Kitchen Fun! Making frothy gels from vinegar and Gaviscon

Here’s a fun experiment for your kitchen, with safe ingredients from the supermarket and a surprising result. You will need: 

• Gaviscon liquid, a common treatment for heartburn
• Vinegar (I’ve used 5% acidity distilled vinegar, but anything should work)

All you have to do is mix 1 teaspoon (5ml) of Gaviscon liquid with 3 tablespoons (45 ml) of water and stir to disperse the Gaviscon into the water. Then, add 1 teaspoon (5ml) of the vinegar and immediately stir the mixture for a few seconds. 

You get an incredible change – within a few seconds, the watery liquid turns into a squishy lump of gel! Over the next minute or so, bubbles form and the gel expands slightly. The gel is quite stiff, but quite easy to break apart with your fingers. It was quite difficult to get out of the cup, since the gel is slowly expanding and gripping the sides. It also tastes quite good, but I should probably recommend that you don’t try that…

What’s going on? I’m not going to let you get away with having fun without learning something, so let’s delve in.

Gaviscon is a treatment for heartburn, which is often caused by stomach acid getting into the wrong place. The main ingredients in Gaviscon are sodium bicarbonate and calcium carbonate, which are both bases. They react with the excess stomach acid to neutralise it (which is why it relieves the heartburn), producing salts, water and CO₂ gas.  Our vinegar is acting in the same way as the stomach acid, which explains where the little bubbles in the gel come from – they are CO₂ generated from the acid+base reaction.

But what about the gel? The other main ingredient apart from water in Gaviscon is sodium alginate, a water soluble polymer extracted from seaweed. Gaviscon is a rather thick liquid, and this is because the sodium alginate polymer chains get tangled up with each other in the liquid, making it harder for the whole thing to flow.

The “-ate” ending in name of sodium alginate tells us this is a salt. That is, the product of an acid+base reaction itself. In this case the parent acid is alginic acid, which has lots of acid groups all along the length of the polymer chains. In the alginate form, these acid groups have a negative charge (as COO-) which helps keep them apart since like charges repel, and stops them getting too tangled. When we add the vinegar, which contains acetic acid, we get another acid+base reaction: sodium alginate + acetic acid --> alginic acid + sodium acetate. Alginic acid no longer has these negative charges (the COO- groups get converted to COOH) and the polymer is no longer very soluble. The polymer chains get strongly bonded together, forming a network throughout the liquid with the water trapped in between the chains. This is why we get a gel! This is probably what also happens in your stomach when you swallow the liquid, helping the Gaviscon to coat and stick to your insides.

You can also do a fun (and messy!) experiment to optimise the amount of Gaviscon and vinegar in the gel, but I’ll report on that in a separate post.

In my real job, I also play with polymers with solubility that depends on pH in the same way as sodium alginate. We've used these polymers to coat particles to produce pH-responsive suspensions, conduct fundamental studies into the swelling of thin films using several different analytical techniques, create sensors and study surfactant assembly.

Enjoy!

Footnote to fellow chemists; I think I’m right about the mechanism here. The pKa of the alginate acid groups is 3.4 and 3.7, as reportedby FMC BioPolymer. The pH of vinegar is apparently about 2.4, and I’m diluting by about 10x, which should give a pH of 2.9, enough to deprotonate the alginate. There could easily be something else going on here though – alginate also gels in the presence of calcium, which is definitely present in the calcium carbonate. Acetic acid + calcium carbonate gives calcium acetate, which would presumably dissolve and also gel the alginate?

Fewer brick walls and more warning signs: teaching students about the limitations of models

A complaint I occasionally hear from science and engineering students at university (and often see expressed online) is that every higher stage of education brings the revelation that things they knew before were "wrong" and this old knowledge is to be replaced by new ideas and models. The students seem to get frustrated with science because of this, become distrustful of their teachers, and ultimately might become turned off to further study in the field.

In some cases, this hierarchy of models feels natural, does no harm and serves everyone well. For example, much of basic chemical structure and bonding can be explained by assuming electrons are little charged orbiting balls being swapped or shared in bonds. Once quantum mechanics is invoked, students get the fuller picture and can explain more phenomena, but the 'little charged balls' model remains useful – even at the highest level, organic chemists still 'push electrons' around!

Image: Pumbaa/Greg Robson - wikimedia commons

However, in many cases, using simple models without warning the students about the limitations of these models causes trouble. I most often see this in online discussions or question-and-answer forums where non-experts ask scientific questions. People often have very "black and white" ideas about how the natural world works, when the truth is the many shades of grey. This all-or-nothing thinking leads people to contradictions, frustrations and misunderstanding, which could have been avoided if the questioner had been properly told the limitations of their model in the first place.

A good example: a questioner wants to know what will happen if they fill a thick metal container up to the brim with water, seal it and freeze the water in their freezer (of course, I know can’t find the original question, but here are a couple of similar examples). They know that water expands when it freezes, but they have also been told that liquids and solids are incompressible (whereas gases are not). This forms a frustrating contradiction: how can the water freeze and expand, if it's got nowhere to expand to? There is also an implicit assumption, that the water will actually freeze in their freezer. In fact, depending on the wall thickness of the metal container, the water will either freeze and cause the container to expand (or break!), or the pressure in the vessel will get so high that the freezing point of the water is depressed, and it will not freeze.

Had the student been properly told in the first place that solids and liquids "can be assumed to be incompressible" and that freezing points can vary, the student might not have been able to solve the problem, but it may remove the contradiction and confusion.

When teaching at university, especially on the foundation course, I like to make it very clear to students when I'm making an assumption which might get overridden at higher levels. This "but you don't need to know that..." approach could be seen as patronizing to the students (and I have been warned off this by at least one colleague), but my approach is not to "wall off" areas of the subject and tell them not to go there, but more like erecting warning signs and hazard tape around it: "only go there if you're happy to learn something more complex and difficult (which won't be in the exam!)". I give students the terms to search for if they want to know more, in clearly marked "advanced topic" boxes. While this may seem unsatisfactory at the time, I believe it will be more satisfactory for students (who don't get the feeling of being lied to) and teachers (who don't have to lie). I’m sure there is some established pedagogical language around this, but I think of it as 'meta-knowledge': some knowledge about the limitations of the knowledge they are being taught.

In an age when nearly every student carries a smart phone in their pocket, and can look up the deeper details in a few seconds of any topic that piques their interest, I would encourage educators to erect a few more "warning signs" and a few less "brick walls". 

Image: Eugene Zemlyanskiy - flickr

Scientists sound boring! Quantifying the 'monotone'

I read a blog post by Alan Mars last week about the excellent Material World science programme on BBC Radio 4, complaining about how all of the scientists interviewed on the show had such boring voices. Alan says,

"Sadly most of the interviewees speak within a very narrow band of auditory frequencies. Monotone in common parlance. It sounds as if the scientists vocal "loudspeakers" are turned in toward their body, rather than outwards towards a public that is thirsty for the latest news."

It doesn't help my scientific colleagues that the show's host, Quentin Cooper (hard at work in the studio, left, picture by bowbrick on Flickr), speaks in such an animated and engaging way.

Listening to the Material World podcast with Alan's quote in mind, the contrast between Quentin's delivery and that of the scientists was unmistakable. The differences become even more stark after a frequency analysis with Audacity.

The vertical axes in these plots are the pitch of the speaker's voice, and the horizontal axes time. The plots were produced using Audacity's EAC autocorrelation function to extract the pitch of the voice. I won't tell you which episode this is!


Here is Quentin (the host), introducing a show:

 

and here's the (male) scientist on immediately after:

The 'monotone' is striking!

Here's another bit of Quentin (this time as a Fourier transform frequency/time plot):

 

and here's the (female) scientist on next:

Flat as a pancake - it's not just the men who have a problem!

When we're training new lecturers, "don't speak in a monotone" is one of the first pieces of advice, followed by the trainer demonstrating a comedic robot-like example of a monotone speaker. However, it's clear that even when a scientist is putting their best foot forward (presumably, since they're on national radio!), they are clearly still speaking in a relatively monotonic voice compared to radio-trained voices.

I'm convinced that a bit of variation in speaking pitch and volume is one of the main factors which determines whether students perceive a lecture as boring or interesting, not necessarily what you're actually saying!