Cambridge O Level Chemistry · Syllabus 5070 · Atoms, Elements and Compounds
Compound
What is Compound?
A substance in which atoms of two or more different elements are chemically bonded together in a fixed ratio. The chemical properties of a compound are completely different from those of the elements that formed it, and it can only be separated back into those elements by a chemical reaction, not by a physical method.
This definition is part of the Atoms, Elements and Compounds chapter in Cambridge O Level Chemistry.
Questions students ask about Compound
What is the difference between an element, a compound and a mixture?
An element contains only one type of atom. Those atoms may be separate, as in argon, joined in molecules such as \(\mathrm{O_2}\), or joined in a giant structure such as diamond — it is still an element. A compound contains atoms of two or more different elements chemically bonded together, so it has its own properties and a fixed formula. A mixture contains two or more substances that are not chemically bonded, so each keeps its own properties and the substances can be separated by physical means.
Why do ionic compounds conduct electricity when molten or dissolved but not when solid?
Conduction needs a charged particle that can move. In a solid ionic compound the ions are held in fixed positions in the giant lattice by strong electrostatic attractions, so although they are charged they cannot move, and the solid does not conduct. When the compound is melted or dissolved in water the lattice breaks down, the ions become mobile and can carry charge, so the molten or aqueous compound conducts well. Name the charged particle and say whether it can move: that answers every conduction question in this chapter.
How do you explain the properties of a substance from its structure in an exam answer?
Walk the chain: identify the particles, follow the outer electrons, name the structure, then read the property off the structure. For melting point, say which particles are held together, by which attraction, and how much energy is needed to overcome it. For conductivity, name the mobile charged particle — delocalised electrons in a metal or graphite, mobile ions in a molten or aqueous ionic compound — or state that there is none. Marks are given for each link in the chain, not for the final word alone.