Atoms, molecules, compounds and mixtures
Publisher Summary
This chapter discusses the atoms, molecules, compounds, and mixtures. Atoms have been defined as the smallest parts of matter that cannot be changed by chemical means. In nature, the inert gases neon, argon, krypton, and xenon together with helium exist in nature as single uncombined atoms. Molecules are the collective term given to substances containing at least two atoms chemically combined together in a simple ratio. Examples of molecules containing one element are hydrogen, nitrogen, oxygen, and fluorine. On the other hand, compounds are substances containing different elements combined in a simple ratio according to the combining powers or valencies of the constituents. Some examples of compounds that are commonly found in everyday life are, table salt or sodium chloride, and sugar or sucrose. Mixtures are combinations of compounds that can be separated by physical methods. These methods include filtering, distillation, evaporation, decanting, separating funnels, magnetism, and fractional crystallization.
1. Atoms have been defined as the smallest parts of matter which cannot be changed by chemical means. In nature, the inert gases neon, argon, krypton and xenon together with helium exist in nature as single uncombined atoms. A few other elements like gold and silver do not exist as single atoms but are not combined with any other element. The majority of elements are found in combination with other elements.
2. Molecules is the collective term given to substances which contain at least two atoms chemically combined together in a simple ratio. Examples of molecules containing one element are hydrogen H2, nitrogen N2, oxygen O2, fluorine F2, phosphorus P4, and sulphur S8. This state of matter can be changed by chemical means during chemical reactions.
3. Compounds are substances which contain different elements combined in a simple ratio according to the combining powers or valencies of the constituents. Some examples of compounds which are commonly found in everyday life are, table salt or sodium chloride (NaCl), sugar or sucrose (C12H22O11), battery acid or sulphuric acid (H2SO4), chalk or calcium carbonate (CaCO3) and camping gas or propane (C3H8). The smallest amount of a compound which can exist and can be changed by chemical methods is called a molecule.
4. Mixtures are combinations of compounds which can be separated by physical methods. These methods include (i) filtering, (ii) distillation, (iii) evaporation, (iv) decanting (v) separating funnels, (vi) magnetism and (vii) fractional crystallisation.
(i) Filtration can be used to separate a mixture of liquid and solid, or by making use of the different solubilities of the compounds in the mixture, by dissolving one substance, but not the other. For example, in a mixture of sand and salt, the salt dissolves in water leaving the sand undissolved. A typical arrangement for filtration is shown in Figure 51.1(i)
(ii) Distillation can also be used to separate a mixture of a solid and a liquid, the liquid being boiled off leaving the solid behind. More commonly fractional distillation is used to separate mixtures of miscible liquids as happens for the separation of the different boiling point fractions of crude oil. A typical arrangement is shown in Figure 51.1(ii)
(iii) Evaporation can be used to separate the components of a solution by allowing the liquid solvent to be removed leaving behind the solid, as shown in Figure 51.1(iii)
(iv) Decanting is a very simple way of separating a solid with a liquid, achieved by carefully pouring off the liquid layer (see Figure 51.1(iv).
(v) Separating funnels are pieces of apparatus designed to separate two immiscible liquids of different density. The liquids from two layers which can be run off into different containers. A typical separating funnel is shown in Figure 51.1(v)
(vi) Magnetism can only be utilised to separate magnetic materials from non-magnetic materials. This technique is used in separating crushed metallic ores or for retrieving iron based articles for recycling. A suitable arrangement is shown in Figure 51.1(vi)
(vii) Fractional crystallisation can be used to separate two solids, which though both soluble in the same solvent, have quite different solubilities. The method involves dissolving the solids and taking ‘crops’ of crystals as they are formed. Finally, the soluble substance is obtained by evaporation.
5. The differences between compounds and mixtures are summarised in Table 51.1.
Table 51.1
The properties of compounds and mixtures
| Compounds | Mixtures |
| 1. The substance has a constant composition. | The composition of the mixture can be varied by mass. |
| 2. The substance can only be separated into its components by chemical methods. | The substances can be separated by physical methods. |
| 3. There can be considerable amounts of heat energy released when the substance is formed. | There is little or no heat energy released when the substances are mixed. |
| 4. The properties of a compound are quite different to the properties of the constituent elements. | The properties are those of each of the substances forming the mixture. |