Answers for Dr Karl #1: Plastic Bags and Chicken Juice

Dr Karl is a smart guy. He has an extraordinary broad knowledge of science in general, as well as human and animal physiology.

The reason for this, of course, is that as a professional educator he gets to read a lot. I am, of course, intensely jealous – and one day hope to have the kind of job that he has.

I love listening to his regular segments on the ABC on Thursdays where people call in and ask him questions. Mostly he has an answer, but occasionally he gets asked a question relating to chemistry that he is unable to answer.

This should not surprise anyone, of course, – Dr Karl himself would be the first to admit that no one knows everything, and anyone who has a call-in radio show and is never stumped is having a lend of their audience. When I had my radio show on 6PR for example, it was quite common for me to say “I’ll have to look into that and get back to you next week.”

Anyhow, for the chemistry questions that Dr Karl can’t answer, luckily I am here.

Last week a caller asked why “chicken juice” leaks through plastic bags. What he was getting at was that whereas if you put water into a plastic bag it wouldn’t leak, if you had some chicken (or meat) in a plastic bag sitting on your kitchen bench top, when you pick it up later on invariably some of the juice has seeped through.

Why is it so?

The answer is simply that plastics are not completely impervious. That is, if you zoom down into the molecular microstructure of plastic bags you will see that it’s essentially a jumbled mess of strands with holes everywhere. The plastic bags that we get from the supermarket are PVC and because they are so thin there are enough faults in the microstructure to allow some things to seep through.

Imagine if you cooked up a kilogram of spaghetti and then tipped it out onto the floor and let it dry. When it had tried it would be a solid lump spread out on the floor. But although it would be a solid lump, between all the overlapping strands there would be gaps. Plastic is essentially the same. At a microscopic level it is made up of polymer strands all jumbled together with only weak interactions between them.

We know that there is weak interaction between the polymer chains, because the bag is flexible. That is, the polymer chains are able to move and flex in relation to each other.

And the reason that some liquids will leak through the bag, whereas water won’t, is probably related to the surface tension of water. If you put a drop of water on a hard surface and then put a drop of metho next to it you would observe a curious phenomenon: whereas the metho drop would flatten out on the surface, the drop of water would sit proud of the surface and would appear round on the edges.

The reason for this is the surface tension of water, which, because it is a polar compound, is very high. This surface tension would mean that the water would not be able to squeeze through the little faults in the plastic, but if there were other substances dissolved in the water they would tend to have the effect of decreasing the surface tension. This would mean that the liquid might be able to seep more readily through the pores.

Incidentally, anyone who works in the chemical industry knows this. Gloves ain’t gloves. If you are working with a toxic chemical that you need to protect your hands from, you need to know what type of glove to use, as some chemicals will seep right through certain gloves, and the most expensive chemical gloves are made up of multi layers of different plastics, to cover all their bases.

So that clears up one of life’s great mysteries – stay tuned for more “answers for Dr Karl.”

Book Extract #9: Oil & Water Chemistry #2

Examples of organic molecules are carbohydrates, proteins,  enzymes, oils and fats. These are of course all naturally-occurring molecules (and most are contained in foods you eat) but there are many organic molecules that are either non-edible (crude oil and all its derivatives – petrol and plastics) or fully synthetic (prescription drugs and specialised industrial chemicals).

You can see that carbon-based molecules are all around us – that’s why they have their own category.

We can demonstrate this ability of carbon to link to itself in long chains by looking at a molecule called octane:



If we count the atoms in this molecule, we arrive at the formula C8H18. Now, since scientists are lazy (but not evil and stupid like marketing people) we like to abbreviate structures like this to make them easier to draw:



By omitting the atoms all we have to draw are the lines linking them.  The zigzag structure tells us that at each elbow there is a carbon atom, with one on each end.  The hydrogens aren’t considered important enough to put on these abbreviated structures, but if there were any oxygen, sulphur or nitrogens, they would be drawn in.

Octane of course is the chemical that is used when classifying petrol grades.

The other major structure that carbon compounds adopt is the formation of rings, like xylene (paint thinners):



Once again we can abbreviate this to:



And of course the sky is the limit in terms of how big these molecules can get



Inorganic chemicals, on the other hand, tend to have far simpler structures, like common table salt (sodium chloride), which simply has the formula NaCl.


You will note the plus sign next to the Na and the minus sign next to the Cl.

This is very important and the whole purpose of this section.  Inorganic compounds have an uneven distribution of charge, whereas organic compounds have an even distribution of charge as they are composed of the same element (carbon).

This is important for one reason alone – the most common liquid on the planet (water) itself has an uneven distribution of charge, and it is impossible to understand the science of stain removal without understanding the chemistry of this remarkable liquid.

You see, strange as it may seem, water is a very unusual molecule.  It is only because it is so abundant that it doesn’t seem unusual, but if it was a lot less abundant than it was, it would be considered quite exotic.  The reason simply is that it is the second most polar liquid in existence (behind ammonia).

By the use of the term polar we simply mean that the charges on the molecule are unevenly distributed.  That is, in just the same way that the earth has the North Pole and the South Pole, and magnets have north and south ends, the water molecule has a positive end and a negative end.



The oxygen (the blue atom) sucks all the electrons up its end, thereby causing a negative charge.  At the other end of the molecule is the hydrogen (the red atoms).  There is a net positive charge at this end as the oxygen has taken all the electrons away from the hydrogens (we say that the oxygen is more “electronegative” – that is, it likes electrons more).

Another simplified way to look at it might be like this, with a charge separation:



Now, we all know that opposites attract.  A negative charge is attracted towards a positive charge and a positive charge is attracted towards a negative charge.

So the molecules in water will align themselves to look like this:



In other words, the negative end of one molecule will be attracted towards the positive end of another molecule, and this provides a very energetically stable structure for water as it aligns itself in this manner.

Now that’s all very well in the bulk of the liquid, but what happens on the surface?  The water molecules want to align themselves with other water molecules to balance their charge, but what happens to the molecules that draw the short straw and find themselves on the surface of the water?

Well, since air contains no charges, the charges on the surface are not balanced.  This causes tension, and is the reason that water has surface tension at all.  They are not at all happy with life, and if such a thing were possible, these molecules would need to see molecular counsellors to deal with the stress that this causes.

Now consider what happens if we mix the octane with the water. If you are a water molecule and you bump into an octane molecule, you’re not going to get along.  The reason is that as a water molecule, you are going to be looking for other water molecules so you can balance your charges.

So if you mix water and octane together all the water molecules will run around until they bump into each other and link up so that the positive and negative charges can be balanced out.  That’s why we get two separate layers – the water molecules want as little interaction with the oil as possible because they want as much interaction with each other.

Or to put it another way, they want to minimise the surface area of the interaction between the water and the oil, as this minimises the number of unbalanced charges.

We could put it this way – the octane is a pretty easy-going molecule – it has no problem with water. it has no particular agendas or issues.  But the water – well – it sure has some agendas and issues.  It needs to balance its charges. So the water has a problem with the octane because it can’t help it balance its charges.

And this affinity or hatred actually expresses itself in chemical terms such as hydrophilic (water loving), hydrophobic (water hating), lipophilic (oil loving), and lipophobic (oil hating).

So in general

hydrophobic = lipohilic


hydrophilic = lipophobic.

This is for example why there are two types of paints – solvent based and water-based.  Solvent based paints require turps to clean the brushes whereas water based paints require water.

So the simple rule of thumb is that like dissolves like – polar solids dissolve in polar liquids and nonpolar solids dissolve in nonpolar liquids. An example of a polar substance dissolving in a polar liquid would be table salt (sodium chloride) dissolving in water. If you tried to dissolve table salt in oil, you’d find it wouldn’t dissolve.

We saw above that sodium chloride has and uneven charge distribution.  In fact, the uneven charge distribution is so great that it entirely accounts for the very existence of the molecule.  In other words, they are stuck together by this very charge and nothing else – kind of like two magnets that are stuck together.

Consequently, if they find another source of charge, they will readily separate, and this is exactly what happens when you dissolve salt in water – the positively charged sodium ions surround themselves with the negative ends of many water molecules and the negatively charged chloride ions surround themselves with the positive ends of many water molecules:



Water is the only polar liquid on the planet in abundance.  Virtually every other liquid that occurs naturally is an oil, and it is the difference between these two, more than anything else, that explains why things stain.

To put it another way, most stains are non-polar, so they don’t wash out in water very well.  That’s why we need detergents, but that’s another story.

Book Extract #8: Oil and Water Chemistry #1

I remember the first time I saw kerosene and water mixed together.  We used to have an old kerosene heater and I was curious to see what would happen if you mixed the kerosene and water together.  Since the kerosene was a nice sky blue colour, I wondered whether it would make the water blue.

Much to my surprise, the kerosene and water separated into two discrete layers, with the kerosene on top.  I was fascinated by this – I’d never seen this before.  Two liquids were mixed together, but not mixing at all.

This phenomenon explains an awful lot of what we see in the world around us, particularly in the kitchen and laundry, so let’s look at – why don’t oil and water mix?

To understand this, we have to go right back to the beginning.  And we’ll work our way slowly forward from there.

The most basic things on earth are elements – everything in existence is composed of one or more of these. These can be represented in a table:


Now, here’s the funny bit. In terms of all the things that we see in the world around us, we can essentially divide everything into two categories.

If you look at the table above, in the second row you will see that element number 6 has the chemical symbol “C” – which of course refers to Carbon.

Carbon has a unique property – the ability to link to itself in long chains or rings of infinite length and with an infinite number of structures and shapes. Think of it as chemical Lego – you buy a box of Lego bricks and you can fashion them into any shape you want. So it is with carbon.

For this reason, carbon-based molecules have their own category – organic.

Organic chemistry is also the chemistry of life (hence the term “organic”). We are organic, as we are carbon-based life forms. Inorganic chemistry is everything else.

In other words, the two major branches of chemistry are organic and inorganic. Organic chemistry concerns itself with the chemistry of one element only – carbon, and inorganic chemistry concerns itself with the chemistry of the other 109 elements in the table above.

Why is Fertilizer Explosive?

Why is fertilizer explosive?

The answer is not obvious, and for years after the invention of ammonium nitrate, people didn’t actually know it was explosive. It was only after the Texas City Disaster that the penny dropped.

Ammonium nitrate is of course a completely synthetic product, which was only created after Fritz Haber in 1913 worked out how to convert nitrogen from air into ammonia. This process opened the way for the manufacture of nitrogen containing compounds as fertilizers.

So why is it explosive? Well it’s not too hard to understand if we look at the anatomy of an explosion. An explosion is simply a rapid combustion (oxidation) process. Coal burns, wearers coal dust explodes. The only difference is the rate.

When something burns, two components are required – the fuel and the oxidant. Usually, the oxidant is simply the oxygen in air. If we were to consider, for example, the combustion of the simplest hydrocarbon, methane, the reaction would be:

CH4 + 2O2 = CO2 + 2H2O

In other words, the flammable methane requires air to burn. This is why your car needs air to run. Without air, the fuel cannot burn. This is also the reason why people put turbochargers on engines – the extra air gives you extra oxygen and therefore a more rapid combustion rate.

Well, this is the chemical structure of ammonium nitrate

you will notice something interesting about it – it contains three oxygens already. What this means is that for it to burn, (or explode) it doesn’t need any air, as it supplies its own oxygen. This means that once the combustion process begins, there is nothing to stop the rate escalating rapidly, and hence the explosion.

Incidentally, this is the same principle behind rocket fuels, but that’s another story.

Book Extract #7: Irreversible Stains

There are some stains that you just can’t do anything about.  This is because you have not added something that is causing an unwanted colour or odour, but because you have removed (or damaged) the thing that is producing the colour that you want.

Almost exclusively this is referring to things that have been attacked either by bleach or acid.  If you have spilled bleach onto something that has discoloured it, then unfortunately there is nothing you can do.  The process of bleaching is irreversible.  Bleach (section 6.1) is an extremely strong oxidising agent that will destroy most pigments and dyes (particularly in clothes) and will irreversibly discolour some types of kitchen bench tops (particularly cheaper type synthetic laminexes) and paints.

The bottom line is this – if bleach has caused the problem then nothing will fix it.  Sometimes if you are lucky you can restore the surface by physically removing the first few layers by mechanical abrasion (see stain removing strategies) but generally it will never be what it was.  Another approach that may have some limited success is to attempt to steam out as much as you can.  Place a cotton tee shirt or paper towel over the damaged area, then put your iron on top and blast it with some steam.  With a bit of luck you may remove some of the staining.

The same is true of acids.  The two strongest acids that are encountered by the general public are hydrochloric acid (otherwise known as Muriatic Acid and freely available from hardware stores) and sulphuric acid (battery acid).

Both of these chemicals will vigorously attack anything metallic, and if you spill them on pavers, particularly those of a darker colour, they will discolour them irreversibly.

The take home lesson from this is that bleaches and acids, although they can be useful stain removing chemicals, are very strong chemicals and must be handled appropriately.