Unit outcomes

By the end of this unit you will be able to:

• Identify, define, and give examples of:
  • a pure substance
  • an element
  • a compound
  • a mixture
  • a homogenous mixture
  • a heterogenous mixture
  • an alloy.

What you should know:

Before you start this unit, make sure you can:

• Identify an atom, ion and a molecule in Subject outcome 5.4, Unit 1: Particles.

Introduction

You will learn to identify, define, and give examples of pure substances that are either elements or compounds, and other substances called mixtures. Mixtures can either be homogenous or heterogenous, in other words, they are either uniform in appearance or not. Alloys are mixtures made up of particular metals and form homogenous mixtures. Brass and steel are examples of alloys.

Elements, compounds, and mixtures

All the objects that we see in the world around us are made of matter. Matter makes up the air we breathe, the ground we walk on, the food we eat and the animals and plants that live around us. Even our bodies are made of matter. Matter can be separated into two different groups, mixtures, and pure substances.

Elements and compounds are pure substances, which means that they are not mixtures. Pure substances are substances that are made up of only one kind of particle and have a fixed composition.

• Elements are the most basic form of matter. They are made up of one type of atom and cannot be broken down into anything simpler.
• Compounds are made from two or more elements which are chemically joined together. Although compounds can be separated, it is very difficult to do so.
• Mixtures are made up of elements and/or compounds that are not chemically joined together and are easily separated. A mixture can either be homogenous or heterogenous.

Common elements, mixtures and compounds are listed in Table 1.

Table 1: Examples of common elements, mixtures, and compounds

Elements

Elements are chemical substances that cannot be divided or changed into other chemical substances by any ordinary chemical means. The smallest unit of an element is the atom. Figure 2 shows a particle diagram of an element.

There are 118 known elements. Most of these are natural, but some are man-made.

As we have previously learnt in SO 5.3 Unit 1, elements are found on the periodic table and have a chemical symbol, for example Oxygen (O) and Calcium (Ca).

Compounds

A compound is a chemical substance that forms when two or more different elements combine in a fixed ratio. An important characteristic of a compound is that it has a chemical formula, which describes the ratio in which the atoms of each element in the compound occur.

Carbon dioxide has the formula CO₂ and is made up of one carbon atom and two oxygen atoms.

Ethanoic acid, which is more commonly known as vinegar, is made up of two carbon atoms, four hydrogen atoms and two oxygen atoms.

Water is a compound and is made up of two hydrogen atoms for every one oxygen atom.

Sodium chloride is a compound made up of one sodium atom for every chlorine atom.

Mixtures

A mixture is a substance which is made up of elements and/or compounds, which are not chemically joined together.

In a mixture, the substances that make up the mixture:

• are not in a fixed ratio – imagine, for example, that you have 250 ml of water and you add sand to the water. It does not matter whether you add 20 g, 40 g, 100 g, or any other mass of sand to the water, it will still be called a mixture of sand and water.
• keep their physical properties – in the example we used of sand and water, neither of these substances has changed in any way when they are mixed together. The sand is still sand and the water is still water.
• can be separated by physical means – to separate something by ‘physical means’ means that there is no chemical process involved. In our sand and water example, it is possible to separate the mixture by simply pouring the water through a filter. Something physical is done to the mixture, rather than something chemical.

Exercise 2.1

State whether the following statements are true of an element, mixture or compound:

1. Made up of two or more elements chemically joined together
2. Made up of one type of atom and can be found on the periodic table
3. Can be separated by mechanical means
4. Substance has a chemical formula
5. Cannot be broken down into anything simpler
6. Made up of elements and/or compounds which are not chemically joined together

The full solutions are at the end of the unit.

Different types of mixtures

We can group mixtures further by dividing them into those that are heterogeneous and those that are homogeneous.

Homogenous mixtures

A homogeneous mixture has a definite composition and specific properties. In a homogeneous mixture, the different parts cannot be seen. Homogenous mixtures are mixtures where all the substances which make up the mixture are in the same state. A homogenous mixture of a soluble solid and a suitable solvent will form a solution, as will a mixture of gases like the air.

Salt water is a solution of salt dissolved in water and is an example of a homogeneous mixture. Figure 7 shows that when the salt (sodium chloride) dissolves, its particles spread evenly through the water so that all parts of the solution are the same, and you can no longer see the salt as being separate from the water.

The air we breathe is another example of a homogeneous mixture as it is made up of different gases which are in a constant ratio, and which cannot be visually distinguished from each other. In other words, you cannot see the different components. Homogenous mixtures tend to be transparent, and can be either solids, liquids or gases.

Alloys are homogenous mixtures of different metals, and in some instances non-metals, used to make a variety of objects including coins, car wheels and bells.

Heterogeneous mixtures

A heterogeneous mixture does not have a definite composition. Cereal in milk is an example of a heterogeneous mixture. Soil is another example. Soil has pebbles, plant matter and sand in it. Although you may add one substance to the other, they will stay separate in the mixture. We say that these heterogeneous mixtures are non-uniform, in other words they are not exactly the same throughout.

Milk is a heterogenous mixture, and so is oil and water. Milk is a colloid of water proteins, fats, lactose, and minerals. The proteins, fats, lactose, and minerals are spread out though the water. The oil and water mixture is also a colloid because the oil and water do not mix and the oil ‘sits’ on top of the water. A common example of a suspension is muddy water. A suspension is a mixture of solid particles in a liquid which will settle out quickly once the liquid is still.

Summary

In this unit you have learnt the following:

• All the objects and substances that we see in the world are made of matter.
• This matter can be classified according to whether it is a mixture or a pure substance.
• A mixture is a combination of two or more substances, where these substances are not bonded (or joined) to each other and no chemical reaction occurs between the substances. Examples of mixtures are air (a mixture of different gases) and cereal in milk.
• The main characteristics of mixtures are that the substances that make them up are not in a fixed ratio, these substances keep their physical properties, and these substances can be separated from each other using mechanical means.
• A heterogeneous mixture is one that consists of two or more substances. It is nonuniform and the different components of the mixture can be seen. An example would be a mixture of sand and water.
• A homogeneous mixture is one that is uniform, and where the different components of the mixture cannot be seen. An example would be salt in water.
• Pure substances can be further divided into elements and compounds.
• An element is a substance that cannot be broken down into other substances through chemical means.
• All the elements are found on the periodic table. Each element has its own chemical symbol. Examples are iron (Fe), sulfur (S), calcium (Ca), magnesium (Mg) and fluorine (F).
• A compound is a substance made up of two or more different elements that are joined together in a fixed ratio. Examples of compounds are sodium chloride (NaCl), iron sulfide (FeS), calcium carbonate (CaCO₃) and water (H₂O).

Unit 2: Assessment

Suggested time to complete: 25 minutes

1. Copy and complete the following table into your notebook.
   

   Substance	Element, mixture, or compound?	Type of mixture (if applicable)
   Water
   Brass
   Jelly
   Aluminium
   Table salt
   Sugar water
   Cooking oil

2. In your notebook, draw particle diagrams to show an element, a heterogenous mixture and the compound water.
3. Give a one-word term for the following:
   1. a mixture with a uniform composition
   2. a substance made up of only one type of atom
   3. a substance which has a chemical formula
   4. a mixture where undissolved substances are spread throughout the substance

The full solutions are at the end of the unit.

Unit 2: Solutions

Exercise 2.1

1. compound
2. element
3. mixture
4. compound
5. element
6. mixture

Back to Exercise 2.1

Unit 2: Assessment

1. .
   

   Substance	Element, mixture, or compound?	Type of mixture (if applicable)
   Water	Compound	N/A
   Brass	Mixture	homogenous
   Jelly	Mixture	heterogenous
   Aluminium	Element	N/A
   Table salt	Compound	N/A
   Sugar water	Mixture	homogenous
   Cooking oil	Mixture	homogenous

2. .
   
3. .
   1. homogenous
   2. element
   3. compound
   4. colloid/heterogenous

Back to Unit 2: Assessment