Changes happen around us all the time — ice melts, milk turns sour, iron rusts. Chemists sort every change into two kinds and then sort matter itself into elements, compounds and mixtures.
Physical change
A physical change alters only the physical state or appearance of a substance; no new substance is formed and the change is usually easily reversible. Melting of ice, boiling of water, dissolving of sugar and breaking of glass are physical changes — the chemical composition does not change. Water (H2O) remains water whether it is ice, liquid or steam.
Chemical change
A chemical change (a chemical reaction) forms one or more new substances with new properties, and is usually difficult to reverse. The rusting of iron, burning of a candle, cooking of food and souring of milk are chemical changes. Tell-tale signs of a chemical change include a change in colour, evolution of a gas, formation of a precipitate, a change in smell, and a release or absorption of heat or light.
Elements
An element is a pure substance made of only one kind of atom; it cannot be broken into anything simpler by chemical means. Elements are grouped as:
- Metals — shiny (lustrous), good conductors of heat and electricity, malleable, ductile and sonorous (e.g. copper, iron, gold).
- Non-metals — usually dull, poor conductors, brittle if solid (e.g. sulphur, carbon, oxygen).
- Metalloids — show properties between metals and non-metals (e.g. silicon, boron, germanium).
Compounds
A compound is a pure substance formed when two or more elements combine chemically in a fixed ratio by mass. Its properties are entirely different from those of its constituent elements, and the elements cannot be separated by physical methods — only by chemical reaction. Water (hydrogen + oxygen) and common salt (sodium + chlorine) are compounds.
The iron and sulphur experiment
This classic experiment shows the difference between a mixture and a compound. If iron filings and sulphur powder are simply mixed, the iron can still be picked out with a magnet and each keeps its own colour — that is a mixture. But if the mixture is heated strongly, it glows and forms iron sulphide (FeS), a new black solid. A magnet no longer attracts it and dilute acid gives a smelly gas — the elements have combined chemically to form a compound with new properties.
Comparing compounds and mixtures
A compound has a fixed composition, uniform properties throughout, and its parts can only be separated chemically; making it usually involves heat or light. A mixture has a variable composition, keeps the properties of its components, and can be separated by physical methods with no energy change.
Classify each change as physical or chemical: (a) melting of wax, (b) burning of wax, (c) dissolving salt in water.
Solution- Melting of wax changes only the state; no new substance forms — physical.
- Burning of wax forms new substances (carbon dioxide and water) with heat and light — chemical.
- Dissolving salt makes a solution but the salt is unchanged and can be recovered — physical.
Answer: (a) physical, (b) chemical, (c) physical.
Give two observable signs that confirm rusting of iron is a chemical change.
Solution- A new substance (reddish-brown rust, iron oxide) forms with properties different from shiny iron.
- There is a clear change in colour from grey to reddish-brown.
- The change cannot easily be reversed — rust will not turn back into iron by simple means.
Answer: Formation of a new substance (rust) and a colour change (grey to reddish-brown); it is also not easily reversible.
Classify these elements as metal, non-metal or metalloid: copper, sulphur, silicon, oxygen.
Solution- Copper is lustrous and a good conductor — a metal.
- Sulphur is dull, brittle and a poor conductor — a non-metal.
- Silicon has properties between the two — a metalloid.
- Oxygen is a gas and a non-metal.
Answer: Copper = metal; sulphur = non-metal; silicon = metalloid; oxygen = non-metal.
In the iron and sulphur experiment, how can you tell the difference between the unheated mixture and the heated product?
Solution- In the mixture, a magnet pulls out the iron filings and the yellow sulphur is still visible — each keeps its own properties.
- After strong heating, a black solid, iron sulphide (FeS), forms.
- A magnet no longer attracts it, and adding dilute acid gives a foul-smelling gas (hydrogen sulphide), confirming a new compound.
Answer: The mixture's iron is magnetic and separable; the heated product (FeS) is black, non-magnetic and gives a smelly gas with acid.
State two differences between a compound and a mixture, using water and air as examples.
Solution- A compound (water) has a fixed composition (hydrogen and oxygen in a fixed ratio); a mixture (air) has a variable composition.
- A compound's properties differ from its elements; a mixture keeps the properties of its components.
- A compound is separated only by chemical means; a mixture is separated by physical methods.
Answer: Water has a fixed composition and new properties separable only chemically; air has a variable composition, keeps its components' properties and is separable physically.
Why is the formation of curd from milk considered a chemical change while freezing of milk is a physical change?
Solution- When curd forms, new substances (acids and proteins changed in structure) are produced and the change cannot be reversed.
- This satisfies the test for a chemical change — a new substance with new properties.
- Freezing only changes milk from liquid to solid; warming it back gives the same milk, so no new substance forms.
Answer: Curd formation makes a new, irreversible substance (chemical change); freezing only changes state and is reversible (physical change).