physics

Molten salt

Molten salt is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Molten salt rather than just read about it. In short: Molten salt is salt which is solid at standard temperature and pressure but liquefied due to elevated temperature. A salt that is liquid even at standard temperature and pressure is usually called a room-temperature ionic liquid, and molten salts are technically a class of ionic liquids.

Key takeaways

Molten salt belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Molten salt to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Molten salt from memory before moving on to harder problems.

Reference excerpt

Molten salt is salt which is solid at standard temperature and pressure but liquefied due to elevated temperature. A salt that is liquid even at standard temperature and pressure is usually called a room-temperature ionic liquid, and molten salts are technically a class of ionic liquids.

Examples As a reference, molten sodium chloride, table salt, has a melting point (m.p.) of 801 °C (1,474 °F). A variety of eutectic mixtures have been developed with lower melting points:

Chlorides Lithium chloride and potassium chloride, m.p. 450 °C (842 °F).

Nitrates Alkali metal nitrates are relatively low melting and thermally stable. The least stable, LiNO3 (m.p. 255 °C (491 °F)) decomposes only at 474 °C (885 °F). At the other extreme, cesium nitrate melts at 414 °C (777 °F) and decomposes at 584 °C.

60:40 mixture of sodium nitrate and potassium nitrate is a liquid between 260 and 550 °C (500 and 1,022 °F). It has a heat of fusion of 161 J/g, and a heat capacity of 1.53 J/(g·K). 1:1 mixture LiNO3:KNO3, m.p. 125 °C (257 °F). 40:7:53 NaNO2:NaNO3:KNO3, m. p. 142 °C (288 °F), stable to 600 °C (1,112 °F).

Uses Molten salts have a variety of uses.

Production of magnesium and aluminium One industrial application is the production of magnesium, which begins with production of magnesium chloride by chlorination of magnesium oxide:

MgO + C + Cl2 → MgCl2 + CO Electrolysis of the resulting molten magnesium chloride is conducted at 700 °C (1,292 °F):

MgCl2 → Mg + Cl2 Aluminium metal is produced from aluminium oxides by electrolysis of a molten mixture of sodium hexafluoroaluminate and alumina at 950 °C (1,740 °F). This conversion is called the Hall-Héroult process.

Heat transfer Molten salts (fluoride, chloride, and nitrate) can be used as heat transfer fluids as well as for thermal storage. This thermal storage is used in concentrated solar power plants. Molten-salt reactors are a type of nuclear reactor that uses molten salt(s) as a coolant or as a solvent in which the fissile material is dissolved. Experimental salts using lithium can be formed that have a melting point of 116 °C while still having a heat capacity of 1.54 J/(g·K).

Other uses Molten chloride salt mixtures are commonly used as quenching baths for various alloy heat treatments, such as annealing and martempering of steel. Cyanide and chloride salt mixtures are used for surface modification of alloys such as carburizing and nitrocarburizing of steel. Cryolite (a fluoride salt) is used as a solvent for aluminium oxide in the production of aluminium in the Hall-Héroult process. Fluoride, chloride, and hydroxide salts can be used as solvents in pyroprocessing of nuclear fuel.

Ambient-temperature molten salts Ambient-temperature molten salts (also known as ionic liquids) are present in the liquid phase at standard conditions for temperature and pressure. Examples of such salts include N-ethylpyridinium bromide and aluminium chloride mix, discovered in 1951, and ethylammonium nitrate discovered by Paul Walden. Other ionic liquids take advantage of asymmetrical quaternary ammonium cations like alkylated imidazolium ions, and large, branched anions like the bistriflimide ion.

See also Electromagnetic pump Ionic liquid Molten-salt battery Molten salt oxidation Molten-salt reactor Parabolic trough United States Department of Energy International Energy Storage Database

References

Bibliography C.F. Baes, The chemistry and thermodynamics of molten salt reactor fuels, Proc. AIME Nuclear Fuel Reprocessing Symposium, Ames, Iowa, USA, 1969 (August 25), doi:10.1016/0022-3115(74)90124-X

External links

[1] Proc. Roy. Soc.

Illustrations

Worked examples

Example 1 — a first encounter with Molten salt

Start with the simplest possible case. Write down what Molten salt claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Molten salt before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Molten salt ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Molten salt

In research: Molten salt appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Molten salt in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Molten salt is common in secondary-school and first-year university syllabi. It links to neighbouring topics Energy storage, Inorganic solvents, Ionic liquids, so understanding it makes those chapters shorter.
In everyday life: Look for Molten salt outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Molten salt in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Molten salt means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Molten salt out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Molten salt in simple terms?

Why does Molten salt matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Molten salt?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Molten salt.