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Leaving Cert Engineering: Thermal equilibrium diagrams

How often Thermal equilibrium diagrams comes up on the Engineering papers, every year it was asked, and questions to try.

HL Asked on 9 of the last 9 Higher Level papers, most recently in 2025. banker

June 2028 is the first exam on the new Engineering course. These come from the 2016–2025 papers, so read them as a guide.

Thermal equilibrium diagrams, Higher Level(8 marks)

Quick ones on Thermal equilibrium diagrams.

(a)An alloy is 40% B. At temperature T the liquid is 60% B and the solid 10% B. What percentage of the alloy is liquid?
  1. 40%
  2. 60%
  3. 75%
(b)Line on an equilibrium diagram marking the limit of solid solubility?
  1. Liquidus
  2. Solidus
  3. Solvus
(c)Below the eutectic temperature, α is 5% B and β is 95% B. What percentage of a 23% B alloy is α?
  1. 80%
  2. 23%
  3. 20%
Show the answers

(a) 60%

(b) Solvus

(c) 80%

Your turn: Higher Level questions on Thermal equilibrium diagrams.

Higher Level

Asked on 9 of the last 9 Higher Level papers, most recently in 2025. banker

Every paper, year by year

YearWhere it came up
2025Q5
2024Q5
2023Q5
2022Q5
2021Q5
2019Q4
2018Q4
2017Q4
2016Q4

Links open the State Examinations Commission’s paper for that year.

More Thermal equilibrium diagrams questions

Thermal equilibrium diagrams, 2 marks

What happens in a eutectic reaction on cooling?

  1. One solid changes into two other solids at a fixed temperature
  2. Liquid freezes gradually into one solid over a range
  3. Liquid changes to two solids at one fixed temperature
Show the answer

Liquid changes to two solids at one fixed temperature

At the eutectic point the liquid freezes at the lowest temperature in the system, forming a fine mix of two solid phases. Solid to two solids is a eutectoid reaction.

Thermal equilibrium diagrams, 2 marks

How many phases are present between the liquidus and solidus of a solid-solution diagram?

  1. Three, liquid and two solids
  2. Two, liquid and solid
  3. One, a single liquid
Show the answer

Two, liquid and solid

Between the two lines the alloy is partly frozen: solid crystals plus remaining liquid. Above the liquidus it is all liquid; below the solidus all solid.

Thermal equilibrium diagrams, 3 marks

A solid-solution alloy cooled quickly has grains whose composition changes from centre to edge. This is?

  1. Coring
  2. Allotropy
  3. Eutectic formation
Show the answer

Coring

Fast cooling gives no time for diffusion, so the first-formed centre is richer in the higher-melting metal. A homogenising anneal evens out the composition.

Other Engineering topics

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