University of Edinburgh
 

BSL Chemistry Glossary - pH - example

Video: This demonstration is about acid, alkali and neutral. 

The first bottle holds an acid and the one on the right holds an alkali.  The middle one contains water which is neutral. I will explain more about them.

The pH scale is from 1 to 14 as seen on this colourful chart. The different colours will help us to identify whether it is an acid or alkali. For an acid, the pH value is less than 7 and for an alkali the pH is above 7. The colours change over the pH scale. Yellow – orange – red for acids and blue to purple for alkalis. 

I will show you 2 different ways that will help us to find out which solution is an acid or alkali.

1. pH paper
You can tear a strip of pH paper and dip it in the solution. Then try to match the colour with the chart to find the pH value and to see whether it is an alkali or acid. I have the 3 different solutions ready in conical flasks for testing. Remember for an acid, the colours are yellow – orange – red. What colour if I dip this in? This paper will change colour. It is red! Let's see with the chart – it is red so it must be an acid. So it is not safe to drink this solution! You must check the pH value by using this pH paper.  It is red, so it is a strong acid, same as car battery acid. It is very corrosive and can burn your hand.

Let's try the other solution. It is dark blue so it is a strong alkali. Same as bleach or oven cleaner, we use regularly to clean our homes. If you pour an alkali onto your hands, it will feel soapy.

Now let’s try the water. It is green – in the middle of the pH scale – pH7 – so it is a neutral liquid. The pH for water is always 7 if pure.

If you happen to mix up these solutions, you will not be able to know which is an acid, alkali or neutral! But you can use the pH paper to help you to find out!

Let’s try another technique:
2.  Universal indicator
I will use another indicator. Now, I will mix them up! Watch! Now, which is an acid or alkali or neutral? I will use this Universal Indicator and it will change colour with the addition of these 3 solutions. We can use the pH scale to help us.

Now! Which is neutral? This one - because it is green and according to the pH scale its pH value is 7 so it is neutral.
The second flask is red, and the pH is 1 so it is an acid. The last one is an alkali and the pH is about 14.