ACID AND ALKALINE
Substances are classified as acidic, alkaline or neutral. Litmus paper can be used to determine these properties. Litmus papers are chemical indicators. There are blue and red litmus papers. They change colour when they come into contact with acidic or alkaline substances.
If a substance changes the colour of a blue litmus paper to red, it is acidic.
Properties of acids
 |
| Acidic : Turns blue litmus paper to red |
1. They are liquids.
2. They are solutions of compounds in water.
3. If concentrated they can be corrosive.
4. Acids taste sour.
5. Turn blue litmus paper red.
6. Usually react with metals to form salts.
 |
| Pineapple |
 |
| Vinegar |
 | |
| Tamarinds |
 |
| Limes |
If a substance changes the colour of a red litmus paper to blue, it is alkaline.
Properties of alkalines
1. They feel soapy to touch.  |
| Alkaline : Turns red litmus paper to blue |
2. They are soluble bases.
3. Like acids, they can burn the skin.
4. They turn red litmus blue.
5. They taste bitter.
If both litmus papers do not change colour, the substance is neutral.
Properties of neutral substances
1. Litmus paper is not affected by neutral paper.  |
| Neutral : Do not change the colour of litmus papers |
2. Tend to be harmless
 |
| Sugar |
 |
| Salt |
 |
| Mineral water |
The Strength of an Acid or Alkaline
Acids and alkalis can be strong or weak!
So how can we measure their strength?
The strength of an acid or alkali is shown using a scale of numbers
called the pH scale.
The numbers go from
0-14.
The pH scale
On the scale it follows that:
- An acidic solution has a pH number less than 7
- An alkaline solution has a pH number greater than 7.
- A neutral solution has a pH number of exactly 7.
You can find the pH of any solution using universal indicator.
Universal indicator is a mixture of dyes. It comes as a solution or in paper.
Universal indicator will change
from green to a different colour depending on the pH of the
solution you place it in.
NOTES:
- All the images above were taken from Google's images
