How good are AI generated responses to typical chemistry questions?
No better than the source material used for the answer ... which can include common misconceptions.
One such question is "What is the pH of water?". To a chemist this is a nonsense question (comparable to asking what colour an apple is or what shape a building is) yet it is often asked in school science classrooms around the world.
I asked ChatGPT "What is the pH of water?" This was the answer ...
The pH of pure water at 25 degrees Celsius (77 degrees Fahrenheit) is considered neutral and is defined as 7. A pH value below 7 indicates acidity, while a pH value above 7 indicates alkalinity. It's important to note that the pH of water can be influenced by dissolved substances, and natural water sources may have varying pH levels due to the presence of minerals and other compounds.
I asked perplexity.ai "What is the pH of water?" This was the answer ...
The pH of water is 7, which is considered neutral. A pH of 7 is neither acidic nor basic. It is important to note that the pH of pure water can change due to the presence of impurities or the absorption of carbon dioxide from the air, which can make it slightly acidic. However, under normal conditions, the pH of pure water is 7, indicating neutrality2
Surprise, surprise, both answers are inaccurate and riddled with common misconceptions (although, to its credit, perplexity.ai does provide sources, but to its discredit it does not refer to a specific temperature which is critical!). ChatGPT emphasizes "pure water", while perplexity.ai is content with just "water". To a chemist, both terms are the same, "water" is a pure substance. If water has substances dissolved in it it is no longer "water" it is an "aqueous solution". Perplexity.ai's referral to "normal conditions" is baffling, perhaps it refers to standard conditions for aqueous solutions? After these issues, both AIs seem to perpetuate the misconception that acidity, basicity (alkalinity) and neutrality are defined by a pH value. This is not true. They are two entirely different concepts (although both are based on an Arrhenius description of aqueous solutions). pH is a measure of aqueous hydrogen ion concentration, independent of whether the aqueous solution has been designated acidic, basic or neutral. Acidic, basic, neutral are terms to describe a the relative amounts of hydrogen ions and hydroxide ions in an aqueous solution.
So, let's start by talking about the neutrality of "pure water". Pure water is always neutral (regardless of its pH). This is because the concentration of acidic protons in the water, H+(aq) or H3O+(aq), is always equal to the concentration of basic hydroxide ions in the water, OH-(aq), that is [H+(aq)] = [OH-(aq)]
An acidic aqueous solution is one in which the concentration of acidic protons is greater than the concentration of basic hydroxide ions, that is, [H+(aq)] > [OH-(aq)]
A basic aqueous solution is one in which the concentration of acidic protons is less than the concentration of basic hydroxide ions, that is, [H+(aq)] < [OH-(aq)]
So "pure water" is always neutral, but what is its pH? pH is a measure of the concentration of aqueous hydrogen ions in
solution, the greater the concentration of hydrogen ions in aqueous solution, the lower the pH. The concentration of these hydrogen ions is very much dependent on the water temperature. The pH of pure
water at 10oC is approximately 7.27 but the water is still described as neutral because [H+(aq)] = [OH-(aq)]. The pH of pure water at 50oC is about 6.63 but the water is still neutral because [H+(aq)] = [OH-(aq)].
The pH of water at 25oC is approximately 7 and is based, NOT on it being defined this way, but on a calculation of the hydrogen ion concentration of water at this temperature.
So, what happens to pH if other substances are dissolved in water to form aqueous solutions at 25oC? Using an Arrhenius definition of acids and bases (and a pH of 7 which has 1 significant figure but even that digit is uncertain), if an aqueous solution has pH < 7 at 25oC it could be described as acidic, but if the temperature is greater than 25oC the solution may be neutral, it may even be basic, we simply do not have enough information to decide. Similarly, If an aqueous solution has pH > 7 at 25oC it could be described as basic, but if the temperature is less than 25oC it is not a justifiable conclusion.
It is important to note that if there are dissolved "substances", including "impurities", "minerals" or "carbon dioxide", then the "water" is not "pure water", it is an aqueous solution, and the AI discussions are not relevant. Furthermore, some solutes dissolve in water at 25oC and increase its pH (aqueous solution of sodium hydroxide), some decrease its pH (aqueous solution of hydrogen chloride), and some may have no effect at all on its pH (aqueous solution of sodium chloride).
There is no simple, correct answer for the question "What is the pH of water?". The question does not contain enough information in order for it to be answered. IF the AIs simply stated that the pH of water at 25oC is approximately 7, that would be a pretty good response, however, by providing additional information in the answer they are repeating, and encouraging, common misconceptions about the nature of "pure water", "pH", "acidity", "basicity" (or "alkalinity") and "neutrality".
For a discussion on why pure water is always neutral and why its pH varies, see https://www.ausetute.com.au/kw.html
For a discussion on what determines whether an aqueous solution is acidic, basic or neutral, see https://www.ausetute.com.au/abneutral.html
