Classification of Matter

Materials and the properties of matter

All objects around us are made of matter: the air we breathe, the ground we walk on, the food we eat and the plants and animals around us. The matter from which an object is made is called its material. The properties of a material determine the properties and uses of the object made from it. For example, a cupboard is made from wood, metal hinges and nails; the strength and durability of the cupboard depend on the properties of these materials.

Key properties of materials that are important to know include:

• Mechanical strength and flexibility - some materials resist bending and breaking (e.g. bricks, rocks), while others bend easily (e.g. cloth).
• Thermal conductivity - materials that conduct heat well (e.g. metals) are called thermal conductors; poor conductors are thermal insulators.
• Electrical conductivity - materials that allow electric current to pass through (e.g. copper) are electrical conductors; those that do not are insulators.
• Brittleness, malleability and ductility - brittle materials break easily; malleable materials can be hammered or rolled into sheets (e.g. gold, copper); ductile materials can be drawn into wires (e.g. copper).
• Magnetism - some materials (notably iron, cobalt and nickel) are magnetic.
• Density - mass per unit volume. Dense materials include concrete and stone.
• Melting point and boiling point - temperatures at which a substance changes state; useful to classify whether a substance is solid, liquid or gas at a given temperature.

Ways to classify matter

Matter can be classified in several useful ways. One common classification groups matter into mixtures and pure substances; pure substances are further divided into elements and compounds. Another useful classification distinguishes metals, metalloids and non-metals, or groups materials by electrical/thermal conductivity or magnetic behaviour.

Classification tree (outline)

• Matter
  • Mixtures
    • Homogeneous mixtures (solutions)
    • Heterogeneous mixtures
  • Pure substances
    • Elements
    • Compounds

Mixtures

We encounter mixtures frequently. Examples: a stew (meat and vegetables), sea water (water plus dissolved salts and other substances), and air (a mixture of gases such as nitrogen, oxygen and small amounts of other gases).

Definition

Mixture. A mixture is a combination of two or more substances in which the components are not chemically bonded and there is no chemical reaction between them.

Important characteristics of mixtures:

• The components are not present in a fixed ratio. For example, any amount of sand added to a fixed amount of water produces a sand-water mixture.
• The components retain their original physical properties (sand remains sand, water remains water).
• The components can usually be separated by physical (mechanical) methods, e.g. filtration, decantation, evaporation, or chromatography.

Heterogeneous mixtures

A heterogeneous mixture is not uniform throughout; the different components can usually be seen and separated by mechanical means. Examples: cereal in milk, soil (contains pebbles, organic matter and sand), sand and water.

Types and examples of heterogeneous mixtures

Homogeneous mixtures

A homogeneous mixture (solution) is uniform throughout; its different components cannot be visually distinguished. Examples: salt dissolved in water, sugar in water, air (a mixture of gases in roughly constant proportions).

Fact. An alloy is a homogeneous mixture of two or more elements (at least one metal) that has metallic properties. Example: steel is mainly iron with small amounts of carbon, manganese and chromium to change hardness, strength and corrosion resistance.

Activities (classroom)

Suggested classroom investigations:

• Make mixtures such as sand + water, potassium dichromate + water, iodine + ethanol, iodine + water. Classify each as homogeneous or heterogeneous and give reasons.
• Choose any two from: sand, water, stones, cereal, salt, sugar. Make as many different mixtures as possible and classify each one.
• Try a simple chromatography (e.g. coloured sweets on filter paper with a small amount of solvent) to observe separation of components in a mixture.

Pure substances

A pure substance is a material that has a fixed composition and cannot be separated into other substances by simple physical methods. Pure substances are either elements or compounds. Their melting and boiling points are sharp (occur at a definite temperature) and chromatographic separation of a pure substance produces only one spot.

Elements

An element is a substance that cannot be broken down into simpler substances by chemical means. The smallest particle of an element that retains its chemical identity is an atom. Elements are listed in the periodic table; each element has a chemical symbol such as H, He, Li, C, O, Fe.

Note. There are more than 100 known elements. New elements with high atomic numbers have been discovered and named in recent decades (examples: flerovium, livermorium, moscovium). The periodic table is the systematic arrangement of these elements.

Compounds

A compound is a substance formed when two or more different elements chemically combine in a fixed ratio. Compounds have definite chemical formulas that show the ratio of atoms. Examples: water (H₂O), sodium chloride (NaCl), calcium carbonate (CaCO₃).

When elements form a compound, the resulting substance has properties different from those of the constituent elements (for example, sodium metal and chlorine gas react to form sodium chloride, common salt).

Using models and representations

Submicroscopic (particle) representations help to show differences among elements, compounds and mixtures. In such diagrams atoms are shown as circles; a compound is shown by atoms joined together in fixed groups (molecules) while a mixture shows different atoms or molecules not chemically joined.

Names and formulae of substances

Chemical names and formulae allow scientists to communicate unambiguously. Elements use the symbols from the periodic table. Compounds are represented by chemical formulae which indicate the kinds and relative numbers of atoms present.

Rules and guidelines for naming and writing formulae

1. A compound name includes the names of the elements present. Examples: iron sulphide (FeS), potassium bromide (KBr), sodium chloride (NaCl).
2. When naming a binary compound, the element that is placed to the left in the periodic table (usually a metal) is named first.
3. Chemical formulae use element symbols with subscripts to show the number of atoms, e.g. H₂O (two H atoms, one O atom).
4. Ionic compounds are formed from cations (positive ions) and anions (negative ions). Common ions and their charges should be memorised for writing formulae.
5. For covalent compounds of non-metals, prefixes (mono-, di-, tri-, etc.) indicate the number of atoms of each element (e.g. CO is carbon monoxide, CO₂ is carbon dioxide).

Common cations (positive ions)

Common anions (negative ions)

Worked examples: writing chemical formulae

Tip. When a polyatomic ion (such as NO₃^-) appears more than once in a formula, enclose it in parentheses and write the appropriate subscript after the parentheses, e.g. Cu(NO₃)₂.

Common covalent compounds

Metals, metalloids and non-metals

The periodic table can be used to group elements as metals, metalloids (semi-metals) and non-metals. Metals are generally on the left and centre of the periodic table, non-metals on the right, and metalloids lie along the dividing zigzag line.

Metals

Examples: copper (Cu), zinc (Zn), gold (Au), silver (Ag), tin (Sn), lead (Pb).

Typical properties of metals:

• Good thermal conductors (used for cooking utensils).
• Good electrical conductors (used for wires; copper is widely used).
• Shiny metallic lustre.
• Malleable (can be hammered into sheets) and ductile (can be drawn into wires).
• Usually high melting points and relatively high density.
• Only a few metals (iron, cobalt, nickel) are strongly magnetic (ferromagnetic).

Non-metals

Examples: sulphur (S), phosphorus (P), nitrogen (N), oxygen (O).

Typical properties of non-metals:

• Poor thermal conductors and electrical insulators.
• Usually lack metallic lustre; brittle in solid form and not ductile.

Metalloids (semiconductors)

Metalloids or semi-metals show properties intermediate between metals and non-metals. Their electrical conductivity increases with temperature; this behaviour makes them useful as semiconductors in electronic devices (examples: silicon (Si), germanium (Ge)).

Electrical conductors, semiconductors and insulators

Electrical conductor. A substance that allows an electric current to pass through it (usually metals such as copper and aluminium).

Insulator. A material that does not conduct electricity (examples: plastic, wood, ceramic).

Semiconductor. A material (often a metalloid) that behaves like an insulator at low temperature but conducts better at higher temperatures (silicon and germanium).

Simple experiment: test electrical conductivity

A circuit with a cell and a lamp can be used: place a test substance in the circuit. If the lamp lights, the substance conducts electricity. Metals typically allow the lamp to glow (conductors); non-metals do not (insulators).

Thermal conductors and insulators

Thermal conductor. A material that transfers heat readily (metals are generally good thermal conductors).

Thermal insulator. A material that resists the transfer of heat (examples: air trapped in mineral wool, polystyrene foam).

Simple demonstration

Place a metal spoon in boiling water and a plastic spoon in another cup of boiling water. The metal spoon becomes hot much faster than the plastic spoon - this shows that the metal conducts heat more effectively.

Thermal conductivity data (typical values)

From these values: silver and stainless steel are good thermal conductors; polystyrene and air are good thermal insulators.

Magnetic and non-magnetic materials

Magnetism is a force exerted by certain materials (magnetic materials) on other magnetic materials without direct contact. A magnet produces a magnetic field around it. Materials such as iron, cobalt and nickel are ferromagnetic and can be magnetised. Steel is often used for permanent magnets because it retains magnetism longer than soft iron.

Magnets have many applications: sorting metals, compasses, data storage (magnetic strips), electric motors and generators.

Simple test for magnetism

Bring a magnet near an object. If the object is attracted, it is magnetic.

Exercises and classroom tasks

Exercise: classify substances

In the table below, indicate whether each substance is a pure substance or a mixture. If it is a mixture, specify whether it is homogeneous or heterogeneous.

Exercise: element, compound or mixture?

Decide whether each item is an element, a compound or a mixture:

1. Fizzy cold drink
2. Steel
3. Oxygen
4. Iron filings
5. Smoke
6. Limestone (CaCO₃)

Exercise: name and formula practice

1. What is the name of KBr?
2. What is the formula for potassium nitrate?
3. Is CaCO₃ an element or a compound? Give the ratio of Ca : C : O atoms.
4. Give the chemical formula for aluminium chloride.
5. Write the formula for sodium oxide.

Classroom activities suggested earlier (making mixtures, chromatography, conductivity tests, model-building with coloured balls) reinforce these concepts through practical work and model-based thinking.

Summary

• All visible objects are made of matter. Matter can be classified as mixtures or pure substances.
• A mixture combines two or more substances that retain their own properties, are not in a fixed ratio and can be separated by physical means.
• Mixtures are either homogeneous (uniform, solutions) or heterogeneous (non-uniform).
• Pure substances are elements or compounds. An element cannot be broken down chemically into simpler substances; a compound forms when two or more elements chemically combine in a fixed ratio and has a definite formula.
• Chemical names and formulae are systematic; ions (cations and anions) are used to form ionic compounds and prefixes or subscripts indicate numbers of atoms in covalent compounds.
• Elements can be grouped as metals, metalloids and non-metals. Materials may also be classified by electrical conductivity (conductors, semiconductors, insulators), thermal conductivity (conductors or insulators) and magnetic behaviour (magnetic or non-magnetic).