Q.An ornament of gold having 87.5% of gold, is of ............... carat.
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Alloy Composition
Alloy Composition – From Intuition to Precision
Think of a metal like gold. Pure gold (24 karat) is too soft for jewellery that needs to hold its shape. So jewellers mix in small amounts of copper or silver. That mixture is an alloy — a material made by combining two or more elements, at least one of which is a metal.
The composition of an alloy simply tells you what elements are present and how much of each there is. It's the recipe.
The Intuition: It's Like Making Dough
Imagine you're making chapati dough. You have:
- Wheat flour (the base)
- Water
- A pinch of salt
If you use too much water, the dough is sticky. Too little, it's dry and cracks. The composition of the dough — the exact ratio of flour to water — determines its properties.
Alloys work the same way. By changing the composition, you change:
- Strength (brass is stronger than pure copper)
- Hardness (steel is harder than pure iron)
- Melting point (solder melts at a lower temperature than pure tin)
- Corrosion resistance (stainless steel doesn't rust like pure iron)
The Precise Statement
Alloy composition is the quantitative description of the elements present in an alloy, usually expressed as a weight percent (wt%) or atomic percent (at%) of each constituent.
Weight percent tells you: "Out of 100 grams of this alloy, how many grams are element X?"
wt% of A=total mass of alloymass of A×100
Atomic percent tells you: "Out of every 100 atoms in the alloy, how many are atoms of element X?"
at% of A=total moles of all elementsnumber of moles of A×100
For most engineering purposes, weight percent is used because it's easier to measure (just weigh the ingredients). Atomic percent matters when you care about the arrangement of atoms in the crystal structure.
A Concrete Example: Brass
Brass is an alloy of copper (Cu) and zinc (Zn).
- Common composition: 70 wt% Cu, 30 wt% Zn
- What this means: If you have 100 g of this brass, it contains 70 g of copper and 30 g of zinc.
Now, why not 50-50? Because at 70-30, the alloy is strong yet ductile (can be drawn into wires). At 50-50, it becomes brittle. The composition determines the properties.
Don't confuse composition with phase. Composition tells you the overall recipe. A phase is a distinct region within the alloy that has its own composition and structure. For example, in a 70-30 brass, the entire alloy is one phase (a solid solution). But in a 60-40 brass, two different phases can coexist — each with its own composition.
Why This Matters for Exams …
Why this formula?
Alloy Composition: Why the Formulas Work
Alloy composition is about how much of each metal is present in a mixture. The key formulas come from two simple ideas: mass conservation and volume additivity (approximately). Let's break down the reasoning.
1. Mass-Based Composition (Weight Percent)
The Formula
Weight percent of element A=total mass of alloymass of A×100%
Why this holds
- Mass is conserved when metals are mixed. If you take 20 g of copper and 30 g of zinc, the total mass is exactly 20+30=50 g.
- The fraction of copper is simply its share of the total mass: 5020=0.4 or 40%.
- This is directly proportional — double the copper mass (keeping zinc fixed), and the weight percent doubles.
Key insight: Mass percent is the most reliable because mass doesn't change when metals are melted together (assuming no loss).
2. Atom-Based Composition (Atomic Percent)
The Formula
Atomic percent of element A=total number of molesnumber of moles of A×100%
Why this holds
- Atoms are discrete particles. The number of atoms determines the alloy's properties at the atomic level (e.g., crystal structure).
- To convert from mass to moles:
moles of A=atomic mass of Amass of A
- Example: 20 g of copper (atomic mass = 63.5 g/mol) gives 63.520≈0.315 moles. 30 g of zinc (atomic mass = 65.4 g/mol) gives 65.430≈0.459 moles.
- Total moles = 0.315+0.459=0.774 Atomic percent of Cu = 0.7740.315×100%≈40.7%
Key insight: Atomic percent differs from weight percent because atoms have different masses. A heavy atom contributes more mass but not more atoms.
3. Volume-Based Composition (Volume Percent)
The Formula
Volume percent of A=total volume of alloyvolume of A×100%
Why this holds (approximately)
- Volume is not strictly additive — when metals mix, atoms may pack differently, causing slight volume changes (contraction or expansion).
- However, for many solid alloys, the volume change is small (<1%), so we approximate:
Total volume≈VA+VB
- Volume of each metal = densitymass
Key insight: Volume percent is useful for porosity calculations or when density is critical (e.g., in casting), but it's less fundamental than mass or atomic percent.
4. Converting Between Composition Types
From weight percent to atomic percent …
The carat scale for gold purity is defined out of 24 parts, with pure gold being 24 carat, so converting a given percentage of gold into a carat value just means scaling that percentage onto the 24-part scale. …
Carat purity of gold is defined on a scale of 24, where 24 carat = 100% pure gold; 87.5% gold corresponds to 21 carat.
The carat system expresses the fraction of gold in a gold alloy out of 24 parts:
%Gold = (carat/24) × 100
Given %Gold = 87.5%, so:
carat = (87.5/100) × 24 = 0.875 × 24 = 21
…
- CBSE 2025Set ANNUAL1 markQ.The transition element present in 'bronze' is _____.
›Reveal solutionSolution
Bronze is an alloy of copper and tin; the transition element in it is copper.
Bronze is a well-known alloy, made predominantly of copper (Cu, typically ~88-90%) along with tin (Sn, ~10-12%) and sometimes small amounts of other elements. Of these constituents, copper is a d-block (transition) element — it has the electronic configuration [Ar]3d10 4s1 and shows characteristic transition-metal behaviour (variable oxidation states Cu+/Cu2+, coloured compou …
- CBSE 2024Set ANNUAL1 markMCQQ.The Silver UK coins are an alloy of Copper with:(a) Silver(b) Aluminium(c) Nickel(d) Chromium
›Reveal solutionSolution
Modern British 'silver' coins contain no silver at all — they are cupro-nickel, an alloy of copper (~75%) and nickel (~25%).
Transition metals readily form alloys because their atoms are of similar size and can substitute for one another in a metallic lattice. British coinage switched from real silver (silver-copper alloy) to cupro-nickel (copper + nickel) in 1947 to cut cost, while keepi …
- CBSE 2022Set M1 markQ.Name the transition metal present in bronze alloy.
›Reveal solutionSolution
Bronze is a Cu–Sn alloy; the transition metal in it is copper.
Bronze is an alloy of copper (Cu) and tin (Sn). Of these, copper is the transition metal (tin is a p-block metal), …
- CBSE 2022Set ANNUAL1 markQ.Write the names of metal elements present in brass.
›Reveal solutionSolution
Brass is a well-known alloy made by combining copper with zinc.
Brass is an alloy in which copper (the base/major metal) is mixed with zinc (typically in proportions of roughly 60-80% Cu and 20-40% Zn, though ratios vary by grade). Alloying copper with zinc increases its hardness and strength compared to pure copper while retaining good corrosion resi …
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