The Metal You Can Cut With a Knife

Metals are usually shiny, hard, can be hammered into sheets and drawn into wires, ring when struck, and conduct heat and electricity. Non-metals are generally dull, brittle, and do not conduct. Those are the working rules — and this chapter is largely about testing them and finding where they break.

This page covers everything in the CBSE Class 7 Science chapter on the properties of metals and non-metals: the six defining properties, how to test a sample, the exceptions, and how properties decide everyday uses.

Each property has a precise name that questions expect you to use.

Lustre is shine. Freshly cut metals are lustrous; non-metals are usually dull.

Hardness is resistance to being scratched or dented. Most metals are hard, while non-metals are often soft or brittle.

Malleability is the ability to be hammered into thin sheets without breaking.

Ductility is the ability to be drawn into thin wires.

Sonority is the ability to produce a ringing sound when struck.

Metals also conduct heat and electricity well, while non-metals generally do not.

For example, aluminium foil in your kitchen shows malleability, the copper inside a wire shows ductility, and a temple bell shows sonority.

The pair most often confused is malleability and ductility. Sheets are malleability, wires are ductility — remember that a wire is drawn, and drawing is ductile.

Four simple tests, one for each property, settle most samples.

Hammer it. If it flattens into a sheet, it is malleable and probably a metal. If it shatters into pieces, it is brittle and probably a non-metal.

Draw it. If it can be pulled into a wire, it is ductile.

Strike it. A clear ringing sound means it is sonorous.

Put it in a circuit tester. If the lamp glows, the sample conducts electricity.

So an iron nail flattens, rings and lights the lamp — a metal. A piece of coal crumbles when hammered, makes a dull thud and leaves the lamp dark — a non-metal.

Use the tests together, not singly. A single result can mislead, which is exactly what the next section is about: some samples pass one test and fail another.

Several well-known substances are exceptions, and questions ask about them precisely because they test whether you learned rules or reasons.

Mercury is a metal that is liquid at room temperature, though metals are normally solid. It is used in thermometers.

Sodium and potassium are metals soft enough to be cut with a knife, though metals are normally hard.

Graphite is a non-metal that conducts electricity, though non-metals normally do not. It is used in pencil leads and in electrodes.

Iodine is a non-metal that is lustrous, though non-metals are normally dull.

So a sample can be soft and still be a metal, or conducting and still be a non-metal. That is why identification uses several properties together rather than resting on one.

The lesson behind the exceptions: these are general tendencies, not laws. Nature was not consulted about our categories.

Almost every use of a material follows directly from a property.

Copper and aluminium are used in electric wires because they conduct electricity well and are ductile enough to be drawn into long thin wires.

Iron is used for tools and utensils because it is hard and strong, and it conducts heat well enough to cook with.

Aluminium foil is used to wrap food because it is highly malleable, so it can be rolled into sheets thin enough to fold around a chapati.

Graphite is used in pencil leads because it is soft and leaves a mark on paper, and in electrodes because it conducts.

Metals in bells and utensils are chosen partly for sonority, which is why a steel plate rings and a plastic one does not.

For example, a cooking vessel is metal for conduction while its handle is plastic or wood for insulation — the two materials chosen for opposite reasons in the same object.