AI and chemistry

 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 neutrality

2

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 10^oC is approximately 7.27 but the water is still described as neutral because [H⁺(aq)] = [OH^-(aq)]. The pH of pure water at 50^oC is about 6.63 but the water is still neutral because [H⁺(aq)] = [OH^-(aq)].

 The pH of water at 25^oC 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 25^oC? 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 25^oC it could be described as acidic, but if the temperature is greater than 25^oC 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 25^oC it could be described as basic, but if the temperature is less than 25^oC 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 25^oC 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 25^oC 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

Shapes of Melting Ice

 What shape is submerged ice as it melts?

That depends on temperature apparently ..

 Which suggests that we can infer water temperature in nature by observing the shape of its melting ice.

Scott Weady, Joshua Tong, Alexandra Zidovska, and Leif Ristroph (2022); Anomalous Convective Flows Carve Pinnacles and Scallops in Melting Ice. Phys. Rev. Lett 128(4)  https://doi.org/10.1103/PhysRevLett.128.044502

Micellar Water

Once upon a time you would buy a bar of soap and use it to wash your hands, face, body, and possibly even your hair.
Not today! Now you have hand-wash for your hands, body-wash for your body, shampoo for your hair, and a huge range of different products to clean your face including "micellar water" or "micellar cleansing water".
Unlike other face-cleaning products which need to be washed off with water, the makers of "micellar water" claim that it will cleanse your skin without vigorous rubbing or rinsing.
Intriguing! This "micellar water" sounds like some kind of magic doesn't it?
What's in "micellar water" and how does it work?

Read all about it in the September 2019 edition of AUS-e-NEWS, AUS-e-TUTE's free quarterly newsletter for chemistry students and teachers.

Want to subscribe to AUS-e-NEWS?

Go to https://www.ausetute.com.au/ausenews.html

Gravimetric Analysis

Gravimetric analysis can be used to determine the quantity of an ion present in a solution.
This can be done by adding a reagent that causes the ion under investigation to form an insoluble compound (a precipitate) that precipitates out of the solution.
AUS-e-TUTE has "free-to-view" tutorials currently available on :

1.  Determining the percentage by mass of sulfate in a lawn fertiliser
2.  Determining the concentration of chloride ions in water

AUS-e-TUTE Members should log-on to go to the Members ONLY resources on these topics which include:

• tutorials
• games
• tests (with worked solutions)
• exams (with worked solutions)

Not an AUS-e-TUTE Member?
Contact us to get access to the interactive demonstration resources.

Concentration of Calcium Ions in Hard Water

AUS-e-TUTE has just added new complexometric titration resources for the determination of calcium ions in hard water.
Members should log-in to see the new tutorial, game, test and drill.

Neutral pH?

We hear this term a lot, often in advertising. But what does it mean?

From a Chemist's point of view, there are two different concepts involved in this seemingly harmless "neutral pH" expression. These two different concepts are:

• neutral
• pH

Let's take a look at the Chemist's definition of neutral first.
A solution is neutral if the concentration of hydrogen ions, [H⁺], is equal to the concentration of hydroxide ions, [OH^-].
Chemists often use square brackets to denote concentration, the concentration of  hydrogen ions can be written as [H⁺] and the concentration of hydroxide ions can be written as [OH^-].
So, for a neutral solution:

[H⁺] = [OH^-] = neutral solution 

Pure water is an excellent example of a neutral substance.

Some of the water molecules, actually very few of them, dissociate to form hydrogen ions and hydroxide ions:

H2O

H+ + OH-

Every time a water molecule dissociates, it produces one hydrogen ion, H⁺, and one hydroxide ion, OH^-, so that the concentration of hydrogen ions is always the same as the concentration of hydroxide ions.
Therefore, pure water is always neutral!

The pH of a solution is a measure of the hydrogen ion concentration in the solution. pH can be defined as:

pH = -log₁₀[H⁺]

This equation can be used to calculate the pH of our neutral water, but only if we know the concentration of  hydrogen ions in the water.

The concentration of hydrogen ions in water is not constant!

The concentration of hydrogen ions in water depends on the temperature of the water!
The dissociation of water molecules requires energy:

H2O + energy

H+ + OH-

If you put more energy into the system by heating it, then more water molecules dissociate, the concentration of hydrogen ions increases and the concentration of hydroxide ions also increases.
If you take energy away from the system by cooling it, then fewer water molecules dissociate, the concentration of hydrogen ions decreases and the concentration of hydroxide ions also decreases.
If we were to measure the concentration of hydrogen ions in pure water at various temperatures, we would find the following values:

Water temperature	[H+] x 10−7 M	pH
0°C	0.32	7.50
10°C	0.55	7.26
18°C	0.84	7.08
25°C	1.10	6.96
30°C	1.34	6.87
50°C	2.82	6.55
60°C	3.55	6.46
70°C	4.60	6.34
80°C	5.92	6.23
90°C	7.28	6.14
100°C	8.54	6.07

So what is the pH of water?
The pH of water is dependent on the temperature of the water.
Water is neutral for every value of pH because the concentration of hydrogen ions is always equal to the concentration of the hydroxide ions.
We can ONLY talk about the pH of water IF we state the temperature of the water.
For example, we can talk about water having a pH of approximately 7 at 25^oC, or we could say that the pH of water is approximately 6 at 100^oC.
Pure water is always neutral.
Pure water is neutral at 25^oC.
Pure water is neutral at 100^oC.

As Chemistry students, what we can't say is that water has a pH of 7, or that a neutral aqueous solution has a particular pH, unless we state the temperature of the system.

Further Reading:
Definitions of Acids and Bases
pH
Dissociation Constant for Water

Suggested Study Questions:

1. Plot a graph of temperature versus concentration of hydrogen ions in water. Describe the shape of the line, and write a generalization that links hydrogen ion concentration and temperature.
2. Plot a graph of temperature versus pH of water. Describe the shape of the line and write a generalization linking the  temperature of water and its pH.
3. Use your graph to find the pH of water at:
   • 12^oC
   • 22^oC
   • 32^oC
4. Construct a table giving the concentration of hydroxide ions in water at each of the temperatures shown above.
5. Plot a graph of temperature versus concentration of hydroxide ions in water. Describe the shape of the line, and write a generalization linking hydroxide ion concentration in water and temperature.
6. Use your graph to find the concentration of hydroxide ions in water at:
   • 12^oC
   • 22^oC
   • 32^oC
7. Explain why water is neutral at all temperatures.
8. Explain why the pH of water varies with temperature.

4 Water Experiments to Download

The International Year of Chemistry may be over, but you can still contribute to the Global Experiment until the end of March.

There are 4 experiments to conduct using water:

1. Acidity
2. Salinity
3. Build a water filtration unit using household materials
4. Using solar energy to purify water

You can download the details of all 4 experiments at

http://water.chemistry2011.org/web/iyc/experiments

You will be able to conduct the 4 experiments in class and submit your results to the website until 31^st March.

Even if you can't upload your results to the website, the 4 experiments above make a terrific teaching and learning resource for the future!

What is the pH of water?

The Brisbane Courier Mail ran a lift-out in their newspaper promoting science week on Tuesday 2^nd August 2011, and, within that, they included a Science Quiz of 20 questions.
Question 13 (an ill omen perhaps?) was, "What is the pH value of water?"

Now this is the kind of question that stumps a lot of High School Chemistry students, and unfortunately, most of the Australian population. So it was with some trepidation that I ventured to see what answer the Courier Mail came up with, and was not at all surprised to find out they got it wrong. Their answer was, ofcourse, 7. Feeling quite strongly about the perpetration of this misconception, especially in an attempt to promote Science Week, I emailed them to explain why the pH of water cannot be said to be 7. Unfortunately my explanation must have been insufficient, or the misconecption just too strongly believed, because they then changed their answer to the equally incorrect "the pH of neutral water is 7".

The September 2011 issue of AUS-e-NEWS takes a look at the dissociation of water and indicators.
To subscribe to AUS-e-NEWS, AUS-e-TUTE's newsletter, email
using subscribe as the subject line.

Hydrogen from Water Splitting

The production of hydrogen as an alternative fuel to current fossil fuels relies on the creation of a suitably cheap and efficient way to split water using the power of sunlight. Monash University scientists in Australia, working with UC Davis scientists in the USA, have found that a manganese mineral known as birnessite can be used as a catalyst to speed up the splitting of water into hydrogen and oxygen gases.

Birnessite, a soft, black mineral formed from precipitation reactions in lakes, oceans and groundwater, is predominantly an oxide of manganese, but calcium, potassium and sodium are also present in smaller amounts.
The formula for birnessite is (Na_0.3Ca_0.1K_0.1)(Mn⁴⁺,Mn³⁺)₂O₄ · 1.5 H₂O
As a catalyst for the water splitting reaction, the manganese in the birnessite cycles between oxidation states. First, when a voltage is applied manganese (II) is oxidized to manganese (IV). Then in sunlight, birnessite goes back to the manganese (II) state.

The water splitting reaction has two steps:

1. Two molecules of water are oxidized to form one molecule of oxygen gas, four protons and four electrons.
   
2. The protons and electrons combine to form two molecules of hydrogen gas

Reference:
Rosalie K. Hocking, Robin Brimblecombe, Lan-Yun Chang, Archana Singh, Mun Hon Cheah, Chris Glover, William H. Casey, Leone Spiccia. Water-oxidation catalysis by manganese in a geochemical-like cycle. Nature Chemistry, 2011; DOI: 10.1038/nchem.1049

Further Reading
Oxidation States (Numbers)
Oxidation and Reduction
Balancing Half Equations
Electrolysis - Electrolytic Cells
Percentage Composition

Study Questions:

1. What is meant by the term oxidation state (or oxidation number)?
2. What is the oxidation state (or oxidation number) for each of the following:
   
   • Mn³⁺
   • Mn⁴⁺
   • manganese (II)
   • manganese (IV)
     
3. Write equations to represent each of the following:
   
   • The oxidation of manganese (II) to manganese (IV)
   • The reduction of manganese (IV) to manganese (II)
4. For each reaction in question 3 above, identify:
   • the oxidant
   • the reductant
     
5. Write an equation to represent the first step in the water splitting reaction.
6. Write an equation to represent the second step in the water splitting reaction.
7. Use the equations in question 5 and 6 above to write an overall reaction for the water splitting reaction.
8. For each equation in questions 5 and 6,
   • label the reaction as an oxidation or reduction reaction
   • identify the oxidizing agent(s)
   • identify the reducing agent(s)
9. In the formula of birnessite, (Na_0.3Ca_0.1K_0.1)(Mn⁴⁺,Mn³⁺)₂O₄ · 1.5 H₂O, what does the 1.5 H₂O mean?
   
10. Calculate the percentage composition of birnessite.
    
    

A New Look at Evaporation

As much as 71% of Earth is covered by oceans and seas which evaporate continuously. Since the heat of evaporation of water is very high, the evaporation determines Earth's climate. What is more, the content of water vapour, the main greenhouse gas, in the atmosphere changes as a result of evaporation. Its concentration in air may reach as much as 4%, more than hundred times higher than that of the infamous carbon dioxide. According to various estimates, if there was no water vapour in air, the temperature on Earth would fall by 20-30 degrees.
The first scientific publication concerning the mechanism of evaporation was written by the famous physicist James Clerk Maxwell, but Polish scientists investigating evaporation are questioning how well we understand the phenomenon.
The investigation studied a drop of liquid in a closed vessel in equilibrium with its vapour. During evaporation the most interesting events take place on the border of a liquid and a vapour. The thickness of this interface is more or less equal to the diameter of an atom.
"Maxwell assumed that evaporation took place at constant temperature. It is so, if we look at the initial state, that is a liquid, and the final state, that is a vapour. It is true that their temperatures are equal. But during the evaporation process itself, the nature acts in a completely different way," explains Ph.D. Marek Litniewski from IPC PAS.
The existing description assumed that the heat transfer in the system was stable and the rate of evaporation was limited by the efficiency of the process during which the particles break away from the surface of drops, i.e. diffusion. However, the simulation carried out in the IPC PAS showed that during the evaporation into vacuum or the liquid's own vapour the system gained mechanical equilibrium very quickly. Particles break away from the surface of a liquid and their mechanical recoil allows the equalisation of the pressure inside the drop. If the rate of evaporation on the surface achieved the maximum value and the system was still unable to equalise the pressures, spaces with new surfaces would open inside the drop and it would start to boil. However, it was observed that the mechanical equilibration of pressure can be insufficient and the temperature on the surface of the liquid decreases: the drop aims at maintaining the pressure equilibrium at the cost of its internal energy. This observation suggests that the factor that is crucial during evaporation is not the diffusion of particles into the environment but the heat transfer and the equality of pressures.

Reference:
Institute of Physical Chemistry of the Polish Academy of Sciences (2010, October 20). Everything evaporates, but how?. ScienceDaily. Retrieved October 21, 2010, from http://www.sciencedaily.com­ /releases/2010/10/101020084149.htm

Further Reading
http://www.ausetute.com.au/chemphys.html
http://www.ausetute.com.au/intermof.html
http://www.ausetute.com.au/equilibrium.html
http://www.ausetute.com.au/heatlatent.html
http://www.ausetute.com.au/greenhouse.html

Study Questions

1. Write a chemical equation to describe the evaporation of water.
2. Is the evaporation of water a chemical or a physical change? Explain your answer.
3. Give the names and formulae of 4 natural greenhouse gases.
4. Give the names and formulae of 4 human-induced greenhouse gases.
5. Briefly explain what is meant by the terms Greenhouse Effect and Enhanced Greenhouse Effect.
6. What would be the difference between studying the evaporation of a water droplet in a closed vessel compared to studying the evaporation of a water droplet in a vessel open to the air?
7. Imagine you were undertaking a study of the evaporation of a water droplet in a closed system. In the first experiment you maintain a constant temperature of 25^oC and in the second experiment you maintain a constant temperature of 65^oC. What differences would you expect in the results of your study?