Elements, Compounds and Mixtures ICSE Class 8: Separation and Its Principles

What Elements, Compounds and Mixtures is about

Weightage: CISCE publishes no question paper, marking scheme or chapter weightage for Class VIII Chemistry: your school sets and marks the exam. Elements, Compounds and Mixtures sits in the Theme: Elements, Compounds and Mixtures theme of the CISCE Curriculum for Upper Primary Classes (VI-VIII).Source: CISCE Curriculum for Upper Primary Classes (VI-VIII), RDCD 2016

Key concepts

Elements, compounds and mixtures

	Element	Compound	Mixture
Made of	one kind of atom	two or more elements chemically combined	two or more substances physically mixed
Composition	fixed	fixed ratio by mass	variable, any ratio
Properties	its own	different from its elements	those of its components
Separation	cannot be split by chemical means	only by chemical methods	by physical methods
Examples	iron, oxygen, sulphur	water, sodium chloride	air, soil, salt solution

Mixtures can be homogeneous (same throughout, like salt solution or air) or heterogeneous (parts can be seen, like sand in water).

Worked example

Iron filings and sulphur powder are mixed. Then a similar mixture is heated strongly to form iron sulphide. Compare the two products.

Try it first, then show the solutionSolution

1. The unheated mixture: a magnet pulls out the iron; any ratio can be used; the properties are those of iron and sulphur.
2. Iron sulphide: a new compound; a magnet does not separate iron from it; iron and sulphur combine in a fixed ratio (7 : 4 by mass).

Answer: The first is a mixture; iron sulphide is a compound.

Separating insoluble solids from liquids

• Sedimentation and decantation: heavier insoluble particles settle at the bottom (sediment); the clear liquid is poured off gently. Principle: difference in density; the solid is heavier and insoluble.
• Filtration: the mixture is poured through filter paper in a funnel; the solid (residue) stays on the paper, the liquid (filtrate) passes through. Principle: the solid particles are too large to pass through the pores of the filter paper.
• Centrifugation: spinning very fast makes fine suspended particles collect at the bottom (cream from milk). Principle: denser particles are thrown outwards and settle faster.

Worked example

How would you separate sand from water? State the principle.

Try it first, then show the solutionSolution

1. Allow the sand to settle and decant most of the water, then filter the rest.
2. Principle: sand is insoluble and its particles are too big to pass through the filter paper.

Answer: Sedimentation and decantation, then filtration

Separating a dissolved solid from a liquid

• Evaporation: heat the solution until all the liquid has turned into vapour; the solid is left behind (salt from sea water). Principle: the liquid is volatile (turns into vapour easily); the solid is not.
• Crystallisation: heat the solution only until it is concentrated, then let it cool; pure crystals form and can be filtered off (copper sulphate crystals). Principle: the solid is less soluble in the cold solution and comes out as pure crystals, leaving impurities in solution.

Crystallisation gives purer solids than evaporating to dryness, and avoids decomposing solids that break down on strong heating.

Worked example

150 g of brine from a salt lake is evaporated to dryness and leaves 12 g of salt. What percentage of the brine is salt? How much salt would 400 g give?

Try it first, then show the solutionSolution

1. Percentage $=\frac{12}{150}\times 100=8\%$.
2. Salt from 400 g $=8\%$ of 400 $=32$ g.

Answer: 8%; 32 g

Sublimation and magnetic separation

• Sublimation: used when one solid in the mixture sublimes (changes directly from solid to vapour) and the other does not. Heat the mixture in a china dish covered by an inverted funnel plugged with cotton; the vapour deposits as solid on the cool funnel. Examples: ammonium chloride, camphor, naphthalene or iodine from salt or sand. Principle: one component sublimes, the other does not.
• Magnetic separation: a magnet attracts iron (or other magnetic substances) out of a mixture, such as iron filings from sand or sulphur. Principle: one component is magnetic, the other is not.

Worked example

How would you separate a mixture of common salt and ammonium chloride? State the principle.

Try it first, then show the solutionSolution

1. Heat the mixture in a china dish covered with an inverted funnel.
2. Ammonium chloride sublimes and collects on the cool funnel; salt remains in the dish.
3. Principle: ammonium chloride sublimes; salt does not.

Answer: Sublimation

Separating liquids

• Separating funnel: for immiscible liquids (liquids that do not mix), such as oil and water. The denser liquid forms the lower layer and is run off through the tap. Principle: the liquids do not mix and have different densities.
• Simple distillation: for a solid dissolved in a liquid when we want to collect the liquid (pure water from salt water). The liquid boils, its vapour is cooled in a condenser and collected as the distillate. Principle: the liquid is volatile and boils off; the solid does not.
• Fractional distillation: for miscible liquids with different boiling points (alcohol and water; the gases of air after liquefying). A fractionating column helps separate them. Principle: difference in boiling points; the liquid with the lower boiling point distils over first.

Worked example

How would you separate (i) kerosene and water (ii) alcohol and water?

Try it first, then show the solutionSolution

1. (i) They do not mix: use a separating funnel; water (denser) is run off from the bottom.
2. (ii) They mix but have different boiling points: fractional distillation; alcohol (lower boiling point) distils first.

Answer: (i) Separating funnel (ii) fractional distillation

Chromatography and multi-step separation

Paper chromatography separates coloured components of a mixture, such as the dyes in ink. A spot of ink is placed on filter paper, which is dipped in a solvent (water or alcohol). As the solvent rises, each dye moves a different distance. Principle: the components have different solubilities in the solvent and are held differently by the paper.

Many mixtures need several steps. Plan them in an order that keeps each step simple: remove the magnetic part first, then dissolve the soluble part, filter off the insoluble part, then evaporate or crystallise.

Worked example

Suggest how to separate a mixture of iron filings, sand and common salt.

Try it first, then show the solutionSolution

1. Use a magnet to remove the iron filings.
2. Add water to dissolve the salt; filter off the sand (residue).
3. Evaporate the filtrate to obtain the salt.

Answer: Magnet, then dissolving and filtration, then evaporation

Chemistry in real life: Elements, Compounds and Mixtures

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Topics in this chapter

• Elements, Compounds and Mixtures Compared
• Separation of Mixtures: Methods and Principles

Elements, Compounds and Mixtures formula sheet

Water (fixed ratio)

$\text{hydrogen}:\text{oxygen}=1:8\text{ by mass}$

Compounds have a fixed composition.

Iron sulphide

$Fe+S\to FeS$

Iron and sulphur combine in the ratio 7 : 4 by mass.

Percentage of a component

$\%=\frac{\text{mass of component}}{\text{mass of mixture}}\times 100$

Varies from sample to sample in a mixture.

Elements, Compounds and Mixtures: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Elements, Compounds and Mixtures

The question patterns to expect, and the method that works for each.

Tricks to remember Elements, Compounds and Mixtures

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Find the difference

  Every separation uses one difference: size, solubility, boiling point, sublimation, magnetism or density.

  Use it when: Choosing a method.

• Fixed = compound

  Fixed ratio and new properties mean a compound; any ratio and old properties mean a mixture.

  Use it when: Compound versus mixture.

• Want the liquid? Distil. Want the solid? Evaporate.

  Distillation collects the liquid; evaporation keeps the solid.

  Use it when: Salt-water questions.

• Mag–dis–fil–eva

  Magnet, dissolve, filter, evaporate: the usual order for iron, sand and salt.

  Use it when: Multi-step separations.

Elements, Compounds and Mixtures: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section B style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 8-mark question split into 2 + 3 + 3 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Elements, Compounds and Mixtures

The slips examiners see most often, and what to write instead.

• ✗Mistake: Naming the method without the principle

  The syllabus emphasises the principle of separation.

  ✓Do this: Always add 'Principle: ...' with the property that differs.

  Usually costs: 1 mark per method.

• ✗Mistake: Calling a solution a compound

  Salt solution has a variable composition and can be separated by evaporation.

  ✓Do this: Solutions are mixtures.

  Usually costs: 1 mark.

• ✗Mistake: Using filtration for a dissolved solid

  Dissolved salt passes through filter paper.

  ✓Do this: Use evaporation or crystallisation for dissolved solids.

  Usually costs: The step.

• ✗Mistake: Wrong order of steps

  Dissolving first would mix the iron into the wet sludge; heating first could make substances react.

  ✓Do this: Plan: magnet, sublimation, dissolve, filter, evaporate.

  Usually costs: 1 to 2 marks.

• ✗Mistake: Mixing up residue and filtrate

  Residue stays on the paper; filtrate passes through.

  ✓Do this: Residue = left behind.

  Usually costs: 1 mark.

Practice questions on Elements, Compounds and Mixtures

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   The method used to separate cream from milk is:
   (a) filtration
   (b) centrifugation
   (c) sublimation
   (d) distillation

   Hint

   Spinning.

   Answer

   (b)

2. Q2 · 1 mark · Very short answerExam level

   Name the method to separate iron filings from sand.

   Hint

   Magnetism.

   Answer

   Magnetic separation.

3. Q3 · 2 marks · Short answerExam level

   Why is sea water called a mixture?

   Hint

   Composition.

   Answer

   It contains water, salt and other substances physically mixed in amounts that vary from place to place; they can be separated by evaporation.

4. Q4 · 2 marks · Short answerHarder exam level

   How would you separate a mixture of kerosene and water? State the principle.

   Hint

   They do not mix.

   Answer

   Separating funnel: the liquids are immiscible; water is denser and is run off from the bottom.

5. Q5 · 1 mark · Assertion-reasonHarder exam level

   Assertion (A): The components of a mixture can be separated by physical methods.
   Reason (R): In a mixture the components keep their own properties and are not chemically combined.
   (a) (A) is true and (R) is false.
   (b) (A) is false and (R) is true.
   (c) Both (A) and (R) are true, and (R) is the correct explanation for (A).
   (d) Both (A) and (R) are true, but (R) is not the correct explanation for (A).

   Hint

   Physically mixed.

   Answer

   (c)

6. Q6 · 2 marks · Short answerHarder exam level

   Name the methods to obtain (i) pure water from muddy water (ii) salt from salt solution.

   Hint

   Two steps for (i).

   Answer

   (i) Filtration (then distillation if fully pure water is needed). (ii) Evaporation or crystallisation.

7. Q7 · 3 marks · Short answerHarder exam level

   How are the dyes in black ink separated? State the principle.

   Hint

   Filter paper strip.

   Answer

   Paper chromatography: a spot of ink on filter paper is dipped in water; the dyes travel different distances because they dissolve and are held by the paper to different extents.

8. Q8 · 2 marks · Short answerHarder exam level

   Give the principle of fractional distillation and one example.

   Hint

   Boiling points.

   Answer

   Miscible liquids with different boiling points distil over at different temperatures; e.g. alcohol and water.

9. Q9 · 3 marks · Short answerHarder exam level

   How would you separate common salt from naphthalene? Explain.

   Hint

   One sublimes.

   Answer

   Sublimation: heat the mixture in a dish covered with an inverted funnel; naphthalene sublimes and deposits on the funnel; salt stays in the dish. (Or dissolve in water and filter, since naphthalene is insoluble.)

10. Q10 · 3 marks · Short answerHarder exam level

    Give three differences between an element and a compound.

    Hint

    Atoms, breakdown, examples.

    Answer

    An element has one kind of atom, a compound two or more kinds chemically combined; an element cannot be split into simpler substances, a compound can by chemical methods; e.g. oxygen (element), water (compound).

11. Q11 · 3 marks · Short answerHOTS / competency

    Plan a separation of a mixture of sulphur powder, salt and iron filings. (Sulphur is insoluble in water.)

    Hint

    Magnet first.

    Answer

    Remove iron with a magnet; add water to dissolve the salt; filter off sulphur (residue); evaporate the filtrate to get salt.

12. Q12 · 3 marks · Short answerHOTS / competency

    22 g of iron sulphide contains 14 g of iron. Another sample of iron sulphide weighs 44 g. How much iron does it contain, and what property of compounds does this show?

    Hint

    Fixed ratio.

    Answer

    Twice the mass, so 28 g of iron. A compound always has a fixed composition by mass.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.What is an element?Show answer

   A pure substance made of only one kind of atom.

2. 2.Is air a compound or a mixture?Show answer

   A mixture.

3. 3.Principle of filtration?Show answer

   Insoluble solid particles are too large to pass through filter paper.

4. 4.Method to separate camphor from sand?Show answer

   Sublimation.

5. 5.Method to separate two miscible liquids?Show answer

   Fractional distillation.

6. 6.What is the liquid collected in distillation called?Show answer

   Distillate.

7. 7.Method to separate the dyes in ink?Show answer

   Paper chromatography.

8. 8.Ratio of hydrogen to oxygen by mass in water?Show answer

   1 : 8.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you classify common substances correctly?
• ☐Can you give three differences between a compound and a mixture?
• ☐Do you write the principle for each method?
• ☐Can you plan a multi-step separation in the right order?
• ☐Can you draw and label filtration and distillation?
• ☐Do you know residue, filtrate and distillate?

Elements, Compounds and Mixtures: frequently asked questions