Atoms, compounds, mixtures and equations explained
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Everything around you is made of atoms, but atoms are rarely alone. They join up and mix in different ways. This lesson is about telling those ways apart.
It matters because chemists use these ideas to clean water, make fuels and test products. Separating a mixture is a skill you will use in many experiments. Equations are the language chemists use to describe what happens.
By the end you will be able to name elements, compounds and mixtures. You will choose a method to separate a mixture. You will also write and balance a simple equation.
Atoms, elements and compounds
An atom is the smallest part of an element that can exist. An element contains only one kind of atom. Each element has a symbol, such as O for oxygen, Na for sodium and Fe for iron.
A compound has two or more different elements that are chemically joined in a fixed ratio. Water is H₂O. It has two hydrogen atoms for every one oxygen atom.
A compound has different properties from the elements in it. Sodium is a soft metal that reacts violently with water. Chlorine is a poisonous gas. Joined together they make sodium chloride, which is table salt. The salt is nothing like either element, and you cannot get them back out by filtering or evaporating.
Mixtures
A mixture has two or more substances that are not chemically joined. Each substance keeps its own properties. Air is a mixture of nitrogen, oxygen and other gases, as the bars show.
Sea water is a mixture of water and dissolved salts. The amounts in a mixture can change. The ratio of elements in a compound cannot.
Think of a fruit salad. You can pick out the grapes and the apples again. A mixture can be separated by physical methods. Separating a compound needs a chemical reaction.
Bars showing the approximate percentage of gases in dry air: nitrogen 78, oxygen 21, argon 0.9 and other gases 0.1.
Filtration and crystallisation
Filtration separates an insoluble solid from a liquid. Insoluble means it does not dissolve. The liquid passes through the filter paper. The solid stays behind and is called the residue. Sand can be filtered out of water.
Crystallisation gets a dissolved solid back from a solution. Heat the solution gently to evaporate some of the water. Leave it to cool so crystals form. Then filter the crystals and dry them.
Do not boil off all the water, because that leaves a powder. Think of a tea strainer. The leaves stay behind, but dissolved sugar passes straight through.
Distillation
Simple distillation separates a liquid from a solution. Heat the solution so the solvent boils. The vapour goes into a condenser, where it cools and turns back into a liquid. The dissolved solid stays behind. This gives pure water from salt water.
Fractional distillation separates liquids that mix together and have different boiling points. Ethanol boils at 78 °C and water at 100 °C. The ethanol boils first and rises up the column, so it leaves first.
Think of a bathroom mirror. Steam touches the cold glass and turns back into drops of water. A condenser does the same job.
Bars showing boiling points in degrees Celsius: ethanol 78 and water 100.
Word and symbol equations
A chemical equation shows what happens in a reaction. Reactants are the substances you start with. Products are the substances made. A word equation puts reactants on the left and products on the right: magnesium + oxygen → magnesium oxide.
A symbol equation uses formulae instead: 2Mg + O₂ → 2MgO. State symbols show the physical state. (s) is solid, (l) is liquid, (g) is gas and (aq) is dissolved in water.
Zinc and hydrochloric acid are written as Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g). Read it like a recipe: this plus this makes that plus that.
Balancing equations
Atoms are not made or destroyed in a reaction. So a balanced equation has the same number of each kind of atom on both sides. Balance by changing the big numbers in front of formulae. Never change the small numbers inside a formula.
Count the atoms first. In 2Ca(OH)₂ the 2 at the front multiplies everything. The 2 after the bracket multiplies O and H. That gives 2 Ca, 4 O and 4 H.
H₂ + O₂ → H₂O has 2 O on the left but 1 on the right. Put a 2 in front of H₂O and a 2 in front of H₂. Now 2H₂ + O₂ → 2H₂O is balanced, like a level seesaw.
Recap
An element has one kind of atom. A compound has different atoms chemically joined in a fixed ratio. A mixture is not chemically joined.
Filtration separates an insoluble solid. Crystallisation gets a dissolved solid. Simple distillation gets a liquid from a solution. Fractional distillation separates liquids with different boiling points.
Reactants go on the left of an equation and products on the right.
Balance an equation by changing the big numbers only.
Key points
An element has one kind of atom. A compound has different elements chemically joined in a fixed ratio.
A mixture is not chemically joined and can be separated by physical methods.
Filtration: insoluble solid from liquid. Crystallisation: dissolved solid from solution. Distillation: liquid from solution or from other liquids.
Balance equations with big numbers only. Atoms on both sides must match.
Worked examples
Example 1
Name the method used to get sand out of a mixture of sand and water.
Sand is an insoluble solid and water is a liquid.
Pour the mixture through filter paper.
The method is filtration.
Answer: filtration
Common slips
Slip: Sand is dissolved in the water. Sand does not dissolve. That is why it can be filtered.
Slip: Heat the mixture until the water has gone. That is evaporation. It is not needed when the solid is insoluble.
Slip: The method is distillation. Distillation is used to collect a liquid from a solution.
Slip: Left: 1 Mg, 2 H, 2 Cl. Before balancing, the left side has 1 H and 1 Cl, not 2.
Slip: Change MgCl₂ to MgCl. Never change a formula. The compound is MgCl₂.
Slip: The equation is 2Mg + 2HCl → MgCl₂ + H₂. Putting 2 in front of Mg gives 2 Mg on the left and 1 on the right.
Example 3
A student has a mixture of salt, sand and water. Describe how to get dry sand and pure water from it.
Filter the mixture. The sand stays in the filter paper as the residue. Wash and dry the sand.
The filtrate is salt solution. Heat it in a simple distillation apparatus.
The water boils, is cooled in the condenser and is collected as pure water. The salt is left behind.
Answer: Filter to remove the sand, then distil the salt solution to collect pure water.
Common slips
Slip: Distil the mixture first. Sand would stay in the flask with the salt. Filter first to remove the insoluble solid.
Slip: Evaporate the filtrate to collect the water. Evaporating lets the water escape into the air. Distillation collects it.
Slip: The salt passes through the condenser. Salt does not boil off. It stays behind in the flask.
Questions and answers
Atoms, compounds, mixtures and equations practice questions for GCSE Combined Science Chemistry. Try each one, then open the answer. Easy questions come first.
Question 1 Easy
Name the method that separates an insoluble solid from a liquid.
Show answer
Answer: filtration
The liquid passes through the paper.
The solid is the residue.
Question 2 Easy
How many atoms are shown in the formula H₂O?
Show answer
Answer: 3 atoms
2 hydrogen and 1 oxygen.
2 + 1 = 3.
Question 3 Easy
What is the name for a substance made of one kind of atom only?
Show answer
Answer: an element
It cannot be broken down by chemical reactions.
Only one kind of atom is present.
Question 4 Easy
What state symbol shows that a substance is dissolved in water?
Show answer
Answer: (aq)
Dissolved in water is called aqueous.
Its symbol is (aq).
Question 5 Easy
What is the symbol for iron?
Show answer
Answer: Fe
Iron comes from the Latin word ferrum.
Its symbol is two letters.
Question 6 Easy
What name is given to the solid left behind on the filter paper?
Show answer
Answer: residue
The solid is too big to pass through.
It stays behind on the paper.
Question 7 Medium
How many atoms are shown in the formula Al₂(SO₄)₃?
Show answer
Answer: 17 atoms
Al: 2.
S: 3 × 1 = 3 and O: 3 × 4 = 12.
2 + 3 + 12 = 17.
Question 8 Medium
Balance the equation: __ Al + __ O₂ → __ Al₂O₃. Give the numbers in order, separated by commas.
Show answer
Answer: 4, 3, 2
Oxygen: 3 × 2 = 6 on the right means 3 O₂ on the left.
Aluminium: 4 Al on the left.
4, 3, 2
Exam-style questions
Longer questions in the style of a written paper, with the marking points. These are Yumio practice questions.
Exam-style question 14 marks
A student has a mixture of sand and copper sulfate solution.
(a) Name the method used to separate the sand. [1 mark]
(b) Describe how the student can get crystals of copper sulfate from the solution. [3 marks]
Show answer and marking points
Answer: (a) filtration (b) Heat the solution gently to evaporate some of the water. Leave the solution to cool so that crystals form. Filter off the crystals and dry them.
(a) filtration (1)
(b) heat or evaporate some of the water (1)
(b) leave to cool so crystals form (1)
(b) filter off the crystals and dry them (1)
Exam-style question 24 marks
(a) Balance this equation. [2 marks]
__ Fe + __ Cl₂ → __ FeCl₃
(b) Explain why a mixture of iron and sulfur is not a compound. [2 marks]
Show answer and marking points
Answer: (a) 2, 3, 2 (b) The iron and sulfur are not chemically joined. They can be separated using a magnet.
(a) 6 Cl on each side or 3 Cl₂ with 2 FeCl₃ (1)
(a) 2, 3, 2 (1)
(b) not chemically joined (1)
(b) can be separated by a magnet, or keeps its own properties (1)
Common mistakes
Changing a small number inside a formula to balance an equation
Only change the big number in front. Changing H₂O to H₂O₂ makes a different substance.
Saying air is a compound
Air is a mixture. Its gases are not chemically joined.
Using filtration to separate a dissolved solid
Dissolved solids pass through filter paper. Use crystallisation or distillation.
Writing the wrong state symbol, such as (aq) for a pure liquid
(aq) means dissolved in water. A pure liquid is (l).
Questions students ask
Is Atoms, compounds, mixtures and equations on the Foundation or Higher tier?
Atoms, compounds, mixtures and equations is on both Foundation and Higher papers for GCSE Combined Science Chemistry, aimed at around grade 3.
What is a common mistake on Atoms, compounds, mixtures and equations?
Changing a small number inside a formula to balance an equation. Only change the big number in front. Changing H₂O to H₂O₂ makes a different substance.
Where are the answers to the Atoms, compounds, mixtures and equations questions?
Open Show answer under each question on this page. The questions are Yumio practice questions, not past exam questions.