Chemical Elements, Compounds & Mixtures (DP IB Chemistry) : Revision Note

Elements, compounds & mixtures

What are elements and compounds?

• Elements are substances made from one kind of atom

  • Atoms cannot be chemically broken down into simpler substances

• 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

• Compounds are made from two or more elements chemically bonded

• 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

What is a mixture?

• Mixtures are substances that contain more than one element or compound in no fixed ratio

• The 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 their pure form

  • 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, for example:

  • Air

  • Bronze (an alloy)

• A heterogeneous mixture has non-uniform composition, so its properties are not the same throughout, for example:

  • Concrete

  • Orange juice with pulp

• It is often possible to see the separate components in a heterogeneous mixture, but not in a homogeneous mixture

Separating Mixtures

• The components of a mixture keep their individual properties

• This means that we can often separate them relatively easily

• The separation technique takes advantage of a suitable difference in the physical properties of the components, such as:

  • Boiling point

  • Solubility

  • Magnestism

  • Density

Separating a Mixture of Salt and Sand

• Salt and sand can be separated by first adding water

  • Only the salt dissolves

• When the mixture is filtered:

  • The salt solution passes through as the filtrate

  • The sand stays behind as the residue

• This technique works because the two substances have different solubilities in water

• The water from the salt solution can then be evaporated to leave only salt