The Acid Test

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Late again! Looks like I was wrong when I said things would be back to normal. But now they will be. But, to make up for being late, I thought I’d post something very special. So it’s new section time! In the coming months, I hope to occasionally add to this section a whole host of experiments you can do from the comfort of your own kitchen (nowhere else mind, some of these might get messy). But, before we proceed, you need to go and find a responsible adult. Not just any old adult you understand. Some of them look responsible, but it’s a trick, they’re actually just as accident prone as children, some even more so, and are definitely not to be trusted with knives or fire. Have you found a responsible adult? Good. Then let’s do some science.

First things first… the experiment!

Kitchen Titration.

You might have heard of acids and alkalis. You might not. Briefly, things like vinegar are acids. They taste strongly sour. And thinks like soaps are alkali. I hope you don’t know what they taste like, but you should definitely know what they feel like. If not, that’s disgusting, go and wash your hands! With water. Which is neutral, or neither acidic nor alkaline. At the end of the experiment, I’ll explain why.

So, here is what you are going to need for your experiment. All of these items can be bought at the supermarket, that’s the point. (I went online for the pipettes, but that’s just because I didn’t have time to get them myself. You can find them at various home stores or DIY stores or pet shops or the like).

equip1

You need to be particularly cautious of these:

vindan

Basically, just don’t throw this into your eyes or down the whole bottle. It’s vinegar, you know how to use it. This is our acid. Next:

kinfedan

This is what you need your responsible adult for. They’ll be doing the cutting, or supervising the cutting. It’s sharp, be careful. Finally:

soddan

Again, your designated adult should handle this at all times. This is our alkali. In solid for it is pure and so can be very dangerous to skin and eyes. Your adult should wear gloves when you’re handling this and ALWAYS add the soda to water, not the other way around, to avoid making a super concentrated solution. Do not swallow this, at all, ever.

Ok. Now we’ve got the safety stuff out of the way and we’ve gathered all our equipment we can begin. What we’re going to do is we’re going to mix our acid and our alkali together until they are neutralised, like water is. But we need to know when that has happened so we need some way of telling what the pH (that’s just a measure of how acidic or alkaline something is and I’ll explain all about it at the end) of our solution is. That’s what the red cabbage is for.

Making a pH indicator.

This is really easy. First we need to cut up some of our red cabbage into small pieces. This is to get all the juice out of it so make sure it’s a fresh juicy bit of cabbage. CAUTION! Knives are sharp. You’ll need your responsible adult for this.

Next we put this into a shot glass and fill the glass with water. You can heat up the cabbage in the water in a pan at this point, but your pan needs to be very very VERY clean or you will contaminate your cabbage juice. It works just as well in cold water so we’ll stick with that.

Stir the cabbage in the water for a good five minutes to make a lovely blue/purple cabbage juice. This juice will be our indicator. Now we need to make our acids and alkalis.

cabprep

Making the samples.

Making the acid sample is easy. You want to half fill a shot glass with vinegar. Make sure it is white vinegar because you need to see the colour when you add the juice.

vinprep

Making the alkali sample is a bit trickier. Half fill a shot glass with water. Now you need to add the right amount of caustic soda. And by you, I mean your responsible adult. This should be enough of the soda crystals to cover the bottom of the shot glass, plus a little bit more. Again, put the water in first, then add the soda. You could use another shot glass to measure how much soda to use. I must stress here NOT to drink this solution. It’s cleaning solution, not for drinking. It could make you very VERY ill.

sodprep

Once you’ve added the caustic soda, stir it well. It should go cloudy and then clear.

Now you have your samples, we can add the indicator.

Testing the samples.

Add some indicator to the acid until the clear solution has a colour to it. You’ll see something very cool happen. When you add the purple cabbage juice, in the acid it turns red!

vintest

Add a bit of indicator to the alkali solution, again, just enough to colour it. This one will turn green! Amazing! So far though, this has all been set-up. Now we’re ready for the experiment.

sodtest

Performing a titration.

This experiment is very simple. All you need to do is add the acid to the alkali, or the alkali to the acid, whichever way round you fancy. You should add it slowly bit by bit until you notice a change. After a little while, you’ll see the colour disappears. If you add too much acid to the alkali the solution will go from green to clear to pink. If you add too much alkali to the acid it goes from pink to clear to green. When it is clear the solution has become neutralised. Neither acid nor alkali, just like water. But it is not water. You should NOT drink this. Ever. At all. It is not edible. Got that? Ok.

titrat

So, what is going on inside those solutions?

The Science.

pH stands for the very awesome sounding “power of Hydrogen” and what it basically means is how many hydrogen atoms you have floating around in your solution. It’s easiest to explain it with water first.

Water.

Water is pH 7, which is neutral. As I’m sure we all know, water is made of one oxygen atom and two hydrogen atoms. But it doesn’t all stick around as water. It’s a very lively, dynamic liquid, with the molecules constantly breaking apart and recombining.

h2o

When they break apart there are as many H atoms, which makes the solution acidic, as there are OH fragments. These OH fragments go around sweeping up the loose H atoms and it is these which make a solution alkali. Now you can see, whenever water breaks apart, there are as many H fragments as OH fragments, so they balance each other out and water is neutral.

Caustic Soda.

Caustic soda is made of a substance called sodium hydroxide. Sodium (written Na in chemistry shorthand) is actually a metal, and a very reactive one at that. Hydroxide just means that OH fragment. So when you put caustic soda in solution it breaks apart, like water, and you get a bunch of sodium atoms flying around and a bunch of OH fragments, so the soda is alkaline.

naoh

Vinegar.

Vinegar is a little more complicated. It looks like this.

vinmol

You can see we have that OH group again, so you would think it would be alkaline. But we all know it’s acidic. That’s because the carbon atom holds on to the oxygen atom much more strongly than the hydrogen atom does, so when it breaks apart, it’s the H atom that comes off, rather than the OH group.

vin

Neutralisation.

So when we add the vinegar and the soda solution together, eventually we have as many vinegar molecules as soda molecules and the two balance each other out and the solution becomes neutral.

neut

That explains why the colour changes after we add them together, but why does an acid or an alkali change the colour of the cabbage juice by themselves?

The indicator.

The molecule that gives the cabbage juice its colour is called anthocyanin and looks like this:

cab1

This is a big complicated molecule, but the only bit we care about is the bit right at the top, with two oxygens right next to each other. This molecule can exist in three different forms. As the solution becomes more acidic, there are more H atoms floating around and these join onto the oxygens and the molecule becomes pink. As there are more OH fragments, these scavenge the H atoms off the cabbage molecule, and it becomes green.

cab2

And there you have it! And it’s not just red cabbage juice that does this. Many plants change colour according to the pH of the soil they grow in. Hydrangeas for instance go from a deep blue in acidic soil, to a bright pink in alkaline soils. And now you know why!

Be sure to check back soon for more colourful home experiments!

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