Monday, 4 March 2013

White Lines; Acid Queen

This is going to be serious and focussed, because it's supposed to provide educational materials.

Here are today's two videos:

Introduction to the chemistry of acids, bases and salts. Click to tweet: .

Acids are, according to some definitions, substances which produce hydrogen ions in solution.  Bases, or rather alkalis, are substances which produce hydroxyl ions in the same circumstances.  This is an inaccurate and sketchy definition, but it's the one used at GCSE and IGCSE so for the sake of that i'll stick to it for now.  Some compounds do both.  These are referred to as amphoteric.

Acidity and alkalinity are measured on the pH scale.  This indicates how many hydrogen ions are present in solution on a scale which changes tenfold with each step - a logarithmic scale (multiplies rather than adds).  Acids are low - below 7 - whereas bases are high - above 7.  Neutral is at 7 itself.

When an acid and a base react together, the process is referred to as neutralisation and the compound which results is called a salt.  Although table salt is a salt (for instance of hydrochloric acid and caustic soda), any such compound is also a salt.  The salt i make in this video, which is inaccurately measured for reasons which will be explained in Part II, is citric acid combined with sodium hydrogen carbonate, or sodium citrate.  This combination also indicates the test for carbonates, which tend to release carbon dioxide when combined with acids.

Please watch part II as well, which is here:

Acids And Bases Part II of II.  Click to tweet: .

Please watch this in conjunction with Part I as it follows on from that.  This is how to make indicator solution using red cabbage water, which works because of the anthocyanins.  I also go through some of the properties of acids and bases.  Bases are bitter and have a soapy feel to them because of saponification of the oil in your hands when you handle them, which brings up a whole philosophical issue but let's leave that for now.  Acids are sour, release hydrogen with metals and form metal salts from metal oxides.  The indicator i made goes red with acid, green with alkali.

Then i go on and on about how rubbish GCSEs are, which is probably not very helpful.  Sorry about that.

I've missed out a few things, which i'll cover here.  

Firstly, though i might've said this already, metals in stronger acids liberate hydrogen and replace the hydrogen itself in the acid, so for example sulphuric acid and copper form anhydrous copper sulphate and liberate hydrogen.  However, this doesn't happen in weaker acids.
Secondly, anthocyanin is not the only indicator and some respond better than it does at other pH levels.  One example is methyl orange:

which looks like this:

and is yellow in alkali and red in acid.  Another one is litmus, which is mainly 7-hydroxyphenoxazone, from the lichen Roccella tinctoria (note the species name), and some others.  Lichen is also used to dye tartans, such as ours:

Clearly i'm not that keen on litmus because lichen grows quite slowly.  Litmus turns blue with alkalis and red with acid.  There are a number of others, including phenolphthalein, a laxative which turns fuchsia with bases and orange or colourless (depending on the type) in acid.

Thirdly, here's something i've not emphasised enough, probably:  acids are acids, but bases are not necessarily alkalis.  In order to be an alkali, bases must dissolve in water.  Copper oxide is a base but not alkaline, for example.

Fourthly, i've said "hydroxyl" all the way through these videos when i should've said "hydroxide".  Sorry about that.

Fifthly, salts can be considered to be related to each other in families connected to their acids.  Sulphates are connected to sulphuric acid, nitrates to nitric acid and chlorides to hydrochloric acid.

That's it for the GCSE subject matter, but it's now got to the stage where this is really bugging me, so i'm going to scratch that itch now!

The problems with this:

  • The definition of an acid.
  • The assumption that all this is happening in water.
  • The idea that there are "hydrogen ions", i.e. actual free protons wandering about in water.
Let's start with the last thing first.  What actually happens is that "hydronium ions" form with acids in water, not hydrogen ions.  These are H3O (best i can do on here i think) ions - that's also inaccurate because protons associate with several water molecules at once.

Penultimately, water is not the only solvent.  Another example, oft-quoted, is ammonia, and since ammonia is itself a base, neutral pH in it is in a different place, which means that all acids are strong acids in it and there are relatively few alkalis with respect to it.  This may have consequences for the probability of extraterrestrial life, because it means that the nucleic and amino acids so important for life on this planet would all be strong acids in liquid ammonia.

Antepenultimately, and related to the previous point, acids can be defined in various ways.  The Bronsted-Lowry theory defines acids as proton donors and bases as proton acceptors.  This makes no difference to water but also enables the idea of acids and bases to be extended to other solvents than water, such as ammonia, sulphuric acid and glacial acetic acid (which is pure acetic acid - nasty stuff).  However, for that very reason, the Bronsted-Lowry theory fails to allow a more absolute definition of acid or base - they all depend on the solvent they're in.

One answer to this is the rather extreme version of the definition in which acids accept electron pairs and bases donate them.  This makes almost all chemical reactions acid-base reactions, which is weird and makes the whole notion meaningless.  This is known as the Lewis Acid theory.  An even broader definition is that and acid is anything which reacts with bases, gives up cations or accepts anions or electrons, which includes redox reactions.

Then there is Lux-Flood, which i can't understand right now as it says acids are oxides which accept oxygen and bases are oxides which donate oxygen, and i can't see how this applies to hydrochloric acid, except that hydrochloric acid is actually a gas called hydrogen chloride which dissolves in water to become acid, so maybe that's it.

There's also something called the solvent system.  It's probably the best but i haven't a clue what it is.

Phew!  That's better.

I'm now torn between doing something about urinalysis or something about how much GCSEs bug me because of the dumbing down.