Alloys and their properties

1. Overview

Alloys are a critical category of materials in chemistry and engineering. While pure metals are often too soft or chemically reactive for many practical purposes, mixing them with other elements creates alloys with enhanced physical and chemical properties. This topic explores why alloys are used instead of pure metals and how their internal structure determines their strength.

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Key Definitions

• Alloy: A mixture of a metal with one or more other elements (usually other metals or carbon).
• Brass: An alloy composed of copper ($Cu$) and zinc ($Zn$).
• Stainless Steel: An alloy composed of iron ($Fe$) mixed with chromium ($Cr$), nickel ($Ni$), and carbon ($C$).
• Malleability: The ability of a material to be hammered or pressed into shape without cracking or breaking.

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Core Content

Common Alloys and Their Compositions

You must know the specific composition of these two alloys:

• Brass: Copper + Zinc
• Stainless Steel: Iron + Chromium + Nickel + Carbon

Properties and Uses

Alloys are generally harder and stronger than the pure metals from which they are made. This makes them significantly more useful in construction and manufacturing.

Example: Stainless Steel

• Properties: Extremely hard and highly resistant to rusting (corrosion).
• Uses: Used extensively in cutlery and kitchen sinks because it does not react with food or water and can withstand the physical wear of daily use.
• Reaction context: Unlike pure iron, which reacts with oxygen and water to form rust, the chromium in stainless steel forms a thin, invisible layer of chromium oxide that protects the metal.

Identifying Alloys from Diagrams

In exams, you may be asked to identify an alloy from a particle diagram.

• Pure Metal: Shows regular rows of identical-sized atoms arranged in a lattice.
• Alloy: Shows a lattice where some atoms are replaced by atoms of a different size, disrupting the regular pattern.

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Extended Content (Extended Curriculum Only)

Why Alloys are Harder than Pure Metals

To understand why alloys are stronger, we must look at the arrangement of atoms (the structure):

1. Pure Metals: In a pure metal, all atoms are the same size. They are arranged in regular, repeating layers. When a force is applied, these layers can easily slide over each other. This is why pure metals like gold or lead are soft and easily shaped.
2. Alloys: In an alloy, the added elements have different sized atoms. These different sized atoms distort the regular arrangement of the metal lattice.
3. The Result: Because the layers are no longer uniform, they can no longer slide over each other easily. This "locks" the structure in place, making the material much harder and less malleable.

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Key Equations

While alloys are mixtures and not compounds (so they don't have a single chemical formula), the process of rusting is often discussed in contrast to the resistance of stainless steel.

The Rusting of Iron (which stainless steel prevents):

• Word Equation: Iron + Oxygen + Water → Hydrated Iron(III) Oxide
• Symbol Equation: $4Fe(s) + 3O_2(g) + 2nH_2O(l) \rightarrow 2Fe_2O_3 \cdot nH_2O(s)$

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Common Mistakes to Avoid

• ❌ Wrong: Describing an alloy as a "compound."
• ✓ Right: An alloy is a mixture. The elements are not chemically bonded in fixed proportions.
• ❌ Wrong: Thinking that adding any element makes a metal softer.
• ✓ Right: Adding different sized atoms almost always makes the metal harder because it prevents layers from sliding.
• ❌ Wrong: Forgetting that carbon is often an ingredient in steel alloys.
• ✓ Right: Steel is a mixture of iron and carbon; stainless steel adds chromium and nickel to that mix.

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Exam Tips

• Command Words: If a question asks you to "Describe" the structure of an alloy, mention the different sized atoms. If it asks you to "Explain" why it is harder, you must mention that the different sized atoms prevent the layers from sliding.
• Contextual Questions: Be prepared to explain why stainless steel is used for medical instruments or cutlery—the answer is always its hardness and resistance to corrosion/rusting.
• Identification: If shown a diagram with two different types of atoms, it is an alloy. If all atoms are the same, it is a pure metal.
• Composition: Memorize that Brass = $Cu + Zn$. A common trick is to swap it with Bronze ($Cu + Sn$), but you only need to know Brass for the IGCSE syllabus.

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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) Define the term 'alloy'. [2]

(b) State two properties of alloys that make them more useful than pure metals in many applications. [2]

(c) Give one example of an alloy and state one of its uses. [1]

Worked Solution:

(a)

1. An alloy is a mixture Identifies the presence of more than one component.
2. of a metal with other elements, usually other metals or non-metals. Specifies that the alloy is a mixture of a metal and other elements.

How to earn full marks:

• Must mention it is a MIXTURE (not a compound).
• Must specify the mixture contains a metal AND other elements.

(b)

1. Harder than pure metals. States a property of alloys.
2. Stronger than pure metals. States another property of alloys.

How to earn full marks:

• Must state TWO correct properties.
• 'More resistant to corrosion' or 'higher melting point' are also acceptable.

(c)

1. Example: Stainless steel. Use: Cutlery (or surgical instruments, etc.). Gives a correct alloy and a valid use related to its properties.

How to earn full marks:

• Must give a valid alloy (e.g. brass, steel, bronze).
• Must give a use that is plausible for the stated alloy.
• The use MUST relate to a property of the alloy.
• 'Construction' is not specific enough; "steel girders in building frames" is better.

Common Pitfall: Make sure you understand the difference between a mixture and a compound. Alloys are mixtures, meaning the elements are physically combined but not chemically bonded. Also, when giving an example, be specific about the use and link it to a property of the alloy.

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

Question:

(a) Explain, in terms of their structure, why alloys are generally harder than pure metals. [4]

(b) Identify which of the following diagrams, A, B, or C, represents the structure of an alloy. [1]

(c) State the name of the metal that is alloyed with copper to make brass. [1]

Worked Solution:

(a)

1. Pure metals have atoms arranged in regular layers. Describes the structure of pure metals.
2. These layers can slide over each other easily. Explains why pure metals are malleable and ductile.
3. Alloys contain atoms of different sizes. Describes the structure of alloys.
4. The different sized atoms disrupt the regular arrangement, making it more difficult for layers to slide. Explains why the layers cannot slide easily in alloys.

How to earn full marks:

• Must mention the REGULAR layers in the pure metal.
• Must say the layers can SLIDE.
• Must mention that alloys have atoms of DIFFERENT SIZES.
• Must link the different sizes to the disruption of the layers, and the prevention of sliding.

(b)

1. Diagram C Correctly identifies the diagram of an alloy.

How to earn full marks:

• Must be exactly 'C'. No other answer is acceptable.

(c)

1. Zinc Correctly identifies the other metal in brass.

How to earn full marks:

• Spelling must be correct.

Common Pitfall: When explaining the hardness of alloys, focus on the disruption of the regular arrangement of atoms. It's not enough to just say "different sized atoms"; you need to explain how this prevents the layers from sliding easily.

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

Question:

A student investigates the properties of a pure metal, M, and an alloy containing M. They perform two tests: a scratch test for hardness and a bending test for strength.

(a) Describe the procedure for a scratch test to compare the hardness of the two materials. Include how the results would be interpreted. [3]

(b) Describe the procedure for a bending test to compare the strength of the two materials. Include how the results would be interpreted. [3]

(c) Predict the results of the two tests, stating which material (M or the alloy) would be harder and which would be stronger. Explain your reasoning. [2]

Worked Solution:

(a)

1. Use a sharp object (e.g., a nail or a file) to scratch the surface of each material. Describes the action of scratching the materials.
2. Apply the same force/pressure when scratching each material. Mentions controlling a variable.
3. Observe the depth/width of the scratch. The material with the smaller scratch is harder. Describes how to interpret the observations.

How to earn full marks:

• Must mention scratching BOTH materials.
• Must mention controlling the FORCE applied.
• Must link the scratch size to hardness. 'The material that scratches less is harder' is acceptable.

(b)

1. Fix each material at one end and apply a force to the other end to try and bend it. Describes the bending action.
2. Measure the force required to bend each material to a specific angle (or until it breaks). Mentions measuring force or angle.
3. The material requiring more force to bend (or the material that bends less before breaking) is stronger. Describes how to interpret the observations.

How to earn full marks:

• Must mention bending BOTH materials.
• Must mention measuring the FORCE required OR the ANGLE of bend.
• Must link force (or angle) to strength.

(c)

1. The alloy would be harder and stronger. Correctly predicts the results.
2. The different sized atoms in the alloy disrupt the regular arrangement of atoms, making it more difficult for the layers to slide, increasing hardness and strength. Explains the results in terms of atomic structure.

How to earn full marks:

• Must correctly predict that the alloy is harder AND stronger.
• Must link the hardness and strength to the disruption of the layers by different sized atoms.

Common Pitfall: When describing experimental procedures, remember to mention the importance of controlling variables to ensure a fair comparison. Also, be clear about how your observations relate to the property you are testing (e.g., smaller scratch = harder material).

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

Question:

Stainless steel is an alloy of iron, chromium, nickel, and carbon. It is widely used in cutlery and surgical instruments.

(a) State two properties of stainless steel that make it suitable for use in cutlery. [2]

(b) Explain why the presence of chromium in stainless steel makes it more resistant to rusting (corrosion) than pure iron. [3]

(c) A student wants to determine the percentage of iron in a sample of stainless steel cutlery. They dissolve 5.00 g of the stainless steel in excess hydrochloric acid. All the iron reacts to form iron(II) chloride, $FeCl_2(aq)$. The other metals in the alloy do not react. The iron(II) chloride solution is then titrated with potassium manganate(VII) solution, $KMnO_4(aq)$.

The equation for the reaction is:

$5Fe^{2+}(aq) + MnO_4^-(aq) + 8H^+(aq) \rightarrow 5Fe^{3+}(aq) + Mn^{2+}(aq) + 4H_2O(l)$

The student finds that 20.0 cm$^3$ of 0.0200 mol/dm$^3$ potassium manganate(VII) solution is required to react completely with the iron(II) chloride solution.

Calculate the percentage by mass of iron in the stainless steel sample. [4]

Worked Solution:

(a)

1. Hard. States one property.
2. Resistant to rusting (corrosion). States another property.

How to earn full marks:

• Must state TWO correct properties (hardness, resistance to corrosion, strong).
• 'Shiny' is not acceptable as it is not a key property that makes it suitable for CUTLERY.

(b)

1. Chromium forms a layer of chromium oxide on the surface of the steel. Describes the formation of a protective layer.
2. This layer is impermeable/prevents oxygen and water from reaching the iron. Explains how the layer protects the iron.
3. Thus, preventing the iron from rusting. Links the protective layer to the prevention of rusting.

How to earn full marks:

• Must mention chromium OXIDE.
• Must say the layer is IMPERMEABLE (or prevents water/oxygen reaching the iron).
• Must link the layer to the PREVENTION of rusting.

(c)

1. Moles of $MnO_4^-$ = $(20.0/1000) \times 0.0200 = 0.000400$ mol Calculates moles of $MnO_4^-$.
2. Moles of $Fe^{2+}$ = $5 \times 0.000400 = 0.00200$ mol Calculates moles of $Fe^{2+}$ using the stoichiometry of the equation.
3. Mass of $Fe$ = $0.00200 \times 56.0 = 0.112$ g Calculates mass of iron.
4. Percentage of $Fe$ = $(0.112/5.00) \times 100 = 2.24$% Calculates percentage of iron.

How to earn full marks:

• Must calculate the moles of $MnO_4^-$ correctly: $\boxed{0.000400 \text{ mol}}$
• Must use the stoichiometry of the equation (5:1 ratio) to find moles of $Fe^{2+}$: $\boxed{0.00200 \text{ mol}}$
• Must calculate the mass of iron correctly: $\boxed{0.112 \text{ g}}$
• Must calculate the percentage by mass of iron: $\boxed{2.24 %}$

Common Pitfall: In calculations involving titrations, pay close attention to the stoichiometry of the reaction. Make sure you use the correct mole ratio to convert between the moles of the known substance and the moles of the unknown substance. Also, remember to convert volumes from cm$^3$ to dm$^3$ before using them in calculations.