Talk:Amount of substance

Awagadro number is {6.022*10res power 23 — Preceding unsigned comment added by Chinmayjangid9 (talk • contribs) 13:08, 20 November 2022 (UTC)

Message to Ed Cormany
Ed. May I ask your reasons for redirecting _Amount of substance_ to _Mole (unit)_ ? The former is a physical quantity; the latter is a unit for measuring the physical quantity. As such they are very different and, from the physical perspective, we need two different articles. We have this with _Length_ and _Metre_, _Mass_ and _Kilogram_, _Time_ and _Second_, _Temperature_ and _Kelvin_, etc. etc. All of these are Physical Quantities and their respective SI base units. Thanks Ian Cairns 21:19, 7 Jul 2004 (UTC)


 * Ian, when I created the redirect every piece of information that was present in the amount of substance article was also present in the mole article, sometimes word-for-word. As such, it seemed redundant to me and I redirected it.  If you have some additional content to add to amount of substance to make it a better standalone article, please edit over the redirect.  Even if you feel that the redirect is inappropriate and should simply be reverted to the previous page edit, go ahead.  Be bold in editing pages; that's what I was when I first created the redirect.  &#151;Ed Cormany 00:38, 8 Jul 2004 (UTC)


 * Ed, Thanks for that. I'm happy to revert it. The physical quantity and the unit belong to different categories within the same subject area, and it helps to separate the descriptions as much as possible. Some overlap is unavoidable. However, it may well be that there is some redistribution of content possible between the articles, and I'll action this. Ian Cairns 11:14, 8 Jul 2004 (UTC)

Substance or Matter?

Amount rather than quantity
The existence of this article is necessitated by the failure to use the term Avogadro's number as the name of the physical quantity identified by the chemical unit mole (unit), and also by the unfortunate use of the term in NIST and BIPM publications. Using the term "amount" requires the additional explanation that it really means "quantity", and that it does not refer to mass, neither of which would have been required if the the terms "quantity of particles" or "number of particles" had been used instead. --Blainster 17:49, 24 October 2005 (UTC)


 * Some of this I believe I can explain. First, the SI quantity, at least, is new. Its purpose is dimensional analysis. Dimensional analysis breaks units into their fundamental parts; and this has several good uses in research. Examples are mass, composed of the dimension M and length, composed of L. Mass & length used to be called primary quantities (now base quantities). Momentum, MLT-1, and energy, ML2T-2 are derived quantities. If you suspect a scientific function, before investigating it break the units into their primary dimensions. This should yield zero, as in f(x,y) = 0. Geologist (talk) 01:04, 2 October 2009 (UTC)


 * Problem. One has many units (scientific operations) that define an equivalent concept, such as time, T. (We used to think.) To be primary, a unit can't be confused with other primary ones. Mass is out, though it is the most primitive (and no reason was given). All other primary units are the primitive units of science, such as mass & length. We measure them with the simplest of tools. Choosing number of atoms, molecules, or isotopes is like stating the four elements from which all are composed are Earth, Wind, Water, & iPods. iPods required a theory to construct. They may be discarded tomorrow with the theory used to make them. Geologist (talk) 01:04, 2 October 2009 (UTC)


 * Solution. However, I have convinced myself that a mole of oxygen can be defined (with scientific operations) without the need for an atomic theory. One can perform chemical operations in the laboratory, as Dalton did. I choose to ignore the 'number of' in the SI definition, and convince myself that N, of which 'mole' is one unit, is a dimension both primary--and primitive, in that 1 mole of He gas can survive the loss of any theory. Everyday quantities can be safely built from these primary, base ones. Being theory independent, they can safely be used to test theories. For this reason, I should minimize counting as the operation used to build a mole: this defines it to be a number of molecules, a theoretical concept. Using a theory to test a theory seems questionable methodology. Geologist (talk) 01:04, 2 October 2009 (UTC)

Votes for deletion

 * Keep. The keepers of our standards insist on making a distinction.  This is made clear in both BIPM's SI brochure  and in NIST's Guide for the Use of the Internaiotnal System of Units (SI).  Gene Nygaard 19:30, 24 October 2005 (UTC)
 * I see that the term "amount of substance" is used in the NIST guide under 8.6.1 here but was unable to readily find it in the BIPM site. Nevertheless, the term itself is ambiguous and thus inaccurate.  What the writers intend by using the term "amount of substance" is actually "number of particles", for which the defined quantity would be Avogadro's number.  I agree that the distinction you refer to does exist between the number or quantity of particles and the units (mole) used to measure them, and this is retained by using the term Avogadros number instead of amount of substance.  As I have noted, it would be acceptable to me to retain the article with a redirect to Avogadro's number. --Blainster 20:08, 24 October 2005 (UTC)


 * Keep. Now that I understand the subtlety of what is being defined: the SI base quantity (unit, dimension) of a substance as mole, there are many aspects of this quantity to be discussed. In addition to whether this 'base quantity' must be thought of as a theoretical term (for those who dislike this), its symbol in mathematical equations, its symbol in dimensional analysis, &c, there are other topics. Should it be expressed n(NaCl) or ni, where i = NaCl. In the latter case, in my experience, the molecular species or formula unit is a right subscript, the phase (v, l, s, x, &c) is a right superscript, and the isotope is indicated by a left superscript. Is this standard? Should the choices of formula unit be discussed here? Some thermodynamic theorems only work with a particular choice of formula unit. What should one do with older terms, such as gram-atoms, gram-cations, &c? Each choice of formula unit and each choice of derived unit (such as molality) have their purposes, and could be numerous in different fields of study. This is my latest opinion on 'substance' or 'amount of substance' or amount of C12. There would appear to be many conventions to discuss. Perhaps this is the place? 'Amount of substance' is just one arbitrary name for the mole, which is the definition of the SI's base unit for a chemical substance. The subtlety of what is a definition of what confused me greatly. It still may. :-) Geologist (talk) 05:55, 1 October 2009 (UTC)

Symbol n?
Does anyone know where it comes? —Preceding unsigned comment added by 128.214.98.177 (talk) 08:55, 9 September 2008 (UTC)


 * This is an excellent question. Researchers tend to adopt the usage in texts, and books on phase equilibrium are the first to be culled by libraries for space. This is occurring before a good history of the subject has been written. Early works always refer to mi, the mass-fraction of the designated unit of matter i, such as (H20)/3 (the gram-atom unit), a choice that allows reaction coefficients to sum to 0, and allows many theorems to easily be written (theorems that fail with mole-fraction). One assumes n followed m. A good history is needed. Geologist (talk) 01:14, 29 September 2009 (UTC)

"It is possible to measure the ratios of the numbers of entities in chemical samples more precisely than it is possible to count the absolute numbers of entities"
This is true. However, it might be worth letting the reader know that in the everyday work of maybe 99% of chemists and other scientists, this is irrelevant. Avogadro's constant is known to about eight significant figures, whereas typical mass measurements in the lab have no more than six. So in most cases the precision of the calculated number of entities is not limited by our knowledge of Avogadro's constant. (I say this because the section about the "Rationale for preferring amount-of-substance to absolute numbers" provides examples that talk about "chemists", and not about metrologists!) --Itub (talk) 13:22, 15 June 2009 (UTC)
 * Let's not forget that the purity of most lab reagents is only three significant figures, which the BIPM puts down as the most usual limitation to the measurement of amount of substance. Physchim62 (talk) 13:32, 15 June 2009 (UTC)

Motivation for not using Mass Fraction
...Many paragraphs deleted by the writer.

My own very long criticism of this article, an article which has spawned Amount concentration and Amount fraction (which have their own criticisms) has been removed by me. My criticism was a long list of unacceptable conclusions that I believe follow from the article (and still do). My conclusion was that, if the SI recommended it, it should be mentioned; but it need not be adopted. All this was based on this sentence:

The International System of Units (SI) defines the amount of substance to be a base physical quantity that is proportional to the number of elementary entities present.

When I read the original SI document, I found that the amount of substance can instead be defined to be any reasonable unit one wishes to create, not just defined to be in moles. Perhaps this can be clarified.

My own notebooks on thermodynamic studies contain, I discovered, 'amount of substance' written many times by me. The erroneous 'definition', the very long article, and the articles spawned confused me so much that I didn't recognize that I had long ago chosen the same (arbitrary) term 'amount of substance' that the SI chose.

When writing about fundamentals, one must be extremely careful, careful in natural language, in logic, and in mathematics. The SI never defined the amount of substance to be a mole. They simply added a new quantity (new to me, at least) to be used when symbolically writing chemical relations that they propose to be a base dimension (a primary dimension) in Dimensional analysis. This chart tells all:

http://en.wikipedia.org/wiki/Physical_quantity#Base_quantities.2C_derived_quantities_and_dimensions

We already had mass (M) and volume (L3), and substance (N) now extends dimensional analyses to proposed relations among chemical phenomena. To best distinguish amounts of carbon from nitrogen, the SI reasonably chose the number of its moles.

That's it.

One may use amount of substance, mass, & volume to create any secondary dimension or any unit one finds convenient. One need not exclusively use moles (and mole fractions). Some units, such as molarity, ppm, ppb, &c, are invariant in being the quotient of ni and V. These, for an unknown reason, were described as being the same amount concentration by the SI. I can't imagine using it.

One may wonder why the SI chose a theoretical term, 'mole'. My guess would be that all other expressions for the concept of an amount of a substance contain secondary or derived quantities. The other reason may be that this need not be a theoretical term. The number of grams to choose for the amount of one mole of carbon can be deduced from assigning 1 to H, and using chemistry to complete the periodic table. Atomic theory need not be assumed.

This arbitrary term 'amount of substance', chosen to describe the fundamental quantity 'mole', doesn't really deserve, IMO, this very lengthly exposition that apparently did not clarity a concept that was actually trivial.

Below are responses to my queries about why I could no longer express the amount of Pb by its mass. Physchim62 clearly misunderstood my confusion, and I his. I suggest the article be severely curtailed, and amount concentration & amount fraction either deleted or kept contained within this article. They are being misunderstood elsewhere as replacements for molarity & mole fraction.

The digressions on the ideal gas model, melting-point depression, and osmotic pressure confused me into thinking Physchim62 was explaining why it was convenient to use the theory-laden term 'mole'.

Geologist (talk) 01:25, 1 October 2009 (UTC)


 * The usual version of the ideal gas law is pV = nRT, where n is the amount of substance in the system (as defined in this article). You could rewrite the ideal gas law as pV = NkT, where N is the number of molecules, but you run into a problem: how do you count the number of molecules?
 * The concept of amount of substance comes about because we can easily find physical relationships which depend on the number of molecules, such as melting-point depression or osmosis, but we don't measure the number of molecules directly, merely a ratio of numbers of molecules in different samples. Electrical measurements are exactly analogous: we know that electric current is a certain number of electrons per second, but we don't know exactly how many. Electric charge at the macroscopic scale is measure in coulombs and not as a multiple of the elementary charge. Amount of substance at the macroscopic scale is measured in moles and not in numbers of molecules. The distinction arises from how we measure these quantities, it is not artificial at all.
 * I see no reason why Wikipedia should chose to prefer your point of view that amount of substance is an artificial definition over and above the views of the various professional bodies which state otherwise. Physchim62 (talk) 12:01, 29 September 2009 (UTC)

...Many paragraphs removed by writer.

We are mis-communicating. Geologist (talk) 01:25, 1 October 2009 (UTC)
 * The power to "carefully decide whether it wishes to follow it" implies the power to carefully decide whether it wishes to ignore it. I shall take the latter route, thank you. Physchim62 (talk) 19:52, 30 September 2009 (UTC)

OK. Now that I read that the IUPAC and SI want to discuss completely different aspects of what can be descriptively called an 'amount of substance', I've decided perhaps this is the place to discuss many aspects of notation, formula-units, derived units (molality), and -- essentially -- all aspects of amounts of chemical substances. I've always found all this extremely complicated; perhaps this is the place to lay out all the latest conventions. Geologist (talk) 06:05, 1 October 2009 (UTC)
 * This is one place we could do that, yes, and this talk page is as good a place as any to discuss any miscomprehensions we might have. Stoichiometry is another article which talks about similar matters, or you can look at law of definite proportions and law of multiple proportions for a more historical overview. Physchim62 (talk) 11:11, 1 October 2009 (UTC)

Avogadro's number vs. Avogadro constant
In many wikipedia articles (Chemistry, Mole (unit), Amount of substance, Avogadro constant), there is confusion about the units of Avogadro's number N0 (should be dimensionless) and the Avogadro constant NA (dimension 1/mol). These are two different constants and should not be used as synonyms. The Avogadro constant is an official physical constant used in the SI system to convert between Amount of substance and number of entities. Because it has a unit, it is not a number. It appears in conversion of macroscopic properties (quantity per mole) to atomic properties (quantity based on a single atom or molecule).

Avogadro's number was used widely before the unit mole for the quantity amount of substance was added to the basic SI units in the early seventies. Textbooks and teachers have not all made this transition fully, especially when it comes to problem sets and model answers. You will find many instances of the statement 1 mol = 6.022x10E23, but according to the current definitions, that statement is wrong. Saying that the name Avogadro's number was changed to the Avogadro constant is also wrong. It was a switch to different units necessitated by the new system of base units. They do have the same numeric values when using mole as the unit, but they have different dimensions, so they are different.

I'll check in a couple of days to see if anyone wishes to comment, and then revise the article accordingly if there is no objection. Theislikerice (talk) 00:44, 18 August 2010 (UTC)

Dimensionless quantity
It is a dimensionless quantity like the plane angle, altough it would appear dimensional. See details/explanations in mole (unit) about (pseudo)-dimensionality. Or see section above which specifies it is a ratio of number of entities.--188.26.22.131 (talk) 11:28, 24 February 2014 (UTC)

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"Orders of magnitude (amount of substance)" listed at Redirects for discussion
An editor has asked for a discussion to address the redirect Orders of magnitude (amount of substance). Please participate in the redirect discussion if you wish to do so. Utopes (talk / cont) 23:39, 11 April 2020 (UTC)