0620 · 2.1Elements, compounds and mixtures

Elements and compounds

0:00Iron and sulfur can be mixed together, yet a magnet can still pull out the iron. After they react through heating, the resulting compound cannot be separated in that way. The same starting elements can therefore produce very different materials. Understanding this difference will help you distinguish elements, compounds and mixtures. All matter is made from tiny particles. For this topic, we need the idea of an atom, which is a tiny building block of an element.

0:25Different elements contain different types of atom. Particle diagrams help us understand substances by showing which atoms are present and whether they are chemically joined. An element is a substance containing only one type of atom. Helium is made only of helium atoms, and iron is made only of iron atoms. There may be enormous numbers of atoms in either sample. What matters is that every atom belongs to the same element, not how many atoms are present.

0:52Atoms of the same element can also join together. Oxygen gas contains molecules made from pairs of oxygen atoms. A molecule is a group of atoms chemically bonded together. Oxygen is still an element because those molecules contain only one type of atom, even though each molecule contains two atoms. A compound contains two or more elements chemically combined in a fixed ratio. It has different properties from the elements that form it.

1:16Chemically combined means that chemical bonds hold the atoms together within the substance. This creates a new substance rather than simply putting the original substances beside one another. Water is a compound containing hydrogen and oxygen. Each water molecule contains two hydrogen atoms chemically bonded to one oxygen atom. That gives a fixed ratio of two hydrogen atoms to one oxygen atom. Every water molecule has that same composition, whether the sample is a drop or a swimming pool.

1:45The fixed ratio is part of the compound’s identity. If hydrogen reacts with oxygen while too much hydrogen is present, the water formed still has two hydrogen atoms to one oxygen atom. The leftover hydrogen simply stays as a separate substance. That sample would then be a mixture of water and hydrogen, rather than pure water. A compound’s properties are not simply an average of its elements’ properties. Hydrogen burns, and oxygen supports combustion, but water has neither of those properties.

2:14Chemical combination changes how the atoms are joined. This explains why identifying the elements present is not enough to predict the compound’s behaviour.

Mixtures and physical separation

2:22A mixture contains two or more substances that are not chemically combined with one another. Its components can be separated by physical methods. The substances might be elements, compounds, or a combination of both. Unlike a compound, a mixture does not have a fixed composition, so the proportions of its components can vary. You can mix iron with a little sulfur, or with a much larger amount. Both samples are mixtures because the original substances remain chemically separate.

2:49The iron still responds to a magnet. Separation works because we can use a property of one component without changing it into a different substance. A mixture does not have to look uneven. Salt water can look uniform, but it still contains water and dissolved sodium chloride. Dissolving does not turn those substances into one new compound. Physical methods can recover each component again. You will see those techniques in the video on separation and purification.

3:16In a teacher demonstration, heating iron and sulfur can make them react to form iron(II) sulfide. This product is a compound because the elements are now chemically combined in a fixed ratio. Its properties differ from those of the starting elements. A magnet cannot separate the iron from fully reacted iron(II) sulfide. Before the reaction, the magnet moves iron that is already present as a separate substance. After the reaction, the iron is chemically combined with sulfur inside the compound.

3:44Separating those elements requires a chemical change, not simply moving one component away. Mixing and chemical combination are therefore fundamentally different processes.

Classifying particle diagrams

3:54Let us work through these three particle diagrams. Each circle represents an atom, and touching circles represent atoms chemically bonded together. First, identify the types of atom in each box. Then check whether the sample contains one substance or more than one. Box one contains pairs of oxygen atoms. Although the atoms are joined, only one type of atom is present. The sample is therefore an element. Counting the atoms in each particle is not enough.

4:21An element can contain molecules made from several atoms of the same type. Box two contains identical water molecules throughout. Each molecule has two different elements chemically combined in the same fixed ratio. This sample is therefore a compound. Box three contains both oxygen molecules and water molecules, so it is a mixture of an element and a compound. Notice that some atoms inside a mixture can still be chemically bonded.

4:47The water molecules in box three do not lose their bonds. The important point is that water and oxygen are not chemically combined with each other. Saying that a mixture contains no chemical bonds would therefore be incorrect. Another common mistake is classifying a sample by appearance alone. A uniform appearance does not prove that something is a compound. Likewise, several atoms joined together do not automatically make a compound.

5:10Use the types of atom, chemical combination and number of substances to justify your classification. An exam question may ask you to describe the differences between a compound and a mixture. Give matching points rather than saying they are different. A compound contains two or more elements chemically combined in a fixed ratio. A mixture contains two or more substances that are not chemically combined, and their proportions can vary.

5:34A compound has different properties from its constituent elements. Substances in a mixture retain their own chemical properties. A mixture can be separated by physical methods. Separating a compound into its constituent elements requires chemical changes. The central distinction is what is present and how it is combined. An element contains only one type of atom. In a compound, two or more elements are chemically combined in a fixed ratio, producing a substance with different properties.

6:02A mixture holds separate substances whose proportions can vary, and physical methods can separate them.