Mixing Two Metals Makes Something Better Than Either

Because the mixture is usually stronger and more useful. Pure iron is comparatively soft and rusts readily, while steel — iron with a little carbon — is strong enough to build with.

Almost every metal object around you is an alloy rather than a pure element. This page covers everything in the CBSE Class 8 Science chapter's second part: elements against compounds, metals and non-metals, alloys and their uses, and minerals and ores.

An element is a pure substance made of one kind of atom, and it cannot be broken down into anything simpler by chemical means. A compound is a pure substance formed when two or more elements combine chemically, and it can be broken down into those elements.

Elements: hydrogen, oxygen, nitrogen, carbon, sulphur, iron, copper, aluminium, gold, zinc.

Compounds: water (hydrogen and oxygen), common salt (sodium and chlorine), carbon dioxide (carbon and oxygen), sugar, chalk, baking soda.

Classifying given substances:

- Element — copper wire, a gold ring, sulphur powder, the graphite in a pencil
- Compound — water, salt, sugar, chalk, rust
- Mixture — air, milk, brass, sea water

The striking feature of a compound is that its properties are nothing like those of its elements. Water is made from hydrogen, which burns, and oxygen, which supports burning — yet water puts fires out. Salt is made from sodium, a metal so reactive it must be kept away from water, and chlorine, a poisonous gas — yet salt is eaten every day.

That loss of the original properties is what separates a compound from a mixture, and it is the test to apply. In brass the copper still conducts and the zinc is still zinc, because they have merely been mixed. In salt, neither sodium nor chlorine survives as itself, because they combined.

By testing a few physical properties.

Metals are:

- Lustrous — shiny when freshly cut or polished
- Malleable — can be beaten into thin sheets, as aluminium foil is
- Ductile — can be drawn into wires, as copper is
- Good conductors of heat and electricity
- Sonorous — give a ringing sound when struck
- Generally hard, dense and solid at room temperature

Examples: iron, copper, aluminium, gold, silver, zinc, sodium.

Non-metals are the opposite — dull, brittle when solid, and poor conductors. They occur as solids, liquids or gases.

Examples: carbon, sulphur, phosphorus, oxygen, nitrogen, chlorine, iodine.

A steel vessel shows the metallic properties together: it shines, conducts heat to the food and rings when tapped, while the plastic on its handle does none of these.

The exceptions carry marks of their own and are worth naming:

- Mercury is a metal that is liquid at room temperature
- Bromine is a non-metal that is liquid
- Graphite, a form of carbon, conducts electricity although it is a non-metal
- Sodium and potassium are metals soft enough to cut with a knife
- Diamond, a non-metal, is the hardest natural substance

So the tests describe a general pattern rather than a rule without exceptions, which is why an answer should name a property and its exception.

An alloy is a mixture of two or more metals, or of a metal with a small amount of a non-metal, made to improve on the properties of the pure metal.

- Brass — copper and zinc. Used for utensils, taps, door fittings, locks and musical instruments.
- Bronze — copper and tin. Used for statues, medals, bells and coins.
- Steel — iron and carbon. Used for construction, rails, tools, vehicles and utensils.
- Stainless steel — iron with chromium and nickel. Used for cutlery and kitchen vessels because it does not rust.
- Solder — lead and tin. Used for joining electrical wires, because it melts at a low temperature.

Alloys are preferred because they are:

- Stronger and harder — steel over pure iron
- More resistant to rusting and corrosion — stainless steel over iron
- Better in appearance, and often cheaper than the pure metal
- Sometimes lower melting, which is exactly what solder needs

Every Indian kitchen makes the case: vessels are stainless steel rather than iron, so they neither rust nor react with food.

Also worth knowing are the everyday uses of the pure substances — copper for electrical wiring because it conducts, aluminium for foil and aircraft because it is light and malleable, graphite for pencil leads, and sulphur for making sulphuric acid.

The point examiners test is that an alloy is a mixture, not a compound. Brass has no fixed formula, its proportions can be varied to make it harder or softer, and its copper and zinc are not chemically combined — which is precisely why alloys can be tuned for a particular job.

Metals are rarely found pure in the earth. Most occur combined with other elements as compounds.

- A mineral is any naturally occurring substance in the earth's crust containing a metal in combined form.
- An ore is a mineral from which the metal can be extracted profitably and in useful quantity.

So every ore is a mineral, but not every mineral is an ore. A mineral holding too little metal, or holding it too tightly to extract cheaply, is not called an ore.

Examples: bauxite is the ore of aluminium, haematite of iron, and copper pyrites of copper. A few metals such as gold, silver and platinum are found in the free state because they are so unreactive.

Obtaining a metal from its ore has three broad stages:

1. Mining — digging the ore out of the earth.
2. Concentration — removing the earthy impurities, called gangue, so the ore is enriched.
3. Extraction — separating the metal chemically from its compound, usually by heating the concentrated ore with a suitable substance.
4. Refining — purifying the metal obtained.

The metal is then shaped into sheets, wires or castings, or alloyed for a particular use — which is how iron ore eventually becomes the steel in a railway line.

The word that defines an ore is therefore profitably. It is an economic judgement as much as a chemical one, which is why a mineral can stop being worth mining, or become worth mining, without its chemistry changing at all.