3.2Unit 5: Atoms, purity and climate
Purity — Cambridge Checkpoint Earth & Space 0893 Revision Notes & Topical Questions
5 min read 10 parts Lesson 2 of 5 1 quiz
Lesson summary
3.2 Purity — CHECKPOINT EARTH AND SPACE 0893 notes: Explain purity, calculate percentage purity and describe why pure products are difficult to obtain.Pure element = every atom is exactly the same and of just one typePure gold — it is made only of gold atomsAlloy — an alloy of go…
- Mixed substances
- Comparison
- Pure diamond
- Composition
- Salt content
- Sodium chloride
Section 1 of 10Pure substances
Pure element
Pure element every atom is exactly the same and of just one type
Pure gold
- Pure gold it is made only of gold atoms
Alloy
- Alloy an alloy of gold may contain other metals such as copper or silver
Mixed substances
- Mixed substances seawater, soil, orange juice and black ink are mixtures and not pure
Section 2 of 10Purity of gold in carats
| Gold | Parts gold out of 24 | Purity |
|---|---|---|
| 24 carat | 24 | 100% |
| 18 carat | 18 | 75% |
| 14 carat | 14 | 58.3% |
| 9 carat | 9 | 37.5% |
Section 3 of 10Calculating percentage purity
Formula
- Formula percentage purity = × 100
- (mass or parts of the pure substance ÷ total mass or parts)
Gold
- 18 carat gold has 18 parts out of 24 gold, so 18 ÷ 24 × 100 = 75%
Silver
- Silver silver marked 925 has 925 parts silver out of 1000, which is 92.5% pure
Comparison
- Comparison silver marked 900 is of lower purity than silver marked 925
Section 4 of 10Pure and impure diamonds
Pure diamond
- Pure diamond it is made only of carbon atoms in a particular arrangement and is colourless and transparent
Yellow
- Yellow a few carbon atoms per million replaced by nitrogen
Blue
- Blue some carbon atoms replaced by boron
Green
- Green about one atom per 1000 replaced by nitrogen, nickel or hydrogen, the rarest colour
Section 5 of 10Seawater and salts
Composition
- Composition seawater is water with various dissolved salts such as sodium chloride
Salt content
- Salt content about 350 g of each 1000 g of seawater is salts
Sodium chloride
- Sodium chloride about 68% of the salts is sodium chloride, about 238 g per 1000 g of seawater
Other salts
- Other salts magnesium chloride 14.6%, sodium sulfate 11.4%, calcium chloride 3.19% and other salts 2.9%
Problem
- Problem evaporating seawater gives a mixture of salts, not pure sodium chloride
Section 6 of 10Finding the mass of salts in seawater
1
Weigh an evaporating basin on a top pan balance.
2
Add seawater and find the mass of the seawater.
3
Heat the seawater until it begins to spit, then remove it from the heat and let the rest of the water evaporate.
4
When the basin is cool and dry, find the mass of the salts.
5
Calculate the mass of salts and their percentage of the seawater.
6
Estimate sodium chloride as about 68% of the salts.
Section 7 of 10Pure products
Importance
- Importance a pure product matters, for example medicines must be free of impurities that harm the patient
One product
- One product in a simple reaction such as magnesium + oxygen → magnesium oxide there is only one product
More than one product
- More than one product the products are mixed and must be separated and purified
Left-over reactants
- Left-over reactants products may also be mixed with reactants not used up
Section 8 of 10Reactions with more than one product
| Reactants | Products |
|---|---|
| Silver nitrate + sodium chloride | Sodium nitrate + silver chloride |
| Sulfuric acid + sodium hydroxide | Sodium sulfate + water |
| Barium chloride + sodium sulfate | Barium sulfate + sodium chloride |
| Lead nitrate + potassium iodide | Potassium nitrate + lead iodide |
| Copper carbonate + hydrochloric acid | Copper chloride + water + carbon dioxide |
Section 9 of 10Key terms
| Term | Meaning |
|---|---|
| Pure | Containing only one type of atom or substance. |
| Alloy | Mixture of a metal with other metals. |
| Percentage purity | Mass of pure substance as a percentage of the total. |
| Carat | Measure of gold purity in parts out of 24. |
| Impurity | Unwanted substance mixed with a product. |
| Reactant | Substance that takes part in a reaction. |
| Product | Substance made in a reaction. |
Section 10 of 10Exam technique
1
Thinking a mixture is pure because it looks the same — pure means one type of atom or substance only.
2
Thinking 9 carat gold is mostly gold — it is 37.5% gold.
3
Using the wrong total in the purity calculation — divide the pure part by the whole.
4
Thinking evaporating seawater gives pure sodium chloride — other salts are also left.
Marked practiceQuizzes for this lesson
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