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KS3 Science · Year 7Chapter 7 · Compounds
Year 7 · Chapter 7

Compounds

By the end of this chapter you will be able to

  • Say what a compound is, and how it differs from an element
  • Read a formula and count the atoms of each element in it
  • Use the fixed ratio of elements in a compound
  • Describe how the properties of a compound differ from those of its elements
  • Explain why a compound cannot be separated by a physical method

You should already know

  • Elements and symbols, and the periodic table (Chapter 6 of this book)
  • Chemical bonds are introduced here at a simple level only

Where this leads

  • Mixtures and separating methods, which compare compounds with mixtures (Chapter 8)

National Curriculum for England, Key Stage 3 science, Year 7. Content: C5 compounds are formed from elements that are chemically bonded in fixed ratios, and cannot be separated by physical methods, C5 formulae of simple compounds, such as H2O, CO2 and NaCl; counting the atoms in a formula.

Exercise P  Do you remember?

1
FWhat is an element? Write the words.
2
FWhat is the symbol for sodium? Write the symbol.
3
FWhat is the symbol for oxygen? Write the symbol.
4
FWhat is the atomic number of sodium? Write the number.
5
FWhat is 2 times 3? Write the number.
7.1

Elements combine to form compounds

Year 7
C5: compounds are formed when two or more elements are chemically bonded together
Video lesson: Elements combine to form compoundsComing soon
Story. Ali is cleaning a metal bowl. “The shiny metal is sodium, and the gas in the bottle is chlorine,” says his teacher. “Neither of these is safe to eat or touch. But when they are joined together, the new substance is table salt.”

A compound is a substance made of two or more different elements that are chemically bonded together. The atoms are held together by bonds, in a fixed ratio.

A compound has different properties from the elements it is made of. Sodium and chlorine are both dangerous to handle, but sodium chloride is a safe white solid.

NasodiumClchlorinesodium chloride
Sodium and chlorine atoms bond together to form sodium chloride.

Key ideas

  • A compound is made of two or more elements, chemically bonded.
  • The elements in a compound are in a fixed ratio.
  • A compound has different properties from its elements.
  • An element has only one type of atom; a compound has at least two types.
Worked example 7.1.1
Is sodium chloride a compound or an element? Write the word.

Method. Count the types of atom.

  1. Count the types. Sodium chloride is made of sodium atoms and chlorine atoms, so two types.
    two types
  2. Decide. A substance with two or more types of atom, chemically bonded, is a compound.
    compound
  3. Write the answer.
    compound

Check. An element has only one type of atom. ✓

Worked example 7.1.2
Is chlorine a compound or an element? Write the word.

Method. Count the types of atom.

  1. Count the types. Chlorine is made of only one type of atom, chlorine.
    one type
  2. Decide. So it is an element.
    element
  3. Write the answer.
    element

Check. Chlorine is a gas made of chlorine atoms. ✓

Worked example 7.1.3
Name the two elements that make up sodium chloride. Write the two names.

Method. Read the description.

  1. Read the picture. The picture shows sodium atoms and chlorine atoms bonded together.
    sodium and chlorine
  2. Name them. The two elements are sodium and chlorine.
    sodium and chlorine
  3. Write the answer.
    sodium and chlorine

Check. Both are elements. ✓

Watch out

Watch out: a compound is not a mixture. In a compound the elements are chemically bonded in a fixed ratio; in a mixture they are not bonded together.

Exam tip

Exam tip. When you say a substance is a compound, name at least two elements that are chemically bonded.

Try it (with your teacher, or a grown-up at home). Look at a labelled diagram of sodium chloride and a diagram of sodium metal on its own. Describe one way the two are different. Do not handle or mix any substances; this is a drawing task.

Exercise 7A  Elements combine to form compounds

1
FWhat is a compound made of? Write the words.
2
FIs sodium chloride a compound? Write yes or no.
3
FIs sodium an element? Write yes or no.
4
FIs water a compound or an element? Write the word.
5
FIs oxygen a compound or an element? Write the word.
6
FName the two elements in water. Write the two names.
7
FAre the elements in a compound chemically bonded? Write yes or no.
8
PSAli says sodium chloride is an element because it contains sodium. Is Ali right? Write yes or no.
9
PSRuby says a mixture of sand and iron filings is a compound. Is Ruby right? Write yes or no.
10
PSWorking scientifically: a pupil says a compound is always a gas. Give one example from this section that shows this is not true. Write the name.
11
RExplain why sodium chloride has different properties from sodium and chlorine. Write one sentence.
12
RChallenge: explain why a compound is not the same as a mixture of its elements. Write one sentence.
7.2

Formulae and fixed ratios

Year 7
C5: the formula of a compound shows the elements it contains and the fixed ratio of their atoms
Video lesson: Formulae and fixed ratiosComing soon
Story. Priya is writing the formula for water. “Is it HO or H2O?” she asks. Her teacher says, “Water has two hydrogen atoms for every oxygen atom. The formula must show that ratio, so write H2O.”

A formula uses the symbols of the elements, with a small number, called a subscript, to show how many atoms of each element are in the formula. If there is only one atom, no number is written.

A compound always has the same ratio of elements. This is why it is described as having a fixed ratio.

OHHH2O: 2 hydrogen atoms and 1 oxygen atom
Water: H2O, two hydrogen atoms and one oxygen atom.
COOCO2: 1 carbon atom and 2 oxygen atoms
Carbon dioxide: CO2, one carbon atom and two oxygen atoms.

The table gives the formula of each compound, the number of atoms of each element in it, and the total number of atoms.

CompoundFormulaAtoms in the formulaTotal number of atoms
waterH2OH 2, O 13
carbon dioxideCO2C 1, O 23
sodium chlorideNaClNa 1, Cl 12
magnesium oxideMgOMg 1, O 12
aluminium oxideAl2O3Al 2, O 35
methaneCH4C 1, H 45
ammoniaNH3N 1, H 34

Key ideas

  • A formula shows the symbols of the elements in the compound.
  • A small number after a symbol shows how many atoms of that element are there.
  • No number written means one atom.
  • The ratio of elements in a compound is fixed.
Worked example 7.2.1
How many hydrogen atoms are in one formula of water, H2O? Write the number.

Method. Read the subscript after H.

  1. Read the formula. H2O has the symbol H with the number 2 after it.
    2
  2. Write the answer. So there are 2 hydrogen atoms.
    2 hydrogen atoms
  3. Write the answer.
    2

Check. Two hydrogen atoms and one oxygen atom make H2O. ✓

Worked example 7.2.2
How many oxygen atoms are in one formula of carbon dioxide, CO2? Write the number.

Method. Read the subscript after O.

  1. Read the formula. CO2 has the symbol O with the number 2 after it.
    2
  2. Write the answer. So there are 2 oxygen atoms.
    2 oxygen atoms
  3. Write the answer.
    2

Check. There is one carbon atom, with no number written. ✓

Worked example 7.2.3
How many atoms of each element are in methane, CH4? Write the two numbers: carbon, then hydrogen.

Method. Read each symbol and its subscript.

  1. Read the carbon. C has no number written, so there is 1 carbon atom.
    C = 1
  2. Read the hydrogen. H has the number 4, so there are 4 hydrogen atoms.
    H = 4
  3. Write the answer. So the numbers are 1 carbon and 4 hydrogen.
    1 and 4
  4. Write the answer.
    1 carbon and 4 hydrogen

Check. Methane has one carbon and four hydrogen atoms. ✓

Watch out

Watch out: the subscript applies only to the symbol just before it. In H2O, the 2 applies to H, not to O, and no number means one atom of O.

Exam tip

Exam tip. When you count atoms in a formula, read each symbol and its number, one at a time.

Try it (with your teacher, or a grown-up at home). Look at a labelled picture of water and carbon dioxide. Write down how many atoms of each element are in each formula. Check your answers by reading the formula out loud.

Exercise 7B  Formulae and fixed ratios

1
FHow many hydrogen atoms are in H2O? Write the number.
2
FHow many oxygen atoms are in H2O? Write the number.
3
FHow many oxygen atoms are in CO2? Write the number.
4
FHow many carbon atoms are in CO2? Write the number.
5
FHow many sodium atoms are in NaCl? Write the number.
6
FHow many hydrogen atoms are in NH3? Write the number.
7
FHow many hydrogen atoms are in CH4? Write the number.
8
PSWhat is the symbol for the element in water that has the number 2 in its formula? Write the symbol.
9
PSAli writes the formula of water as HO. Is this correct? Write yes or no.
10
PSPriya says the formula H2O means two oxygen atoms and one hydrogen atom. Is Priya right? Write yes or no.
11
PSWorking scientifically: a formula is written as XY2. How many atoms of Y are there in one formula? Write the number.
12
RExplain why the ratio of hydrogen to oxygen in water is always 2 to 1. Write one sentence.
13
RChallenge: a formula is X3Y. If the formula has 4 atoms in total, how many atoms of X are there? Write the number.
7.3

Counting atoms and ratios in compounds

Year 7
C5: count the atoms of each element in a simple formula, and use the fixed ratio of atoms
Video lesson: Counting atoms and ratios in compoundsComing soon
Story. Omar is asked to count the atoms in the formula Al2O3. “How many atoms are there in total?” he asks. His teacher says, “Count the atoms of each element, then add them. Be careful: the 2 belongs to aluminium, and the 3 belongs to oxygen.”

To find the total number of atoms in a formula, add the numbers of atoms of each element. For example, in CO2 there is 1 carbon atom and 2 oxygen atoms, so the total is 3.

The ratio of atoms is found from the numbers in the formula. In Al2O3, the ratio of aluminium to oxygen is 2 : 3.

NaClNaClClNaClNaNaClNaClNaCl: sodium and chlorine in a 1 : 1 ratio
Sodium and chlorine atoms alternate in a regular pattern, in a 1 to 1 ratio.

The table gives the ratio of the elements in five compounds.

CompoundRatio of the elements
waterH : O = 2 : 1
carbon dioxideC : O = 1 : 2
sodium chlorideNa : Cl = 1 : 1
magnesium oxideMg : O = 1 : 1
aluminium oxideAl : O = 2 : 3

Key ideas

  • Total atoms = the sum of the numbers of atoms of each element.
  • The ratio of atoms comes from the numbers in the formula.
  • In NaCl, the ratio of sodium to chlorine is 1 : 1.
  • In Al2O3, the ratio of aluminium to oxygen is 2 : 3.
Worked example 7.3.1
How many atoms are in one formula of aluminium oxide, Al2O3? Write the total number of atoms.

Method. Add the numbers of atoms.

  1. Count the aluminium. Al has a 2 after it, so there are 2 aluminium atoms.
    2
  2. Count the oxygen. O has a 3 after it, so there are 3 oxygen atoms.
    3
  3. Add. The total is 2 + 3.
    2 + 3 = 5
  4. Write the answer.
    5

Check. 2 aluminium and 3 oxygen make 5 atoms. ✓

Worked example 7.3.2
How many atoms are in one formula of ammonia, NH3? Write the total number of atoms.

Method. Add the numbers of atoms.

  1. Count the nitrogen. N has no number, so there is 1 nitrogen atom.
    1
  2. Count the hydrogen. H has a 3 after it, so there are 3 hydrogen atoms.
    3
  3. Add. The total is 1 + 3.
    1 + 3 = 4
  4. Write the answer.
    4

Check. 1 nitrogen and 3 hydrogen make 4 atoms. ✓

Worked example 7.3.3
What is the ratio of carbon to oxygen in CO2? Write the ratio as two numbers.

Method. Read the numbers in the formula.

  1. Read the numbers. There is 1 carbon atom and 2 oxygen atoms.
    1 and 2
  2. Write the ratio. So the ratio of carbon to oxygen is 1 : 2.
    1 : 2
  3. Write the answer.
    1 : 2

Check. The ratio is the numbers in the formula. ✓

Watch out

Watch out: the number after a symbol is the number of atoms of that symbol, and it does not change the other elements. In Al2O3 the 2 is not the number of oxygen atoms.

Exam tip

Exam tip. When you find the total, add the numbers from the formula. Do not add the symbols or the letters.

Try it (with your teacher, or a grown-up at home). Write the formulae of three compounds from the table in this section, in your notebook. For each one, count the atoms of each element, then add them to find the total. Check your totals against the table.

Exercise 7C  Counting atoms and ratios in compounds

1
FHow many atoms are in one formula of water, H2O? Write the total number.
2
FHow many atoms are in one formula of carbon dioxide, CO2? Write the total number.
3
FHow many atoms are in one formula of sodium chloride, NaCl? Write the total number.
4
FHow many atoms are in one formula of methane, CH4? Write the total number.
5
FWhat is the ratio of hydrogen to oxygen in water? Write the ratio as two numbers.
6
FWhat is the ratio of sodium to chlorine in sodium chloride? Write the ratio as two numbers.
7
FHow many atoms of aluminium are in Al2O3? Write the number.
8
FHow many atoms of oxygen are in Al2O3? Write the number.
9
PSWhat is the total number of atoms in one formula of magnesium oxide, MgO? Write the number.
10
PSWhich compound has the larger total number of atoms: CO2, or Al2O3? Write the formula.
11
PSRuby says NH3 has 3 atoms in total, because it has 3 hydrogen atoms. Is Ruby right? Write yes or no.
12
PSWorking scientifically: a pupil says the ratio of aluminium to oxygen in Al2O3 is 2 : 2. Is the pupil right? Write yes or no.
13
RExplain why the ratio of atoms in a compound is fixed. Write one sentence.
14
RChallenge: a compound has the formula X2Y5. How many atoms are there in total? Write the number.
7.4

Compounds have different properties

Year 7
C5: a compound has different properties from the elements it is made of
Video lesson: Compounds have different propertiesComing soon
Story. Sam reads a label on a box of table salt. “Sodium is a dangerous metal and chlorine is a poisonous gas,” he says. “So why is the salt safe to put on food?” His teacher says, “Because the two elements have bonded to make a compound, and the compound has new properties.”

Elements combine to make a compound, and the compound has different properties from the elements. Its appearance, its melting point and how it reacts can all change.

The table compares the elements with the compound.

SubstanceWhat it is like
sodiuma soft, reactive metal
chlorinea poisonous gas
sodium chloridea white solid, which is the table salt we put on food

Key ideas

  • A compound has different properties from the elements it is made of.
  • Sodium is a reactive metal and chlorine is a poisonous gas; sodium chloride is a safe white solid.
  • The change in properties comes from the new bonds between the atoms.
  • You cannot predict the properties of a compound from the elements alone, without testing.
Worked example 7.4.1
What is sodium chloride like: a poisonous gas, or a white solid? Write the words.

Method. Read the table.

  1. Read the table. The table says sodium chloride is a white solid.
    white solid
  2. Decide. So sodium chloride is a white solid.
    white solid
  3. Write the answer.
    a white solid

Check. Chlorine is the poisonous gas, not the compound. ✓

Worked example 7.4.2
Name one element in table salt that is a poisonous gas. Write the name.

Method. Read the table.

  1. Read the table. The table says chlorine is a poisonous gas.
    poisonous gas
  2. Name it. So the element is chlorine.
    chlorine
  3. Write the answer.
    chlorine

Check. Sodium is a soft metal. ✓

Worked example 7.4.3
Is a compound more like its elements, or different from them? Write the word.

Method. Think about the properties.

  1. Think about the properties. Sodium chloride has properties that are different from sodium and chlorine.
    different
  2. Decide. So the compound is different from its elements.
    different
  3. Write the answer.
    different

Check. The new bonds make a new substance. ✓

Watch out

Watch out: do not assume a compound has the properties of its elements. Sodium is a reactive metal and chlorine is a poisonous gas, but sodium chloride is safe to use as salt.

Exam tip

Exam tip. When you compare a compound with its elements, name one property that is different.

Try it (with your teacher, or a grown-up at home). Look at the label on a packet of table salt, or the picture of sodium chloride. Write down one property of the salt, and then say which property of the elements it does not have. Do not taste or heat any substance.

Exercise 7D  Compounds have different properties

1
FWhat is table salt chemically? Write the formula.
2
FIs sodium a soft, reactive metal? Write yes or no.
3
FIs chlorine a poisonous gas? Write yes or no.
4
FWhat state of matter is sodium chloride at room temperature? Write the word.
5
FIs the compound sodium chloride a metal? Write yes or no.
6
FName one of the elements in sodium chloride that is a soft metal. Write the name.
7
PSWhich element in table salt is a gas? Write the name.
8
PSA pupil says sodium chloride is poisonous, because chlorine is poisonous. Is the pupil right? Write yes or no.
9
PSAli says a compound always has the properties of its elements added together. Is Ali right? Write yes or no.
10
PSWorking scientifically: a pupil wants to show that a compound has different properties from its elements. Name one property to test. Accept any one: colour, state, or whether it is poisonous.
11
RExplain why a compound often has different properties from its elements. Write one sentence.
12
RChallenge: explain why table salt is safe to use on food, even though one of its elements is a poisonous gas. Write one sentence.
7.5

Compounds cannot be separated by physical methods

Year 7
C5: compounds cannot be separated into their elements by physical methods; a chemical reaction is needed
Video lesson: Compounds cannot be separated by physical methodsComing soon
Story. Ruby has a mixture of iron filings and sand. “A magnet can pull the iron out,” she says. “Can I do the same with water, to split it into hydrogen and oxygen?” Her teacher says, “No. A magnet can separate a mixture. Water is a compound, and it needs a chemical reaction to split it.”

A physical method separates substances without changing them, for example by a magnet, by a filter or by evaporation. These methods can separate a mixture, because the substances in a mixture are not bonded together.

A compound cannot be separated by a physical method, because its elements are chemically bonded. To split a compound into its elements, a chemical reaction is needed, for example electricity used in a chemical reaction.

Key ideas

  • A physical method separates a mixture without changing the substances.
  • A compound cannot be separated into its elements by a physical method.
  • A chemical reaction is needed to split a compound.
  • Separating a mixture and splitting a compound are different.
Worked example 7.5.1
Can a magnet separate water into hydrogen and oxygen? Write yes or no.

Method. Think about the method.

  1. Think about the method. A magnet is a physical method, and it cannot split a compound.
    physical method
  2. Decide. So the answer is no.
    no
  3. Write the answer.
    no

Check. Water is a compound, so a chemical reaction is needed. ✓

Worked example 7.5.2
Can a magnet separate iron filings from sand? Write yes or no.

Method. Think about the method.

  1. Think about the method. Iron filings and sand are a mixture, so the iron can be pulled out by a magnet.
    mixture
  2. Decide. So the answer is yes.
    yes
  3. Write the answer.
    yes

Check. A mixture can be separated by a physical method. ✓

Worked example 7.5.3
Which type of method splits water into hydrogen and oxygen: physical or chemical? Write the word.

Method. Recall which method splits a compound.

  1. Recall. A compound is split only by a chemical method.
    chemical
  2. Decide. So the answer is chemical.
    chemical
  3. Write the answer.
    chemical

Check. Water needs a chemical reaction to split. ✓

Watch out

Watch out: boiling a compound is not the same as splitting it. When water boils it becomes steam, which is still water, not hydrogen and oxygen.

Exam tip

Exam tip. For a separation question, say whether the substance is a mixture or a compound first. Then choose the method.

Try it (with your teacher, or a grown-up at home). Do not carry out any chemical reaction. Look at a labelled picture of a magnet pulling iron out of sand, and say why this works for a mixture but not for a compound.

Exercise 7E  Compounds cannot be separated by physical methods

1
FWhat kind of method separates substances without changing them? Write the word: physical or chemical.
2
FCan a compound be separated into its elements by a physical method? Write yes or no.
3
FCan a mixture be separated by a physical method? Write yes or no.
4
FIs water a mixture or a compound? Write the word.
5
FWhich method splits a compound into its elements? Write the word: physical or chemical.
6
FWhich method can pull iron out of a mixture of iron filings and sand? Write the word.
7
PSRuby says she can separate a sample of sodium chloride by pouring water through a filter. Is Ruby right? Write yes or no.
8
PSBen says that water boiling into steam means the water has been split into hydrogen and oxygen. Is Ben right? Write yes or no.
9
PSWorking scientifically: a pupil says a compound is a mixture of its elements, so it can be separated by filtering. Is this claim correct? Write yes or no.
10
RExplain why a magnet cannot separate the elements in iron sulphide, a compound. Write one sentence.
11
RChallenge: explain why a physical method can separate sand from water, but cannot separate hydrogen from oxygen in water. Write one sentence.

Chapter summary

SkillMethod
CompoundTwo or more elements chemically bonded in a fixed ratio.
FormulaSymbols with subscripts. No number means one atom.
Counting atomsAdd the numbers of atoms of each element to find the total.
PropertiesA compound has different properties from its elements.
SeparatingA compound cannot be split by a physical method; a chemical reaction is needed.

Can you…?

  • I can say what a compound is, and how it differs from an element
  • I can read a formula and count the atoms of each element
  • I can give the ratio of elements in a compound from its formula
  • I can compare the properties of a compound with its elements
  • I can explain why a compound cannot be separated by a physical method

Key terms

compound
a substance made of two or more elements chemically bonded in a fixed ratio
chemical bond
the link between atoms in a compound
formula
a way of writing a compound using symbols and subscripts
subscript
a small number after a symbol, showing how many atoms there are
fixed ratio
the same ratio of elements every time in a compound
physical method
a way of separating substances without changing them chemically
chemical reaction
a change that makes new substances

Exercise 7Z  Check-up

1
What is a compound made of? Write the words.
2
How many hydrogen atoms are in H2O? Write the number.
3
How many atoms are in one formula of CO2? Write the total number.
4
What is the ratio of aluminium to oxygen in Al2O3? Write the ratio as two numbers.
5
How many atoms of carbon are in CH4? Write the number.
6
Is table salt, NaCl, an element or a compound? Write the word.
7
Is sodium chloride a poisonous gas? Write yes or no.
8
Can a physical method split water into hydrogen and oxygen? Write yes or no.
9
Which element is a soft, reactive metal in sodium chloride? Write the name.
10
Explain why a compound has different properties from its elements. Write one sentence.

Total 17 marks. Allow about 17 minutes.

Exercise 7X  Practice test questions

1
PSHow many atoms are in one formula of ammonia, NH3? Write the total number.[2 marks]
2
FWhat is the ratio of hydrogen to oxygen in water? Write the ratio as two numbers.[1 mark]
3
RExplain why a magnet cannot separate the elements in a compound. Write one sentence.[2 marks]
4
FWhich element in table salt is a poisonous gas? Write the name.[1 mark]
5
FHow many oxygen atoms are in Al2O3? Write the number.[1 mark]
6
PSA pupil says CO2 is an element because it contains carbon. Is the pupil right? Write yes or no.[2 marks]
7
FWrite the formula of water, using the symbols of its elements. Write the formula.[1 mark]
8
PSCan filtering separate the elements in water? Write yes or no.[2 marks]
9
FName the type of method needed to split a compound into its elements. Write the word: physical or chemical.[1 mark]
10
PSWrite the total number of atoms in one formula of magnesium oxide, MgO. Write the number.[1 mark]
11
RExplain why sodium chloride is not the same as sodium and chlorine mixed together. Write one sentence.[3 marks]
← Chapter 6: Atoms, elements and the periodic tableChapter 8: Mixtures and separating →

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