Acids and AlkalisChapter 10Chemistry

2nd week

Detecting acids and alkalis

- Robert Boyle was an Irish scientist who lived 300 years ago.

- He decided to try and find and easy way to identify acids and alkalis.

- He knew that workers in France used plant juices to dye silk clothes.

- So he began testing plant juices to see if they would solve his problem.

- When he used red cabbage juice he found that:

Red cabbage juice

(purple)+ Acid Juice turns red

Red cabbage juice

(purple)+ Alkali Juice turns green

- When he used violets he found that:

+ Acid Juice turns Purple

+ Alkali Juice turns greenish yellow

- He discovered the colour change in litmus (juice from lichen)

- Litmus was used in laboratories around the world.

- It can be used as a solution of absorbed into paper strips.

- There are over 20 indicators that scientists use.

- 2 examples:- Methyl orange:Turns pink in acids.Turns Yellow in Alkalis.

- Phenolphthalein:Turns colourless in acid solutions.Turns Pink in alkaline solutions.

Acid Alkali

AlkaliAcid

pH scale- After indicators where found to identify acids and alkalis,

scientists wanted compare the strength of the acids and alkalis.

- In 1909, a Danish scientist called Soren Sorensen invented a scale called pH scale to compare the strengths.

- pH stand for ‘power of hydrogen’ because all acids hydrogen (active part in the chemical reactions)

- The pH scale starts from 0 – 14.

- The strongest acid is 0 and the strongest alkali is 14.

pH Classification0 - 2 Strong acid3 - 6 Weak acid

7 Neutral8 - 11 Weak alkali

12 - 14 Strong alkali

- N.B. An electrical instrument known as pH meter is used to measure pH accurately.

- The pH indicator is made of a mixture of indicators.

- Each indicator changes colour in the range shown in the scale.

- By combining the indicators a solution is made that gives different colours over the whole range of the scale