Question 1 Medium
Which method gets pure water from salt water?
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Answer: simple distillation
- The water must be collected, so it is cooled in a condenser.
- This is simple distillation.
Atoms, compounds, mixtures and equations: GCSE Combined Science Chemistry practice questions
A second set of questions on atoms, compounds, mixtures and equations. They are different from the ones on the topic page. Try each, then open the answer and compare your working.
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Question 2 Medium
Ethanol boils at 78 °C and water boils at 100 °C. Which liquid is collected first when the mixture is separated by fractional distillation?
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Answer: ethanol
- The liquid with the lower boiling point boils first.
- Ethanol, at 78 °C, is below water at 100 °C.
Question 3 Medium
How many hydrogen atoms are shown in 3H₂O?
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Answer: 6
- The 3 multiplies the whole formula.
- Each H₂O has 2 hydrogen atoms.
- 3 × 2 = 6.
Question 4 Medium
Balance the equation: __ K + __ Cl₂ → __ KCl. Give the numbers in order, separated by commas.
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Answer: 2, 1, 2
- Cl₂ has 2 chlorine atoms, so 2KCl is needed.
- That needs 2 K on the left.
- 2, 1, 2
Question 5 Hard
Balance the equation: __ C₂H₆ + __ O₂ → __ CO₂ + __ H₂O. Give the numbers in order, separated by commas.
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Answer: 2, 7, 4, 6
- C: 2 CO₂ for each C₂H₆, so 2 on the left means 4 CO₂.
- H: 2 C₂H₆ has 12 H, so 6 H₂O.
- O on the right: 8 + 6 = 14, so 7 O₂.
- 2, 7, 4, 6
Question 6 Hard
Write a balanced symbol equation, with no state symbols, for hydrogen reacting with chlorine to make hydrogen chloride (HCl).
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Answer: H₂ + Cl₂ → 2HCl
- Hydrogen and chlorine are both diatomic: H₂ and Cl₂.
- Each side needs 2 H and 2 Cl, so 2 HCl.
- H₂ + Cl₂ → 2HCl
Question 7 Hard
Salt is dissolved in water. A student heats it gently until half the water has gone, then leaves it to cool. What forms in the dish?
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Answer: crystals
- The solution gets more concentrated.
- On cooling the salt comes out of solution as crystals.
Question 8 Hard
A mixture contains iron filings and sulfur. Name one property of iron that could be used to separate it from sulfur.
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Answer: it is magnetic
- Iron is magnetic. Sulfur is not.
- A magnet picks up the iron.
Quick checks
Short questions with options. Choose one before you open the answer.
Check 1
What is an atom?
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The smallest part of an element that can exist.
Check 2
What is an element?
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A substance made of one kind of atom only.
Check 3
What is a compound?
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Two or more different elements chemically joined in a fixed ratio.
Check 4
What is a mixture?
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Two or more substances that are not chemically joined.
Check 5
How is a mixture different from a compound?
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A mixture can be separated by physical methods and has no fixed ratio.
Check 6
What does filtration separate?
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An insoluble solid from a liquid.
Check 7
What is the solid left on filter paper called?
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The residue.
Check 8
What does crystallisation do?
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It gets a dissolved solid out of a solution as crystals.
Check 9
What does simple distillation separate?
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A liquid from a solution, such as pure water from salt water.
Check 10
What does fractional distillation separate?
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Liquids with different boiling points that mix together.
Check 11
Which liquid leaves first in fractional distillation?
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The one with the lowest boiling point.
Check 12
What does (aq) mean in an equation?
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Dissolved in water.
Check 13
What do (s), (l) and (g) mean in an equation?
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Solid, liquid and gas.
Check 14
What numbers can you change to balance an equation?
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Only the big numbers in front of the formulae.
Check 15
What must be true for a balanced equation?
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The same number of each kind of atom on both sides.
Check 16
What is the symbol for sodium?
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Na.
Check 17
What is a solvent?
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The liquid that a solid dissolves in.
Check 18
What does insoluble mean?
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It does not dissolve in a liquid.
Check 19
What is the filtrate?
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The liquid that passes through the filter paper.
Check 20
What happens in a condenser?
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The vapour cools and turns back into a liquid.
Check 21
What is a reactant in an equation?
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A substance you start with, written on the left of the arrow.
Check 22
What is a product in an equation?
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A substance that is made, written on the right of the arrow.
Check 23
What is the symbol for iron?
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Fe.
Check 24
How many atoms are in one molecule of water?
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Three: two hydrogen atoms and one oxygen atom.
Check 25
About what percentage of dry air is nitrogen?
Show answer
About 78%.
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