Elements, compounds and mixtures side by side

The problem

Iron filings stirred into sulphur powder can be pulled out again with a magnet. Heat the same two together and you get iron sulphide, and the magnet no longer works. Same starting elements, so what changed? Your book's table on pp. 9–10 answers this row by row.

Element: the simplest form of matter, a pure substance with one kind of atom, which ordinary chemical reactions cannot break down. When it exists as atoms it is written as a symbol, such as Na and Ca. Gaseous elements exist as independent molecules (N₂, O₂, Cl₂); noble gases are monoatomic (He, Ar).

Compound: a pure substance whose atoms of different elements combine in a fixed ratio by weight; in water, hydrogen and oxygen are always 1 : 8 by weight. Only a chemical reaction can split it back into its elements, as when ammonia is converted to nitrogen and hydrogen. Its properties are different from those of its elements.

Mixture: an impure sample with more than one element or compound in any ratio. Each component keeps its identity and properties, the components are not chemically bound, and physical methods can separate them. A mixture's properties are the sum of its components' properties.

A compound's elements are chemically bonded in a fixed ratio by weight, so it has new properties and needs a chemical reaction to separate. A mixture's parts are not bonded, so they keep their properties and separate physically.

Key terms

Fixed ratio by weight

Every sample of a compound has its elements in the same proportion by mass.

Chemical reaction

The only way to get a compound's elements back out of it.

Physical methods

Separating parts that were never chemically bound.

Monoatomic

How the noble gases exist, unlike other gaseous elements.

Short questions with model answers

1. Q1. Using the book's ratio H : O = 1 : 8 by weight, find the masses of hydrogen and oxygen in 27 g of pure water.

   • Total parts = 1 + 8 = 9.
   • One part = 27 g ÷ 9 = 3 g.
   • Hydrogen = 1 × 3 g = 3 g; oxygen = 8 × 3 g = 24 g.

   Hydrogen 3 g, oxygen 24 g.

2. Q2. Ammonia gas and a mixture of nitrogen and hydrogen gases contain the same elements. Give two differences from the table.

   • Ammonia is a compound: its nitrogen and hydrogen are bonded in a fixed ratio, and it needs a chemical reaction to split.
   • The mixture has the two gases in any ratio, each keeping its own properties, separable by physical methods.

   Fixed ratio and chemical separation for ammonia; any ratio and physical separation for the mixture.

3. Q3. Why is helium written He but nitrogen gas written N₂?

   • Gaseous elements such as nitrogen exist as independent molecules of two atoms.
   • Noble gases such as helium exist as monoatomic molecules.

   Nitrogen gas is diatomic (N₂); helium is a monoatomic noble gas (He).

Common mistakes

• ✗ Reading 1 : 8 in water as the ratio of hydrogen atoms to oxygen atoms.

  ✓ 1 : 8 is by weight. By atoms, H₂O has two hydrogens to one oxygen.

• ✗ Saying a compound can be separated by filtering, a magnet or another physical method.

  ✓ Its elements are chemically bonded; only a chemical reaction separates them.

• ✗ Writing that water shows the properties of hydrogen and oxygen.

  ✓ A compound's properties differ from its elements'. Only a mixture keeps its components' properties.

MCQs

1. 1. In water, hydrogen and oxygen are present in the fixed ratio by weight of:

   1. (a) 2 : 1
   2. (b) 1 : 16
   3. (c) 1 : 8
   4. (d) 1 : 2
   Show answer

   (c) Your book: 1 : 8 by weight (p. 9). 2 : 1 is the ratio of atoms, not of weight.

2. 2. Which can be separated into its components by physical methods?

   1. (a) A compound
   2. (b) A mixture
   3. (c) An element
   4. (d) An allotrope
   Show answer

   (b) A mixture's components are not chemically bound, so physical methods separate them.

3. 3. Using H : O = 1 : 8 by weight, the mass of hydrogen in 45 g of pure water is:

   1. (a) 5 g
   2. (b) 40 g
   3. (c) 9 g
   4. (d) 2.5 g
   Show answer

   (a) 1 + 8 = 9 parts; 45 g ÷ 9 = 5 g per part; hydrogen is 1 part = 5 g (oxygen 40 g).

4. 4. Which element exists as monoatomic molecules?

   1. (a) Nitrogen
   2. (b) Chlorine
   3. (c) Oxygen
   4. (d) Argon
   Show answer

   (d) Argon is a noble gas; the book gives He and Ar as monoatomic. N₂, Cl₂ and O₂ are diatomic.

5. 5. The properties of a compound are:

   1. (a) The sum of its elements' properties
   2. (b) Different from those of its elements
   3. (c) The same as its lightest element's
   4. (d) Changed by adding more of one element
   Show answer

   (b) Water's properties differ from hydrogen's and oxygen's. A sum of properties describes a mixture.

Quick revision

• Element: one kind of atom, not split by ordinary reactions; written as a symbol, N₂-type molecules, or monoatomic noble gases.
• Compound: fixed ratio by weight (water 1 : 8), new properties, split only by a chemical reaction.
• Mixture: any ratio, components keep their properties, separated by physical methods.