Elements, compounds and mixtures

1. Overview

Everything in the universe is made of matter, and all matter is composed of atoms. This topic explains how we classify substances based on whether they contain only one type of atom (elements), different atoms bonded together (compounds), or different substances simply tangled together (mixtures). Understanding these differences is fundamental to predicting how substances will behave and how they can be separated.

---

Key Definitions

• Atom: The smallest particle of an element that can take part in a chemical reaction.
• Element: A pure substance made of only one type of atom. It cannot be broken down into simpler substances by chemical means.
• Compound: A substance made of two or more different elements chemically joined (bonded) together in a fixed ratio.
• Mixture: A combination of two or more substances (elements or compounds) that are physically mixed but not chemically joined.
• Molecule: Two or more atoms chemically bonded together (can be the same element, e.g., $O_2$, or different elements, e.g., $H_2O$).

---

Core Content

Characteristics of Elements

• Elements are the basic building blocks of all matter.
• They are found on the Periodic Table (e.g., Carbon (C), Oxygen (O), Iron (Fe)).
• 📊A box filled with identical single circles representing atoms of a monatomic element like Neon, and a box with pairs of identical joined circles representing a diatomic element like Oxygen ($O_2$)

Characteristics of Compounds

• Fixed Composition: Elements in a compound are always present in a fixed ratio (e.g., $H_2O$ always has two Hydrogen atoms for every one Oxygen atom).
• Chemical Bonding: Elements are joined by strong chemical bonds; energy (usually heat) is often required to form or break them.
• New Properties: A compound has completely different physical and chemical properties from the elements it is made of.
  • Example: Sodium ($Na$) is a reactive metal and Chlorine ($Cl_2$) is a poisonous gas. Together they form Sodium Chloride ($NaCl$), which is harmless table salt.
• Separation: Can only be separated into elements by chemical reactions or electrolysis.

Characteristics of Mixtures

• Variable Composition: Substances can be present in any ratio (e.g., a cup of tea can have one or three spoons of sugar).
• No Chemical Bonding: Substances are not joined together.
• Retained Properties: Each substance in a mixture keeps its own original properties.
• Separation: Can be easily separated by physical methods (e.g., filtration, distillation, chromatography, or using a magnet).
• 📊A box showing a variety of different shapes and colors—some single atoms, some bonded molecules—not organized in any specific pattern.

Summary Comparison Table

Feature	Element	Compound	Mixture
Composition	Only one type of atom	Two or more types of atoms chemically bonded	Two or more substances physically combined
Properties	Unique to that element	Different from its component elements	Same as its components
Separation	Cannot be simplified	Chemical reactions or electrolysis	Physical methods (e.g. filtration)
Melting/Boiling Point	Fixed	Fixed	Over a range of temperatures

---

Extended Content (Extended Only)

There are no specific "Supplement" learning objectives listed for sub-topic 2.1 in the current IGCSE Chemistry syllabus. All content in this section is Core.

---

Key Equations

While this topic is conceptual, equations are used to demonstrate the formation of compounds from elements.

1. Formation of Iron(II) Sulfide

• Word Equation: Iron + Sulfur $\rightarrow$ Iron(II) sulfide
• Symbol Equation: $Fe(s) + S(s) \rightarrow FeS(s)$
• Note: The mixture of Fe and S can be separated by a magnet. Once heated and reacted into $FeS$, it is no longer magnetic.

2. Formation of Water

• Word Equation: Hydrogen + Oxygen $\rightarrow$ Water
• Symbol Equation: $2H_2(g) + O_2(g) \rightarrow 2H_2O(l)$
• Note: Hydrogen and Oxygen are explosive gases; water is a stable liquid used to put out fires.

---

Common Mistakes to Avoid

• ❌ Wrong: Thinking that $O_2$ or $N_2$ are compounds because they have two atoms.
• ✓ Right: $O_2$ is a molecule, but it is still an element because the atoms are the same type.
• ❌ Wrong: Saying that mixtures have a fixed melting point.
• ✓ Right: Only pure elements and compounds have sharp, fixed melting points. Mixtures melt over a range of temperatures.
• ❌ Wrong: Using the word "mixture" to describe a compound like $CO_2$.
• ✓ Right: $CO_2$ is a compound because the Carbon and Oxygen are chemically bonded.

---

Exam Tips

• Command Words: If a question asks you to "Distinguish between a compound and a mixture," always provide a comparison for both (e.g., "A compound has a fixed composition, whereas a mixture has a variable composition").
• Identifying Diagrams: In the exam, you will often see boxes with circles.
  • Identical circles = Element.
  • Two different circles joined together = Compound.
  • Different circles/groups not joined together = Mixture.
• Real-world Context: Air is a common exam example. Remember: Air is a mixture of gases (Nitrogen, Oxygen, Argon, Carbon Dioxide, etc.) because they are not bonded to each other and can be separated by physical means (fractional distillation).
• Fixed Points: If a graph shows a substance melting at exactly $0^\circ C$, it is a pure substance (element or compound). If it melts from $-2^\circ C$ to $0^\circ C$, it is a mixture.

---

Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0620 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

A student investigates the properties of three different substances: sodium chloride, graphite, and methane.

(a) Sodium chloride is an example of a compound. State the meaning of the term compound. [1]

(b) Graphite is an allotrope of carbon and is used as a lubricant. State two physical properties of graphite that make it suitable for this use. [2]

(c) Methane is a simple molecular compound. State two physical properties that are typical of simple molecular compounds. [2]

Worked Solution:

(a)

1. A compound is a substance made from two or more elements chemically combined. Defining a compound.

How to earn full marks:

• Mentioning "two or more elements" worth 1 mark
• Stating "chemically combined" worth 1 mark

(b)

1. Graphite is soft. Explaining the first property.
2. Graphite is slippery. Explaining the second property.

How to earn full marks:

• Stating "soft" worth 1 mark
• Stating "slippery" worth 1 mark

(c)

1. Simple molecular compounds have low melting points. Explaining the first property.
2. Simple molecular compounds are poor conductors of electricity. Explaining the second property.

How to earn full marks:

• Stating "low melting point" worth 1 mark
• Stating "poor conductor of electricity" worth 1 mark

Common Pitfall: Make sure you understand the difference between elements, compounds, and mixtures. Remember that compounds are formed through chemical reactions, while mixtures are physically combined.

Exam-Style Question 2 — Short Answer [6 marks]

Question:

A mixture of sand and iron filings is contaminated with sodium chloride. Describe how you would obtain a pure sample of sodium chloride from this mixture. Your answer should include the processes used and the properties of the substances that allow them to be separated.

Worked Solution:

1. Use a magnet to remove the iron filings. Iron is magnetic, sand and sodium chloride are not. Separating iron filings using magnetism.
2. Add water to the remaining mixture and stir to dissolve the sodium chloride. Sodium chloride is soluble in water, sand is not. Dissolving sodium chloride.
3. Filter the mixture to remove the sand. Sand is insoluble and larger than the pores of the filter paper. Removing the sand by filtration.
4. Evaporate the water to obtain solid sodium chloride. Sodium chloride has a higher boiling point than water. Evaporating the water to leave solid sodium chloride.

How to earn full marks:

• Stating "use a magnet to remove iron filings" worth 1 mark
• Stating "add water to dissolve sodium chloride" worth 1 mark
• Stating "filter the mixture to remove sand" worth 1 mark
• Stating "evaporate the water" worth 1 mark
• Mentioning "magnetism" worth 1 mark
• Mentioning "solubility" worth 1 mark

Common Pitfall: When describing separation techniques, always link the method to the specific property that allows the separation to occur. For example, don't just say "filter the mixture"; explain why filtration works in this case (the sand is insoluble).

Exam-Style Question 3 — Extended Response [8 marks]

Question:

Crude oil is a mixture of hydrocarbons. Fractional distillation is used to separate crude oil into different fractions.

(a) State the meaning of the term hydrocarbon. [1]

(b) Describe the process of fractional distillation of crude oil, explaining how the different fractions are separated. [5]

(c) Give two uses of the fraction called 'bitumen'. [2]

Worked Solution:

(a)

1. A hydrocarbon is a compound containing carbon and hydrogen atoms only. Defining a hydrocarbon.

How to earn full marks:

• Stating "carbon and hydrogen" worth 1 mark
• Stating "only" worth 1 mark

(b)

1. The crude oil is heated to a high temperature, causing most of it to vaporize. Heating the crude oil.
2. The hot vapour is passed into the bottom of a fractionating column which is hotter at the bottom and cooler at the top. Introducing the vapour into the column.
3. As the vapour rises up the column, it cools. The cooling process.
4. Hydrocarbons with high boiling points condense at the bottom of the column, where it is hotter. Condensation of high boiling point fractions.
5. Hydrocarbons with lower boiling points condense higher up the column, where it is cooler. Condensation of low boiling point fractions.
6. Each fraction is collected separately. Collection of fractions.

How to earn full marks:

• Stating "crude oil is heated" worth 1 mark
• Stating "vapour rises and cools" worth 1 mark
• Stating "higher boiling points condense at the bottom" worth 1 mark
• Stating "lower boiling points condense at the top" worth 1 mark
• Stating "each fraction is collected separately" worth 1 mark

(c)

1. Bitumen is used for road surfacing. First use of bitumen.
2. Bitumen is used for roofing. Second use of bitumen.

How to earn full marks:

• Stating "road surfacing" worth 1 mark
• Stating "roofing" worth 1 mark

Common Pitfall: When describing fractional distillation, remember to explain why the separation occurs. It's not enough to say "the vapour cools"; you need to link the cooling to the different boiling points of the hydrocarbons.

Exam-Style Question 4 — Extended Response [9 marks]

Question:

A student investigates the composition of a fertiliser. The fertiliser is a mixture of ammonium sulfate, potassium nitrate and an inert filler.

(a) State the chemical formula of: (i) ammonium sulfate [1] (ii) potassium nitrate [1]

(b) Describe a laboratory procedure to show that the fertiliser contains the nitrate ion ($NO_3^−$). [4]

(c) The student dissolves 5.00 g of the fertiliser in 100 cm$^3$ of water. They then add excess barium chloride solution to precipitate all the sulfate ions ($SO_4^{2−}$) as barium sulfate ($BaSO_4$). After filtering, washing, and drying the precipitate, the student obtains 2.33 g of barium sulfate.

(i) Write the ionic equation, with state symbols, for the precipitation reaction. [2] (ii) Calculate the mass of ammonium sulfate in the 5.00 g sample of fertiliser. [Mr of ammonium sulfate = 132, Mr of barium sulfate = 233] [1]

Worked Solution:

(a) (i)

1. $(NH_4)_2SO_4$ Formula of ammonium sulfate.

How to earn full marks:

• Correct formula worth 1 mark

(ii)

1. $KNO_3$ Formula of potassium nitrate.

How to earn full marks:

• Correct formula worth 1 mark

(b)

1. Add aqueous sodium hydroxide to the fertiliser solution. Preparing the solution for the test. Adding sodium hydroxide.
2. Add aluminium foil to the mixture. Adding the reducing agent. Adding aluminium foil.
3. Warm the mixture. Speeding up the reaction. Warming the mixture.
4. Test the gas produced with damp red litmus paper. If nitrate ions are present, the litmus paper will turn blue. Testing for ammonia. Testing the gas.

How to earn full marks:

• Stating "add sodium hydroxide" worth 1 mark
• Stating "add aluminium foil" worth 1 mark
• Stating "warm the mixture" worth 1 mark
• Stating "test gas with damp red litmus paper, turns blue" worth 1 mark

(c) (i)

1. $Ba^{2+}(aq) + SO_4^{2−}(aq) \rightarrow BaSO_4(s)$ Ionic equation for the precipitation.

How to earn full marks:

• Correct formulae of all species worth 1 mark
• Correct state symbols and balancing worth 1 mark

(ii)

1. Moles of $BaSO_4 = \frac{2.33 \text{ g}}{233 \text{ g/mol}} = 0.01 \text{ mol}$ Calculating moles of barium sulfate.

2. Moles of $(NH_4)_2SO_4$ = Moles of $BaSO_4 = 0.01 \text{ mol}$ Moles of ammonium sulfate equal moles of barium sulfate.

3. Mass of $(NH_4)_2SO_4 = 0.01 \text{ mol} \times 132 \text{ g/mol} = 1.32 \text{ g}$ Calculating the mass of ammonium sulfate.

How to earn full marks:

• Calculating moles of $BaSO_4$ correctly worth 1 mark
• Final answer is $\boxed{1.32 \text{ g}}$

Common Pitfall: Remember to include state symbols in ionic equations. Also, pay close attention to the units in calculations and make sure they are consistent throughout. Forgetting units can lead to incorrect answers.