Classify matter by composition and properties, and separate mixtures using physical properties

Learning goal: Classify a sample of matter as an element, a compound, a homogeneous mixture or a heterogeneous mixture; distinguish physical from chemical properties and changes; explain solutions, solubility and the rate of dissolving with the particle model; place acids, bases and neutral solutions on the pH scale; and design a separation that uses physical properties.

Two ways to classify matter. Matter can be sorted by what it is made of (its composition) or by how it behaves (its properties). By composition, a pure substance has one fixed composition throughout: an element contains one kind of atom (copper, oxygen, sulfur), and a compound contains two or more elements chemically joined in a fixed ratio (water, table salt, carbon dioxide). A mixture contains two or more substances that are physically combined, so its composition can vary from sample to sample and the substances keep their own properties. Mixtures are split into two classes. A homogeneous mixture is uniform: any two spoonfuls have the same composition, and you cannot see separate parts even with a hand lens. Salt water, air, brass and clear apple juice are homogeneous. A heterogeneous mixture has visibly different parts or regions with different composition: chicken noodle soup, granite, a salad, muddy water. The test is composition from point to point, not whether the mixture looks "smooth" from across the room; milk looks uniform but is a heterogeneous mixture of fat droplets in water, which a microscope reveals.

Solutions. A solution is a homogeneous mixture in which a solute is dissolved in a solvent. In salt water the salt is the solute and water is the solvent; in air, oxygen and other gases are dissolved in nitrogen. Water is used as a solvent so often because a very large number of substances dissolve in it. In the particle model, dissolving is the solvent particles surrounding and separating solute particles until they are spread evenly through the liquid. For solutions like salt or sugar in water, nothing new is formed; if the water evaporates, the same salt is left behind, which is why this kind of dissolving is a physical change (some substances react with water as they dissolve, and those are chemical changes). Solubility is the amount of solute that can dissolve in a given amount of solvent at a given temperature. For most solid solutes in water, solubility increases as the temperature rises, because the particles have more kinetic energy and can overcome the attractive forces that hold the solid together. The condition matters: this is the general rule for solids in liquids, and gases behave differently (a warm soda goes flat faster than a cold one). The rate of dissolving is a separate idea from how much can dissolve. Stirring, warming the solvent and crushing the solute to increase its surface area all make a solid dissolve faster, because each brings solvent particles into contact with solute particles more often. A sugar cube and the same mass of granulated sugar dissolve to the same final amount, but the granules finish first.

Physical and chemical properties. A physical property can be observed or measured without changing the substance into something else: color, odor, hardness, density, solubility, conductivity, melting point, boiling point, viscosity and malleability. A chemical property describes how a substance behaves when it changes into a different substance: flammability, reactivity with acids or with oxygen, and pH behavior. A physical change alters form but not identity (melting, dissolving, cutting), while a chemical change produces at least one new substance. A burning candle shows both at once: the heat of the flame melts the solid wax (physical), the liquid wax is drawn up the wick and vaporizes (physical), and the wax vapor then reacts with oxygen in the air to form carbon dioxide and water vapor, releasing thermal energy and light (chemical). The wax that is gone did not vanish; it became gases that left the flame.

Acids, bases and the pH scale. Water solutions can be classified as acidic, neutral or basic using the pH scale. For water solutions at room temperature (about 25 °C), a pH below 7 is acidic, a pH of 7 is neutral, and a pH above 7 is basic. The further a value is from 7, the more acidic or basic the solution. (pH describes the solution; it does not by itself say whether the dissolved acid or base is a "strong" or "weak" one, a distinction made in chemistry.) Household examples measured with a pH meter or with universal indicator paper typically fall roughly here: vinegar and lemon juice near pH 2 to 3, black coffee near 5, pure water 7, a baking soda solution near 8 to 9, and an ammonia-based cleaner near 11. These are approximate; the actual reading for a particular bottle must be measured, not assumed. The explanation of pH in terms of the ions in the solution is left for a chemistry course; here the job is to measure, place on the scale and compare.

Separating mixtures using physical properties. Because the substances in a mixture keep their own properties, a difference in a physical property can be used to pull them apart. A magnet removes iron filings from sand because iron is magnetic and sand is not. Filtering removes an undissolved solid from a liquid because the solid particles are larger than the filter's pores while dissolved particles pass through. Evaporation recovers a dissolved solid because the solvent has a much lower boiling point than the solute. Distillation goes further: the vapor is condensed and collected, so both the solvent and the solute are recovered. Two liquids that do not mix, such as oil and water, form separate layers, and the less dense one can be poured or skimmed off the top; liquids that mix, like alcohol and water, cannot be separated this way. The separation methods in this lesson are all physical changes; if a step produced a new substance, the mixture would not have been separated but reacted. The same ideas are used to purify water where clean supplies are scarce: settling and filtering through sand and cloth remove suspended solids, boiling kills most germs but does not remove dissolved salts (it concentrates them in the water left in the pot), while distillation, including solar distillation (evaporating water under a clear cover and collecting the condensed drops), leaves dissolved salts and most other non-volatile impurities behind, though chemicals that evaporate easily can come over with the water. Water treated in a classroom activity is never drunk; and a method is judged by which impurities it removes, how much energy it needs and whether the materials are available locally. Devising such a method is an application of this lesson, not a separate topic.

A worked classification and separation model. Suppose a jar contains sand, table salt and iron filings, all mixed. The jar is a heterogeneous mixture: three different materials are visible, and a spoonful from the top may have more sand than a spoonful from the bottom. The particle picture of the jar shows three kinds of particles in separate clumps, not evenly spread. Plan: pass a magnet in a plastic bag over the mixture to lift the filings (magnetism); add water and stir so the salt dissolves while the sand does not (solubility); pour through filter paper to keep the sand (particle size); warm the clear salt water gently until the water evaporates and the salt remains (boiling point). After the first two steps the beaker holds a heterogeneous mixture (sand in salt water); after filtering, the liquid is a homogeneous solution. Each step used a difference in a physical property, and at the end the three original substances are back, unchanged.

Investigation: separate a three-part mixture and measure the recovery. Materials: about 5 g each of dry sand, table salt and iron filings (or short pieces of steel wool), a balance, a beaker, a stirring rod, a funnel, filter paper, an evaporating dish, a hot plate or warm-water bath, a bar magnet in a zip-top bag, safety goggles. Steps: record the mass of each component before mixing and the total; combine; pass the bagged magnet through the mixture repeatedly and record the mass of filings recovered; add about 50 mL of warm water, stir for two minutes, filter, rinse the sand with a little water, dry and record its mass; evaporate the filtrate gently to dryness (do not let it spatter) and record the salt's mass. Record: the three starting masses, the three recovered masses and the percentage recovered for each. A result that supports the claim: each substance is recovered with most of its original mass and its original properties (the filings still stick to the magnet, the salt still dissolves, the sand still does not), showing that the components were physically combined and could be separated by physical properties. Safety: goggles throughout; do not touch the hot plate or the hot dish; never taste anything in the laboratory; keep the magnet inside its bag so filings can be released cleanly. What the model cannot show: a small mass loss during transfer is not evidence that matter was destroyed, and this procedure cannot separate two solids dissolved in the same water: evaporating or distilling off the water leaves both behind together, so pulling them apart needs a difference between the two solutes themselves, such as one crystallizing out first as the solution cools because its solubility falls faster with temperature.

Worked example 1 — classify five samples. Air is a homogeneous mixture: every liter near you has the same composition, and its gases are not chemically joined. Distilled water is a compound, a pure substance with fixed composition. A copper wire is an element. Orange juice with pulp is a heterogeneous mixture: pulp settles and is visibly separate. Brass is a homogeneous mixture, a solid solution of copper and zinc, because its composition can vary from one alloy to another yet is uniform within a sample. The deciding question is always fixed composition (pure substance) versus variable composition (mixture), and then uniform (homogeneous) versus non-uniform (heterogeneous).

Worked example 2 — rate versus amount. A student drops a 4 g sugar cube into one glass of cold water and 4 g of granulated sugar into an identical glass, without stirring. The granulated sugar disappears first because its surface area is much larger, so more sugar particles are in contact with water at once. After enough time both glasses hold the same amount of dissolved sugar, because the solubility of sugar in water at that temperature has not changed. If the student instead warmed one glass, the sugar would dissolve faster and more of it could dissolve, because solubility of most solids in water increases with temperature.

Worked example 3 — placing solutions on the pH scale. In this example data set, written for the lesson, a class measures three solutions with a calibrated pH meter: a vinegar sample reads 2.9, a tap-water sample reads 7.2 and a baking soda solution reads 8.4. Vinegar is acidic because 2.9 is below 7; it is also the most strongly acidic of the three because it is farthest below 7. The tap water is very slightly basic (7.2 is above 7) but close to neutral. The baking soda solution is basic. If a fourth solution reads 7.0 at room temperature, it is neutral. Placing the values on a 0 to 14 scale shows the vinegar far left, the water just right of center and the baking soda a little farther right.

Worked example 4 — physical or chemical change? Wax melting near the flame: physical, because liquid wax is still wax. Wax vapor burning: chemical, because carbon dioxide and water are produced and thermal energy is released. Salt dissolving in water: physical; evaporation returns the salt. Iron filings rusting in damp air: chemical; rust is a new substance that no longer sticks to a magnet the way iron does. The deciding sign of a chemical change is a new substance with new properties. Energy released or absorbed, a color change or bubbles often accompany one, but they can accompany physical changes too: melting absorbs energy, dissolving can warm or cool the water, and boiling makes bubbles. So these clues point to a possible chemical change, and the test is whether a new substance formed.

Representation task: a classification key and a particle picture. Draw a decision tree with the questions "Is the composition fixed?" and "Is it uniform throughout?" and place these nine samples on its branches: helium, sugar water, granite, carbon dioxide, a bar of pure gold, soil, tea with no leaves, a salad, iron. Beside the tree, draw particle diagrams of salt water (particles of two kinds evenly mixed) and of sand in water (one kind clumped, the other spread), and write one sentence under each saying which is homogeneous and why.

Independent performance: design and justify a separation. You are given a mixture of small plastic beads, sand and dissolved sugar in water. Write a procedure of at least four steps that recovers all three, and for each step name the physical property that makes the step work. Success criteria: every step uses a physical property (density, particle size, solubility, boiling point or magnetism), no step creates a new substance, the order of steps is justified in one sentence each, and you state one measurement you would take to check how completely each component was recovered. Finish with a paragraph explaining why the sugar cannot be filtered out, using the words solute, solvent and dissolved.