It’s all about language
Careless language can cause confusion. Precise use of language can assist understanding.
In Chemistry, there are terms agreed upon by IUPAC. Particular terms have particular meanings, and we should use them correctly or the receiver of a communication (a listener, or a reader) may become confused, or even develop a wrong understanding of the idea being discussed.
Unfortunately, over the years IUPAC has decided to use terms which have different meaning in everyday life from their meaning in Chemistry. This can be confusing if we don’t recognise which “compartment” (Chemistry, or everyday) that we are operating in.
The terms concentrated, dilute, strong, and weak are often used wrongly (due to either lack of care, or ignorance).
Among the terms concentrated, dilute, strong, and weak, there are two related pairs:
The concentration of a solutions refers to how crowded the solute species are in the solution – regardless of the volume of solution. See Module 0907 Solution concentration.
We use the following terms:
There is no sharp dividing line between concentrations that we refer to as either concentrated or dilute – although it is common to use these terms for solutions whose concentrations are near either extreme.
Mostly, these terms are used in a relative sense, such as:
The terms strong and weak are opposite ends on the scale of extent of ionisation of solutes that are electrolytes (i.e., when the solute dissolves in water, some of the molecules break up into ions). Refer to the previous module 0909 Solutes: Electrolytes or non-electrolytes?
And a reminder from a previous module 0910 Electrolytes: Strong or weak?
Here are some examples that show the distinctly different meanings of the two pairs of words: concentrated - dilute, and strong - weak.
So, whenever we use any of these four terms, we should think “Am I talking about the solution, or about the solute?”
1. The following six diagrams are symbolic portrayals, at the level of molecules and ions, of:
Decode the symbols for (i) solute molecules, (ii) cations, and (iii) anions, and then decide which of the solutions is represented by each of the diagrams.
2. This questions involves comparison of the three solutions A, B, and C, listed here:
A 0.20 M aqueous sucrose solution
B 0.05 M aqueous sucrose solution
C 0.01 M aqueous sucrose solution
(a) Which one of these solutions is the strongest? Which is the weakest?
(b) Make some valid statements about the relative concentrations of the solutions A, B, and C.
3. Consider the following three aqueous solutions of hypothetical weak electrolytes HX, HY, and HZ.
A. 0.10 M solution of HA. 11% of the HX molecules are ionised, and 89% remain un-ionised.
B. 0.10 M solution of HB. 4% of the HY molecules are ionised, and 96% remain un-ionised.
C. 0.10 M solution of HC. 0.5% of the HZ molecules are ionised, and 99.5 % remain un-ionised.
(a) Make some valid statements about the relative strengths of the solutes HX, HY, and HZ.
(b) Why would your ability to answer question (a) be more uncertain if all of the solutions were not 0.10 M?
4. Comment on what this person said: “I’m using quite a strong acetic acid solution – it’s 5.0 M. I think that I should use a weaker solution – like 0.01 M”.
5. Consider the following solutions:
Insert appropriate words in the spaces in the following sentences related to the two aqueous solutions A and B:
A: 0.01 M hydrochloric acid (HCl) solution
B: 0.08 M acetic acid (CH3COOH) solution.
C: 0.05 M sucrose (C12H22O11) solution
Insert appropriate terms in the spaces of the text below. Choose from the following words and phrases:
Solution A is more ……...…..…… (that is, less ……........……) than solution B. Solution B is ………......….. than solution C. The solute in solution A is a ………....…...………. electrolyte than the solute in solution B. In fact, because all of the HCl molecules ionise in solution, hydrochloric acid is a …………..…………. . The solute in solution B is a ……....…..…….. because only some of the molecules ……..........… in solution, but it is a ……..…........…… than the solute in solution C. In solution C, none of the sucrose molecules ionise, so sucrose is called a ….......…....…. .
6. Relational (or Venn) diagrams show the interrelationships among sets of objects or ideas. For example, a relational diagram can portray the relationships such as the following:
(a) Draw a relational (Venn) diagram that shows the interrelationships between the following concepts:
You might, for example, begin with trying to portray diagrammatically that every aqueous solution is either (i) a solutions of an electrolyte, or (ii) a solution of a non-electrolyte.
(b) Put each of the following solutions in the appropriate area of the diagram. That is, consider in which area of the diagram each solution “belongs”.
Remember that there is no single concentration that is a sharp dividing line between solutions that are classified as “concentrated” or “dilute”. Nevertheless, these terms are often used. Make practical decisions.
1. A is a concentrated solution of a non-electrolyte solute.
B is a dilute solution of a weak electrolyte solute.
C is a dilute solution of a strong electrolyte solute.
D is a dilute solution of a non-electrolyte solute.
E is a concentrated solution of a weak electrolyte solute.
F is a A concentrated solution of a strong electrolyte solute
(a) These are nonsense questions. Words describing strength (such as strong, weak, stronger, weaker, …..) are relevant to properties of solutes in solution, not of the solutions.
(b) All of the following are valid statements about the solutions:
(a) All of the following are valid statements about the solutes:
(b) The percentage of a weak electrolyte that ionises depends on the concentration of the solution. [The more dilute the solution, the more of the electrolyte ionises.] So the strengths of electrolytes can only be compared in solutions of the same concentration. (Although, in the example presented, the solutions would need to be of vastly different concentrations for the percentages ionised to be in a different order.]
4. Nonsense. Misuse of terms.
5. Solution A is more dilute … (that is less .. concentrated …) than solution B. Solution B is … more concentrated .. than solution C. The solute in solution A is a ... stronger .. electrolyte than the solute in solution B. In fact, because all of the HCl molecules ionize in solution, hydrochloric acid is a …. strong electrolyte …. . The solute in solution B is a …. weak electrolyte ... because only some of the molecules ... ionise … . in solution, but it is a … stronger electrolyte … than the solute in solution C. In solution C, none of the sucrose molecules ionise, so sucrose is called a … non-electrolyte .. .
6. There are many possible ways to represent the relationships among these categories of solution. One way is shown. The various solutions have been placed in their appropriate areas.
There is usually more than one way of diagrammatically representing relationships. No matter how the relational diagram is constructed, it should show: