinity for a surface for chromatography, magnetism, and density for decanting or centrifuging.
- Separating a mixture is a physical process; separating a compound into its elements is a chemical one and needs a reaction, usually with a considerable energy input.
Where students lose marks: calling a solution a pure substance because it looks uniform. Saltwater is clear and uniform and is still a mixture, because the proportions can be varied continuously and the salt can be recovered unchanged.
The problem. A sample contains sand, sodium chloride and iron filings, all mixed together. Recover all three, dry and separate.
Step one: classify the mixture. All three components are visible as distinct regions, so this is heterogeneous. Nothing is chemically bonded to anything else, so the whole job is physical and no reaction is needed.
Step two: list the property differences available. Iron is magnetic and the other two are not. Sodium chloride is soluble in water and the other two are not. Sand is an insoluble solid. Three components, and each has at least one property the others lack, so a clean separation is possible.
Step three: order the steps so each one is clean. Take the magnetic step first, while everything is dry. Passing a magnet over a wet mixture would work badly, and once water is added the iron would begin to rust. Order matters.
Step four: remove the iron. Pass a magnet over the dry mixture, collecting the filings that jump to it. Repeat until no further filings are picked up. Sand and salt remain.
Step five: dissolve the salt. Add water and stir. The sodium chloride dissolves and the sand does not, because sodium chloride is ionic and water is polar while sand is a covalent network solid. The mixture is now sand suspended in saltwater.
Step six: filter. Pour through filter paper. Sand particles are too large to pass through the pores and stay on the paper as the residue; the saltwater passes through as the filtrate. Rinse the sand with a little distilled water to remove trapped salt solution, then dry it.
Step seven: recover the salt. Evaporate the filtrate, either in an evaporating basin over gentle heat or by leaving it to stand. Sodium chloride crystals remain. If the water is also wanted, use simple distillation instead, which collects the condensed water while the salt stays behind in the flask.
Step eight: check the recovery. Weigh the three recovered components and compare the total with the starting mass. A shortfall points to a specific loss: salt left dissolved in the damp sand, sand left on the filter paper, or spattering during evaporation. Naming which loss you suspect is part of the answer.
Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.
- Name the two kinds of pure substance.
Show the full solution
Elements and compounds
- State the difference between a compound and a mixture in terms of ratio.
Show the full solution
A compound has a fixed ratio of elements chemically bonded; a mixture can have any proportions and is not bonded
- Classify each as homogeneous or heterogeneous: seawater, granite, air, sand in
water.
Show the full solution
Homogeneous, heterogeneous, homogeneous, heterogeneous
- Which property does distillation exploit?
Show the full solution
Difference in boiling point
- Which property does filtration exploit?
Show the full solution
Difference in particle size, separating an insoluble solid from a liquid
- Water is a compound of hydrogen and oxygen. Hydrogen burns explosively and oxygen
supports combustion, yet water extinguishes fires. Explain what this shows about
compounds.
Show the full solution
It shows that a compound's properties are not the sum or average of its elements' properties, because the atoms are chemically bonded and the resulting substance is genuinely new. The hydrogen in water is not present as hydrogen gas; it is bonded into molecules whose behavior has to be determined by experiment rather than predicted from the ingredients. This is the sharpest difference between a compound and a mixture: a mixture of hydrogen and oxygen gases is explosive, and the same atoms bonded as water are not. A compound's properties are new, not inherited from its elements
- Explain how you would separate a mixture of two liquids that dissolve in each
other, such as ethanol and water, and why filtration cannot work.
Show the full solution
Filtration separates by particle size and only works when one component is an undissolved solid. Two miscible liquids form a homogeneous mixture in which the particles are of molecular size and pass through any filter together, so no pore size helps. The property that does differ is boiling point: ethanol boils at about 78 degrees Celsius and water at 100. Fractional distillation exploits this, heating the mixture so the more volatile ethanol vaporizes preferentially, passing up a fractionating column where repeated condensation and vaporization improve the separation, then condensing in the condenser and collecting first. Fractional distillation, using the boiling point difference; filtration cannot separate molecular-scale mixtures
- In the worked example, explain why the magnetic step must come before the water is
added.
Show the full solution
Two reasons, one practical and one chemical. Practically, a magnet works poorly through a wet slurry: the filings clump with damp sand and are dragged along with material that should stay behind, so the recovered iron is contaminated and the recovered sand is depleted. Chemically, iron in contact with water and dissolved oxygen begins to rust, which converts some of the iron into iron oxide. That is a chemical change, so the iron can no longer be recovered unchanged and the mass balance at the end will not close. Ordering the steps to keep every stage physical is part of designing the separation. Wet filings separate badly, and iron in water begins to rust, which is a chemical change
- Chromatography separates the pigments in a leaf extract. Explain what property
difference it exploits and why the spots end up at different heights.
Show the full solution
It exploits the difference in how strongly each pigment is attracted to the stationary phase, the paper, compared with how readily it dissolves in the moving solvent. As the solvent rises through the paper it carries the pigments with it, but each one spends a different fraction of its time stuck to the paper rather than traveling in the solvent. A pigment with a strong attraction to the paper and low solubility in the solvent is held back and travels a short distance; one that dissolves readily and binds weakly travels nearly as far as the solvent front. The heights therefore report a ratio of two competing affinities, which is why the same pigment gives a reproducible position for a given paper and solvent. Competing affinity for the stationary paper against solubility in the moving solvent
- Air is usually called a homogeneous mixture, but air also contains dust and water
droplets. Explain how both statements can be defensible.
Show the full solution
The classification depends on the scale being described and on what is included in the sample. Treated as its gaseous components alone, air is genuinely homogeneous: nitrogen, oxygen, argon and carbon dioxide are mixed at the molecular level and no region differs from another, which is why a sample taken anywhere in a room has the same composition. Include the suspended solids and liquid droplets and the sample now has visibly distinct phases, which makes it heterogeneous, and fog and smoke are exactly that. Neither statement is careless; they answer different questions, and a good answer says which components it is counting. The gaseous components alone are homogeneous; including suspended particles and droplets makes it heterogeneous