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# Mixtures, compounds and elements

## Introduction

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 own human bodies are made of matter!

Different objects can be made of different types of matter, or materials. For example, a cupboard (an object) is made of wood, nails and hinges (the materials). The properties of the materials will affect the properties of the object. In the example of the cupboard, the strength of the wood and metals make the cupboard strong and durable. In the same way, the raincoats that you wear during bad weather, are made of a material that is waterproof. The electrical wires in your home are made of metal because metals are a type of material that is able to conduct electricity. It is very important to understand the properties of materials, so that we can use them in our homes, in industry and in other applications. In this chapter, we will be looking at different types of materials and their properties.

Some of the properties of matter that you should know are:

• Strength: Materials can be strong and resist bending (e.g. iron rods, cement) or weak (e.g. fabrics)
• Thermal and electrical conductivity: Materials that conduct heat (e.g. metals) are called thermal conductors. Materials that conduct electricity are electrical conductors.
• Brittle, malleable or ductile: Brittle materials break easily. Materials that are malleable can be easily formed into different shapes. Ductile materials are able to be formed into long wires.
• Magnetic or non-magnetic: Magnetic materials have a magnetic field.
• Density: Density is the mass per unit volume. An example of a dense material is concrete.
• Boiling and melting points: The boiling and melting points of substance help us to classify substances as solids, liquids or gases at a specific temperature.

The diagram below shows one way in which matter can be classified (grouped) according to its different properties. As you read further in this chapter, you will see that there are also other ways of classifying materials, for example according to whether or not they are good electrical conductors.

## Discussion: Everyday materials

In groups of 3 or 4 look at the labels of medicines, food items, and any other items that you use often. What can you tell about the material inside the container from the list of ingredients? Why is it important to have a list of ingredients on the materials that we use? Do some research on the safety data of the various compounds in the items that you looked at. Are the compounds in the items safe to use? In the food items, what preservatives and additives are there? Are these preservatives and additives good for you? Are there natural alternatives (natural alternatives are usually used by indigenous people groups)?

## Mixtures

We see mixtures all the time in our everyday lives. A stew, for example, is a mixture of different foods such as meat and vegetables; sea water is a mixture of water, salt and other substances, and air is a mixture of gases such as carbon dioxide, oxygen and nitrogen.

Definition 1: Mixture
A mixture is a combination of two or more substances, where these substances are not bonded (or joined) to each other.

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

• are not in a fixed ratio Imagine, for example, that you have a 250 ml beaker of water. It doesn't 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 the sand and water, neither of these substances has changed in any way when they are mixed together. Even though the sand is in water, it still has the same properties as when it was out of the water.
• can be separated by mechanical means To separate something by 'mechanical 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.

Some other examples of mixtures include blood (a mixture of blood cells, platelets and plasma), steel (a mixture of iron and other materials) and the gold that is used to make jewellery. The gold in jewellery is not pure gold but is a mixture of metals. The amount of gold in the jewellery is measured in karats (24 karat would be pure gold, while 18 karat is only 75% gold).

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

### Heterogeneous mixtures

A heterogeneous mixture does not have a definite composition. Think of a pizza, that has a topping of cheese, tomato, mushrooms and peppers (the topping is a mixture). Each slice will probably be slightly different from the next because the toppings (the tomato, cheese, mushrooms and peppers) are not evenly distributed. Another example would be granite, a type of rock. Granite is made up of lots of different mineral substances including quartz and feldspar. But these minerals are not spread evenly through the rock and so some parts of the rock may have more quartz than others. Another example is a mixture of oil and water. 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.

Definition 2: Heterogeneous mixture
A heterogeneous mixture is one that is non-uniform and the different components of the mixture can be seen.

### Homogeneous mixtures

A homogeneous mixture has a definite composition, and specific properties. In a homogeneous mixture, the different parts cannot be seen. A solution of salt dissolved in water is an example of a homogeneous mixture. When the salt dissolves, it will 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. Think also of a powdered drink that you mix with water. Provided you give the container a good shake after you have added the powder to the water, the drink will have the same sweet taste for anyone who drinks it, it won't matter whether they take a sip from the top or from the bottom. The air we breathe is another example of a homogeneous mixture since it is made up of different gases which are in a constant ratio, and which can't be visually distinguished from each other (i.e. you can't see the different components).

Definition 3: Homogeneous mixture
A homogeneous mixture is one that is uniform, and where the different components of the mixture cannot be seen.

An alloy is a homogeneous mixture of two or more elements, at least one of which is a metal, where the resulting material has metallic properties. Alloys are usually made to improve the properties of the elements that make them up. For example steel is much stronger than iron (which is the main component of steel).

### Exercise 1: Mixtures

For each of the following mixtures state whether it is a homogenous or a heterogenous mixture:

1. sugar and water
2. flour and iron filings (small pieces of iron)
3. flour and baking powder
4. smarties, jelly tots and peppermints

#### Solution

1. This is a homogenous mixture since we cannot see the sugar in the water. Also, the two components are mixed uniformly.
2. This is a heterogenous mixture since we are able to make out the pieces of iron in the flour. In this mixture there may be places where there are a lot of iron filings and places where there is more flour, so it is not uniformly mixed.
3. Homogenous mixture since there is no way to distinguish between the flour and the baking powder. The two components of the mixture are uniformly mixed.
4. Heterogenous mixture since we can clearly see each of the components that make up the mixture. The three components of the mixture are not evenly distributed.

### Activity: Classifying materials

Look around your classroom or school. Make a list of all the different materials that you see around you. Try to work out why a particular material was used. Can you classify all the different materials used according to their properties? On your way to school or at home or in the shops, look at the different materials that are used. Why are these materials chosen over other materials?

### Activity: Making mixtures

Make mixtures of sand and water, potassium dichromate and water, iodine and ethanol, iodine and water. Classify these as heterogeneous or homogeneous. Try to make mixtures using other substances. Are the mixtures that you have made heterogeneous or homogeneous? Give reasons for your choice.

### Mixtures

1. Which of the following substances are mixtures?
1. tap water
2. brass (an alloy of copper and zinc)
3. concrete
4. aluminium
5. Coca cola
6. distilled water
2. In each of the examples above, say whether the mixture is homogeneous or heterogeneous.

## Pure Substances: Elements and Compounds

Any material that is not a mixture, is called a pure substance. Pure substances include elements and compounds. It is much more difficult to break down pure substances into their parts, and complex chemical methods are needed to do this.

One way to determine if a substance is pure is to look at its melting or boiling point. Pure substances will have a sharply defined melting or boiling point (i.e. the melting or boiling point will be a single temperature rather than a range of temperatures.) Impure substances have a temperature range over which they melt or boil. We can also use chromatography to determine if a substance is pure or not. Chromatography is the process of separating substances into their individual components. If a substance is pure then chromatography will only produce one substance at the end of the process. If a substance is impure then several substances will be seen at the end of the process.

### Activity: Smartie chromatography

You will need filter paper (or chromatography paper), some smarties in different colours, water and an eye dropper.
Place a smartie in the center of a piece of filter paper. Carefully drop a few drops of water onto the smartie. You should see rings of different colour forming around the smartie. Each colour is one of the individual colours that are used to make up the colour of the smartie.

### Note:

For the above activity you can use ordinary filter paper (that is used in coffee filters) from your local store.

### Elements

An element is a chemical substance that can't be divided or changed into other chemical substances by any ordinary chemical means. The smallest unit of an element is the atom.

Definition 4: Element
An element is a substance that cannot be broken down into other substances through chemical means.

There are 112 officially named elements and about 118 known elements. Most of these are natural, but some are man-made. The elements we know are represented in the Periodic Table of the Elements, where each element is abbreviated to a chemical symbol. Examples of elements are magnesium (MgMg), hydrogen (HH), oxygen (OO) and carbon (CC). On the Periodic Table you will notice that some of the abbreviations do not seem to match the elements they represent. The element iron, for example, has the chemical formula FeFe. This is because the elements were originally given Latin names. Iron has the abbreviation FeFe because its Latin name is 'ferrum'. In the same way, sodium's Latin name is 'natrium' (NaNa) and gold's is 'aurum' (AuAu).

#### Note: Interesting fact:

Recently it was agreed that two more elements would be added to the list of officially named elements. These are elements number 114 and 116. The proposed name for element 114 is flerovium and for element 116 it is moscovium. This brings the total number of officially named elements to 114.

### Compounds

A compound is a chemical substance that forms when two or more elements combine in a fixed ratio. Water (H2OH2O), for example, is a compound that is made up of two hydrogen atoms for every one oxygen atom. Sodium chloride (NaClNaCl) is a compound made up of one sodium atom for every chlorine atom. 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.

Definition 5: Compound
A substance made up of two or more elements that are joined together in a fixed ratio.

Figure 2 might help you to understand the difference between the terms element, mixture and compound. Iron (Fe) and sulphur (S) are two elements. When they are added together, they form a mixture of iron and sulphur. The iron and sulphur are not joined together. However, if the mixture is heated, a new compound is formed, which is called iron sulphide (FeS). In this compound, the iron and sulphur are joined to each other in a ratio of 1:1. In other words, one atom of iron is joined to one atom of sulphur in the compound iron sulphide.

Figure 2 shows the microscopic representation of mixtures and compounds. In a microscopic representation we use circles to represent different elements. To show a compound, we draw several circles joined together. Mixtures are simply shown as two or more individual elements in the same box. The circles are not joined for a mixture.

We can also use symbols to represent elements, mixtures and compounds. The symbols for the elements are all found on the periodic table. Compounds are shown as two or more element names written right next to each other. Subscripts may be used to show that there is more than one atom of a particular element. (e.g. H2OH2O or NaClNaCl). Mixtures are written as: a mixture of element (or compound) A and element (or compound) B. (e.g. a mixture of FeFe and SS).

One way to think of mixtures and compounds is to think of buildings. The building is a mixture of different building materials (e.g. glass, bricks, cement, etc.). The building materials are all compounds. You can also think of the elements as Lego blocks. Each Lego block can be added to other Lego blocks to make new structures, in the same way that elements can combine to make compounds.

#### Exercise 2: Mixtures and pure substances

For each of the following substances state whether it is a pure substance or a mixture. If it is a mixture, is it homogenous or heterogenous? If it is a pure substance is it an element or a compound?

1. Blood
2. Argon
3. Silicon dioxide (SiO2SiO2)
4. Sand and stones

##### Solution
1. Blood is a mixture since it is made up of many different compounds and substances. Blood is a homogenous mixture since you cannot see the individual components and the components are uniformly distributed.
2. Argon is a pure substance. Argon is an element since we can find it on the periodic table.
3. Silicon dioxide is a pure substance. It is a compound since it is made of the elements silicon and oxygen joined in a fixed ratio.
4. Sand and stones form a mixture. There are two distinct compounds. It is a heterogenous mixture since the particles are not uniformly distributed and we can see the individual pieces.

#### Activity: Using models to represent substances

Use coloured balls and sticks to represent elements and compounds. Some examples that you can try to build are:

• Hydrogen
• Oxygen
• Nitrogen
• Neon
• Sodium chloride (salt, NaClNaCl)
• Potassium permanganate (KMnO4KMnO4)
• Water (H2OH2O)
• Iron sulphide (FeSFeS)
Think about the way that we represent substances microscopically. Would you use just one ball to represent an element or many? Why?

#### Elements, mixtures and compounds

1. In the following table, tick whether each of the substances listed is a mixture or a pure substance. If it is a mixture, also say whether it is a homogeneous or heterogeneous mixture.
 Substance Mixture or pure Homogeneous or heterogeneous mixture fizzy colddrink steel oxygen iron filings smoke limestone (CaCO3CaCO3)
2. In each of the following cases, say whether the substance is an element, a mixture or a compound.
1. CuCu
2. iron and sulphur
3. AlAl
4. H2SO4H2SO4
5. SO3SO3

## Giving names and formulae to substances

Think about what you call your friends. Their full name is like the substances name and their nickname is like the substances formulae. Without these names your friends would have no idea which of them you are referring to. In the same way scientists like to have a consistent way of naming things and a short way of describing the thing being named. This helps scientists to communicate efficiently.

It is easy to describe elements and mixtures. We simply use the names that we find on the periodic table for elements and we use words to describe mixtures. But how are compounds named? In the example of iron sulphide that was used earlier, which element is named first, and which 'ending' is given to the compound name (in this case, the ending is -ide)?

The following are some guidelines for naming compounds:

1. The compound name will always include the names of the elements that are part of it.
• A compound of iron (FeFe) and sulphur (SS) is iron

sulphide (FeSFeS)
• A compound of potassium (KK) and bromine (BrBr) is potassium

bromide (KBrKBr)
• A compound of sodium (NaNa) and chlorine (ClCl) is sodium

chloride (NaClNaCl)
2. In a compound, the element that is on the left of the Periodic Table, is used first when naming the compound. In the example of NaClNaCl, sodium is a group 1 element on the left hand side of the table, while chlorine is in group 7 on the right of the table. Sodium therefore comes first in the compound name. The same is true for FeSFeS and KBrKBr.
3. The symbols of the elements can be used to represent compounds e.g. FeSFeS, NaClNaCl, KBrKBr and H2OH2O. These are called chemical formulae. In the first three examples, the ratio of the elements in each compound is 1:1. So, for FeSFeS, there is one atom of iron for every atom of sulphur in the compound. In the last example (H2OH2O) there are two atoms of hydrogen for every atom of oxygen in the compound.
4. A compound may contain compound ions. An ion is an atom that has lost (positive ion) or gained (negative ion) electrons. Some of the more common compound ions and their formulae are given below.
 Name of compound ion Formula Carbonate CO32-CO32- Sulphate SO42-SO42- Hydroxide OH-OH- Ammonium NH4+NH4+ Nitrate NO3-NO3- Hydrogen carbonate HCO3-HCO3- Phosphate PO43-PO43- Chlorate ClO3-ClO3- Cyanide CN-CN- Chromate CrO42-CrO42- Permanganate MnO4-MnO4-
5. When there are only two elements in the compound, the compound is often given a suffix (ending) of -ide. You would have seen this in some of the examples we have used so far. For compound ions, when a non-metal is combined with oxygen to form a negative ion (anion) which then combines with a positive ion (cation) from hydrogen or a metal, then the suffix of the name will be ...ate or ...ite. NO3-NO3- for example, is a negative ion, which may combine with a cation such as hydrogen (HNO3HNO3) or a metal like potassium (KNO3). The NO3-NO3- anion has the name nitrate. SO32-SO32- in a formula is sulphite, e.g. sodium sulphite (Na2SO3Na2SO3).
SO42-SO42- is sulphate and PO43-PO43- is phosphate.
6. Prefixes can be used to describe the ratio of the elements that are in the compound. You should know the following prefixes: 'mono' (one), 'di' (two) and 'tri' (three).
• COCO (carbon monoxide) - There is one atom of oxygen for every one atom of carbon
• NO2NO2 (nitrogen dioxide) - There are two atoms of oxygen for every one atom of nitrogen
• SO3SO3 (sulphur trioxide) - There are three atoms of oxygen for every one atom of sulphur

The above guidelines also help us to work out the formula of a compound from the name of the compound.

When working out the formula of a compound from the name we work backwards. For example, if you are given potassium chloride and were told to give its formula you would start by noting that we having potassium and chloride. Next you write down the formula for each of these ions. Potassium is K+K+ and chloride is Cl-Cl-. The final step is to note the charge on each ion to see how it combines. Since both potassium and chlorine have a charge of 1, they combine in a 1:1 ratio. The formula is KClKCl.

### Tip:

When numbers are written as 'subscripts' in compounds (i.e. they are written below and to the right of the element symbol), this tells us how many atoms of that element there are in relation to other elements in the compound. For example in nitrogen dioxide (NO2NO2) there are two oxygen atoms for every one atom of nitrogen. In sulphur trioxide (SO3SO3), there are three oxygen atoms for every one atom of sulphur in the compound. Later, when we start looking at chemical equations, you will notice that sometimes there are numbers before the compound name. For example, 2H2O2H2O means that there are two molecules of water, and that in each molecule there are two hydrogen atoms for every one oxygen atom.

We can use these rules to help us name both ionic compounds and covalent compounds (more on these compounds will be covered in a later chapter). However, covalent compounds are often given other names by scientists to simplify the name (or because the molecule was named long before its formula was discovered). For example, if we have 2 hydrogen atoms and one oxygen atom the above naming rules would tell us that the substance is dihydrogen monoxide. But this compound is better known as water! Or if we had 1 carbon atom and 4 hydrogen atoms then the name would be carbon tetrahydride, but scientists call this compound methane.

### Exercise 3: Naming compounds

What is the chemical name for

1. KMnO4KMnO4
2. NH4ClNH4Cl

#### Solution

1. Step 1. Write down the names of each element or ion in the compound: For a) we have potassium and the permanganate ion. For b) we have the ammonium ion and chlorine.
2. Step 2. Use the naming rules to write the names: For a) we list the potassium first and the permanganate ion second. So a) is potassium permanganate. For b) we list the ammonium ion first and change the ending of chlorine to -ide. So b) is ammonium chloride.

### Exercise 4

Write the chemical formulae for:

1. sodium sulphate
2. potassium chromate

#### Solution

1. Step 1. Write the symbol for each element or compound: In part a) we have Na+Na+ (sodium) and SO42-SO42- (sulphate). In part b) we have K+K+ (potassium) and CrO42-CrO42- (chromate)
2. Step 2. Look at the charge on the ions: In part a) the charge on sodium is +1+1 and the charge on sulphate is -2-2, so we must have two sodiums for every sulphate. In part b) the charge on potassium is +1+1 and the charge on chromate is -2-2, so we must have two potassiums for every chromate.
3. Step 3. Write the final answer: a) is Na2SO4Na2SO4 and b) is K2CrO4K2CrO4

### Naming compounds

1. The formula for calcium carbonate is CaCO3CaCO3.
1. Is calcium carbonate a mixture or a compound? Give a reason for your answer.
2. What is the ratio of Ca:C:OCa:C:O atoms in the formula?
2. Give the name of each of the following substances.
1. KBrKBr
2. HClHCl
3. KMnO4KMnO4
4. NO2NO2
5. NH4OHNH4OH
6. Na2SO4Na2SO4
3. Give the chemical formula for each of the following compounds.
1. potassium nitrate
2. sodium iodide
3. barium sulphate
4. nitrogen dioxide
5. sodium monosulphate
4. Refer to the diagram below, showing sodium chloride and water, and then answer the questions that follow.
1. What is the chemical formula for water?
2. What is the chemical formula for sodium chloride?
3. Label the water and sodium chloride in the diagram.
4. Give a description of the picture. Focus on whether there are elements or compounds and if it is a mixture or not.

## Metals, Metalloids and Non-metals

The elements in the Periodic Table can also be divided according to whether they are metals, metalloids or non-metals. On the right hand side of the Periodic Table you can draw a 'zigzag' line (This line starts with Boron (BB) and goes down to Polonium (PoPo). This line separates all the elements that are metals from those that are non-metals. Metals are found on the left of the line, and non-metals are those on the right. Along the line you find the metalloids. You should notice that there are more metals then non-metals. Metals, metalloids and non-metals all have their own specific properties.

### Metals

Examples of metals include copper (CuCu), zinc (ZnZn), gold (AuAu), silver (AgAg), tin (SnSn) and lead(PbPb). On the Periodic Table, the metals are on the left of the zig-zag line. There are a large number of elements that are metals. The following are some of the properties of metals:

• Thermal conductors Metals are good conductors of heat. This makes them useful in cooking utensils such as pots and pans.
• Electrical conductors Metals are good conductors of electricity. Metals can be used in electrical conducting wires.
• Shiny metallic lustre Metals have a characteristic shiny appearance and so are often used to make jewellery.
• Malleable This means that they can be bent into shape without breaking.
• Ductile Metals (such as copper) can be stretched into thin wires, which can then be used to conduct electricity, as well as for other uses.
• Melting point Metals usually have a high melting point and can therefore be used to make cooking pots and other equipment that needs to become very hot, without being damaged.

You can see how the properties of metals make them very useful in certain applications.

#### Group Work : Looking at metals

1. Collect a number of metal items from your home or school. Some examples are listed below:
• hammer
• wire
• cooking pots
• jewellery
• nails
• coins
2. In groups of 3-4, combine your collection of metal objects.
3. What is the function of each of these objects?
4. Discuss why you think metal was used to make each object. You should consider the properties of metals when you answer this question.

### Non-metals

In contrast to metals, non-metals are poor thermal conductors, good electrical insulators (meaning that they do not conduct electrical charge) and are neither malleable nor ductile. The non-metals are found on the right hand side of the Periodic Table, and include elements such as sulphur (SS), phosphorus (PP), nitrogen (NN) and oxygen (OO).

### Metalloids

Metalloids or semi-metals have mostly non-metallic properties. One of their distinguishing characteristics is that their conductivity increases as their temperature increases. This is the opposite of what happens in metals. This property is known as semi-conductance and the materials are called semi-conductors. Semi-conductors are important in digital electronics, such as computers. The metalloids include elements such as silicon (SiSi) and germanium (GeGe). Notice where these elements are positioned in the Periodic Table.

You should now be able to take any material and determine whether it is a metal, non-metal or metalloid simply by using its properties.

### Exercise 5: Metals, metalloids and non-metals 1

For each of the following substances state whether they are metals, metalloids or non-metals, using their position on the periodic table.

1. Oxygen
2. Arsenic
4. Potassium

#### Solution

1. Oxygen is on the right of the zigzag line and so is a non-metal.
2. Arsenic is on the zigzag line and is a metalloid.
3. Vandaium is on the left of zigzag line and so is a metal.
4. Potassium is on the left of the zigzag line and so is a metal.

### Exercise 6: Metals, metalloids and non-metals 2

For each of the following substances state whether they are metals, metalloids or non-metals, using the information given.

1. Aluminium in a cooking pot
2. Silicon in a computer chip
3. Plastic insulation around a wire
4. Silver jewellery

#### Solution

1. A cooking pot needs to be able to conduct heat and so the aluminium used must be a metal.
2. Computer chips rely on semi-conductors and all metalloids are semiconductors. So silicon is a metalloid.
3. The plastic around the wire must be insulating to current and so is a non-metal.
4. Silver in the jewellery is chosen for its malleability and shiny lustre. So silver is a metal.

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