Elements, Compounds & Mixtures
- Elements are substances made from one kind of atom
- Compounds are made from two or more elements chemically combined
- Elements take part in chemical reactions in which new substances are made in processes that most often involve an energy change
- In these reactions, atoms combine together in fixed ratios that will give them full outer shells of electrons, producing compounds
- The properties of compounds can be quite different from the elements that form them
Elements into compounds diagram
The properties of sodium chloride are quite different from sodium and chlorine
- In a mixture, elements and compounds are interspersed with each other, but are not chemically combined
- This means the components of a mixture retain the same characteristic properties as when they are in their pure form
- So, for example, the gases nitrogen and oxygen when mixed in air, retain the same characteristic properties as they would have if they were separate
- Substances will burn in air because the oxygen present in the air supports combustion
Mixtures at the molecular level diagram
Particle in boxes diagrams such as these can help you to visualise the difference between elements and compounds at the molecular level
Homogeneous or hetergeneous
- A homogeneous mixture has uniform composition and properties throughout
- A heterogeneous mixture has non-uniform composition, so its properties are not the same throughout
- It is often possible to see the separate components in a heterogeneous mixture, but not in a homogeneous mixture
Types of Mixtures
| Mixture | Homogeneous or Heterogeneous |
| Air | Homogeneous |
| Bronze (an alloy) | Homogeneous |
| Concrete | Heterogeneous |
| Orange juice with pulp | Heterogeneous |
Separating Mixtures
- The components retain their individual properties in a mixture and we can often separate them relatively easily. The technique we choose to achieve this will take advantage of a suitable difference in the physical properties of the components
Some mixtures and examples of separation techniques
| Mixture | What technique can be used to separate the components? | The property that is different in the components |
| Air | (Fractional) distillation | Boiling point |
| Salt & sand | Filtration | Solubility in water |
| Pigments in food colours | Chromatography | Adsorption on cellulose |
| A iron-sulfur mixture | A magnet | Magnetism |