An acid is a corrosive, sour-tasting substance that turns blue litmus paper red.

A base is a slippery, bitter-tasting substance that turns red litmus paper blue.

Substances that are neither acids nor bases are neutral. When you neutralize an acid or base, you make it neutral.

In L1 — A2, you learned that physical and chemical properties allow scientists to identify and classify matter. You have now learned that acids taste sour and bases taste bitter. But scientists do not rely on this property in order to identify acids and bases.

After much investigation, scientists discovered other properties of acids and bases. This allows substances that are too dangerous to touch or taste (such as battery acid) to be classified.

  

 

The table below lists some properties of acids and bases.

Acids	Bases
taste sour change litmus paper from blue to red react with metals corrosive neutralize bases conduct electricity pH less than 7	taste bitter change litmus paper from red to blue feel slippery corrosive neutralize acids conduct electricity pH more than 7

Identifying Acids and Bases

Scientists use indicators to identify acids and bases. An indicator is a natural substance that changes colour in the presence of an acid or base.

Some flowers are indicators. They change colour depending on the acidity of the soil. Lichens, grape juice, and tea will also change colour in the presence of an acid or base.

The most reliable way to identify an acid or a base is to use an indicator such as litmus, which is a dye made from lichen. Litmus paper is a indicator strip that has been treated with a weak solution of litmus.

 

 

You may have noted in the table above that the term pH was used. A substance’s pH tells you how acidic or basic it is. Knowing the pH of a substance helps you use it effectively and also safely.

Chemists have developed a scale that classifies how acidic or basic substances are. This pH scale classifies substances from 0 to 14.

• Acids have a pH of less than 7.

• Bases have a pH greater than 7.

• Substances that fall in the middle of the scale are neither acidic nor basic. They are neutral.

 

You can see where everyday substances fall on the pH scale above.

(Click on the + sign if you would like to see the image clearer.)

 Find milk on the pH scale above. The pH of milk is very close to neutral.
Now move your finger to the left of the scale. As you get closer to 0, the
substances get more acidic. As you move your finger toward 14 on the scale, the substances get more basic.

What is the pH of apple juice? Is it more or less acidic than milk?

 

Have you ever wondered why your hair looks great one day and bad the next? Did you consider that it may be because of the products you use on your hair?

Most shampoos are acidic. But what does that mean to your hair? If the shampoo you are using is too acidic, your hair will feel hard and it will break easily. Shampoos that have a pH of about 6 will make your hair smooth, and your hair will resist breaking.

Shampoo that is too basic will make your hair feel rough and will also break easily. The next time you wash your hair, notice how your hair feels.

Do you think your shampoo is too acidic? Too basic? Or just right?

In 1909, Eugene Schuller created the first commercial hair-colouring product. He was the founder of the L'Oreal company — a giant in the hair care product industry.

Two main chemical ingredients are involved in the hair-colouring process: hydrogen peroxide and ammonia. Both these chemicals are bases. They allow the hair to open up and allow colour to penetrate or move into the hair itself. If you are brave and like to experiment, check out a hair colouring product and try it. If you are not so brave, you can try a strand test: pull about 20 hairs from your head, mix 5 mL of colour and 5 mL of developer (peroxide) in a glass bowl. Apply this mixture to the test hairs and time the process according to the directions. Rinse and dry the sample, then look at the colour in various types of lighting to see if you like the new colour.