What is Universal Indicator and How To Use it
What is Universal Indicator?
Universal indicator is a substance which tells you by means of a colour change whether a substance is an acid or a base. It is used by scientists in laboratories all the time, as they need to know which substances are acids and which are bases.
Not only that, but universal indicator can tell you how strong an acid or base is, so you know which ones to avoid and which ones are safe! Car battery acid would burn your finger to the bone should you go near it!
This topic is always learned off by heart by students all over the world, so I thought it would be helpful to help them by providing some extra material. Of course, this hub is for everyone and even includes some experiments!
Acids, Bases and Litmus
Before we delve into the world of universal indicator, it is time to learn about the pH scale and acids and bases are.
Substances are divided into three categories; acids, bases and neutral substances. An acid is a substance which has a sharp taste and if is strong, it will burn you. An example of an acidic substance is lemon juice. As you can see, lemon juice is sour, making it acidic.
Bases (or alkalis) are substances which have a soapy feel and are good for cleaning. An example would be toothpaste as it washes your teeth or detergent which washes clothes.
Neutral substances are 'in the middle'. They do not burn your skin or have distinct tastes. They are virtually harmless, and the most common neutral substance of all is water.
However we cannot distinguish all substances to be acidic or basic. Using the methodologies outlined above such as seeing if they are soapy or have a sharp taste will not always work. Instead, a substance called 'litmus' is used. It tells us by means of a colour change whether a substance is acidic, basic or neutral.
Experiment: How To Make Litmus Paper
If you want to determine whether substances are acids, bases or neutral, you can make litmus paper. It is an easy and fun experiment and uses common household substances. Here are the things you need:
- Red cabbage
- Blotting Paper
- Hob Pan
- Baking Soda
Now that you have the things you need, it is time to make the litmus paper!
- Chop the red cabbage and add it to a pan filled with hot water. Always be careful when handling hot water!
- As soon as you see red juice coming out from the cabbage, give it a mix and leave it there simmering gently for ten minutes.
- Then, strain the red cabbage from the purple water.
- Cut your blotting paper into strips and immerse into the red solution. Leave them there for a few minutes to soak in the solution.
- Take out and leave to dry.
To test you litmus paper, dip it into a substance of your choice. If the paper turns red, that means it is an acidic substance. If it turns blue or green, then it is a base. If it doesn't change colour at all, then it is a neutral substance. For maximum effect, try these substances:
- Lemon Juice
- Baking Soda (mix a tablespoon with a little water and then dip your litmus paper in.)
- Tomato Juice
- Washing Up Liquid
- Rainwater (great for testing for acid rain in your area)
The pH Scale
Although you may know whether a substance is acidic or basic using litmus paper, you don't know how strong or weak that acid is. For example, the acid in your stomach is so strong it would burn your skin down to the bone! Or sodium hydroxide, a strong base is caustic meaning it will burn your flesh too.
On the other hand, acids like lemon juice and vinegar are very weak, they are virtually harmless. That is why scientists devised a scale known as the pH scale.
Very strong acids are on the lower end of the scale, from 0 to 2 whilst super strong bases are on the upper end of the scale. Neutral substances are at the number 7.
You may be wondering why there is a colour attached with each number of the pH scale. This is where universal indicator comes in! When you add a drop of universal indicator to a substance, it will turn an exact colour or an exact shade of a colour. You then get your copy of the pH scale attached with the universal indicator and match up the colour with the number on the pH scale. That number will then tell you how strong the acid or base really is.
- 0-3 means a very very strong acid!
- 4-6.5 means a weak acid
- 7 is neutral.
- 8-10 is a weak base
- 11-14 is a very strong base!
There are two types of universal indicator. The first type is universal indicator solution, where using a pipette you drop a few drops of it into your substance.
The second type is universal indicator paper. You simply dip it into the substance of your choice and it will change colour. You can dispose these afterwards.