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Unit 5: Atoms, purity and climate

Purity — HIA International - Grade 7 Revision Notes & Topical Questions

5 min read 10 parts Lesson 3 of 6 2 quizzes

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Lesson summary

5.2 Purity — HIA INTERNATIONAL notes: Explain what purity means, calculate percentage purity and describe why pure products are hard to obtain.Pure = a substance that contains only one substance, with nothing else mixed in.Everyday meaning — pure orange juice means natural, but s…

  • Pure element
  • Silver marked 925
  • Lower purity
  • Pure diamond
  • Yellow diamond
  • Blue diamond

Part 1 of 8Pure substances

Pure
Pure a substance that contains only one substance, with nothing else mixed in.
Everyday meaning
  • Everyday meaning pure orange juice means natural, but scientifically it is a mixture.
Pure element
  • Pure element every atom is the same, for example pure gold is made only of gold atoms.
Alloy
  • Alloy a metal mixed with other metals, such as gold with copper or silver.

Part 2 of 8Purity of gold

Carat
  • Carat a measure of purity, where 24 carat is pure gold.
Meaning
  • 18 carat gold has 18 parts in 24 gold and 6 parts of other metals.
Percentage purity
  • 18 ÷ 24 × 100 = 75%.
Colour
  • Colour changes from slightly coppery to yellow-gold as the carat increases.

Part 3 of 8Purity of silver

Marking
  • Marking silver is marked with parts per thousand that are silver.
Silver marked 925
  • 925 parts in 1000, which is 92.5% pure.
Lower purity
  • Lower purity silver marked 900 is of lower purity than 925.

Part 4 of 8Purity of diamonds

Pure diamond
  • Pure diamond carbon atoms arranged in a particular way with no other elements; colourless and translucent.
Yellow diamond
  • Yellow diamond a few carbon atoms per million are replaced by nitrogen.
Blue diamond
  • Blue diamond some carbon atoms replaced by boron.
Green diamond
  • Green diamond the rarest, with one atom in 1000 replaced by nitrogen, nickel or hydrogen.

Part 5 of 8Seawater and its salts

Salts
  • Salts about 350 g of every 1000 g of seawater is dissolved salts.
Sodium chloride
  • Sodium chloride about 68% of those salts.
Salt in seawaterPercentage of the salts
Sodium chloride68%
Magnesium chloride14.6%
Sodium sulfate11.4%
Calcium chloride3.1%
Other salts2.9%
1Obtaining sodium chloride
  • Obtaining sodium chloride evaporating seawater gives only 68% pure sodium chloride, so more work is needed to remove the other salts.
2Salt names
  • Salt names salts are compounds made from acids, for example chlorides from hydrochloric acid and sulfates from sulfuric acid.

Part 6 of 8Finding the mass of salts in seawater

1
Find the mass of an evaporating basin on a top pan balance.
2
Add some seawater and find the mass of the seawater.
3
Heat until the water begins to spit, then remove from the heat and let the rest evaporate.
4
When the basin is cool, find the mass of the salts.
5
Calculate the percentage of salts and estimate the sodium chloride mass.
  • Safety
    • wear safety glasses and write a risk assessment.

Part 7 of 8Pure products from reactions

1One product
  • One product magnesium + oxygen gives magnesium oxide only.
2More than one product
  • More than one product for example silver nitrate + sodium chloride gives sodium nitrate + silver chloride.
3Neutralisation
  • Neutralisation sulfuric acid + sodium hydroxide gives sodium sulfate + water.
4Barium chloride + sodium sulfate
  • Barium chloride + sodium sulfate gives barium sulfate + sodium chloride.
5Lead nitrate + potassium iodide
  • Lead nitrate + potassium iodide gives potassium nitrate + lead iodide.
6Copper carbonate + hydrochloric acid
  • Copper carbonate + hydrochloric acid gives copper chloride + water + carbon dioxide.
7Purifying
  • Purifying products must be separated from each other and from unused reactants.
8Importance
  • Importance medicines must be free from harmful impurities.

Part 8 of 8Melting points and chromatography as tests of purity

Melting point
  • Melting point a pure substance melts at one sharp temperature.
Impurities
  • Impurities lower the melting point and make it melt over a range, and raise the boiling point.
Chromatography
  • Chromatography a pure substance gives one spot and a mixture gives more than one.

Key terms

TermMeaning
PureContaining only one substance.
PurityHow much of a sample is the pure substance.
CaratA measure of the purity of gold.
AlloyA mixture of a metal with other metals.
ImpurityAn unwanted substance mixed with another.
TranslucentLetting light through.
SaltA compound made from an acid.
Percentage purityMass of pure substance ÷ total mass × 100.

Avoid these mistakes

1
Saying pure means natural — scientifically it means only one substance.
2
Saying 18 carat gold is pure — it is 75% gold.
3
Saying evaporating seawater gives pure salt — other salts are present.
4
Saying impurities raise the melting point — they lower it.

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