physics

ISO 31-8

ISO 31-8 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 ISO 31-8 rather than just read about it. In short: ISO 31-8 is the part of international standard ISO 31 that defines names and symbols for quantities and units related to physical chemistry and molecular physics. Quantities and units Notes In the tables of quantities and their units, the ISO 31-8 standard shows symbols for substances as subscripts (e.g., cB, wB, pB).

Key takeaways

ISO 31-8 belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect ISO 31-8 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of ISO 31-8 from memory before moving on to harder problems.

Reference excerpt

ISO 31-8 is the part of international standard ISO 31 that defines names and symbols for quantities and units related to physical chemistry and molecular physics.

Quantities and units

Notes In the tables of quantities and their units, the ISO 31-8 standard shows symbols for substances as subscripts (e.g., cB, wB, pB). It also notes that it is generally advisable to put symbols for substances and their states in parentheses on the same line, as in c(H2SO4).

Normative annexes

Annex A: Names and symbols of the chemical elements This annex contains a list of elements by atomic number, giving the names and standard symbols of the chemical elements from atomic number 1 (hydrogen, H) to 109 (unnilennium, Une). The list given in ISO 31-8:1992 was quoted from the 1998 IUPAC "Green Book" Quantities, Units and Symbols in Physical Chemistry and adds in some cases in parentheses the Latin name for information, where the standard symbol has no relation to the English name of the element. Since the 1992 edition of the standard was published, some elements with atomic number above 103 have been discovered and renamed.

Annex B: Symbols for chemical elements and nucleides Symbols for chemical elements shall be written in roman (upright) type. The symbol is not followed by a full-stop. Examples:

H He C Ca Attached subscripts or superscripts specifying a nucleotide or molecule have the following meanings and positions:

The nucleon number (mass number) is shown in the left superscript position (e.g., 14N) The number of atoms of a nucleotide is shown in the right subscript position (e.g., 14N2) The proton number (atomic number) may be indicated in the left subscript position (e.g., 64Gd) If necessary, a state of ionization or an excited state may be indicated in the right superscript position (e.g., state of ionization Na+)

Annex C: pH pH is defined operationally as follows. For a solution X, first measure the electromotive force EX of the galvanic cell

reference electrode | concentrated solution of KCl | solution X | H2 | Pt and then also measure the electromotive force ES of a galvanic cell that differs from the above one only by the replacement of the solution X of unknown pH, pH(X), by a solution S of a known standard pH, pH(S). Then obtain the pH of X as

pH(X) = pH(S) + (ES − EX) F / (RT ln 10) where

F is the Faraday constant; R is the molar gas constant; T is the thermodynamic temperature. Defined this way, pH is a quantity of dimension 1, that is it has no unit. Values pH(S) for a range of standard solutions S are listed in Definitions of pH scales, standard reference values, measurement of pH, and related terminology. Pure Appl. Chem. (1985), 57, pp 531–542, where further details can be found. pH has no fundamental meaning; its official definition is a practical one. However, in the restricted range of dilute aqueous solutions having amount-of-substance concentrations less than 0.1 mol/L, and being neither strongly alkaline nor strongly acidic (2 < pH < 12), the definition is such that

pH = −log10[c(H+) y1 / (1 mol/L)] ± 0.02 where c(H+) denotes the amount-of-substance concentration of hydrogen ion H+ and y1 denotes the activity coefficient of a typical uni-univalent electrolyte in the solution.

Worked examples

Example 1 — a first encounter with ISO 31-8

Start with the simplest possible case. Write down what ISO 31-8 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 ISO 31-8 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 ISO 31-8 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 ISO 31-8

In research: ISO 31-8 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 ISO 31-8 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: ISO 31-8 is common in secondary-school and first-year university syllabi. It links to neighbouring topics ISO 31, Molecular physics, Physical chemistry, so understanding it makes those chapters shorter.
In everyday life: Look for ISO 31-8 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 ISO 31-8 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what ISO 31-8 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 ISO 31-8 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is ISO 31-8 in simple terms?

ISO 31-8 is the part of international standard ISO 31 that defines names and symbols for quantities and units related to physical chemistry and molecular physics. Quantities and units Notes In the tables of quantities and their units, the ISO 31-8 standard shows symbols for substances as subscripts…

Why does ISO 31-8 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 ISO 31-8?

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 ISO 31-8.