IndicatorsIndicators1. Indicators are dyes which change colour

when acidic or alkaline solutions are added to them.

2. Some common indicators are litmus, methyl orange, phenolphthalein and universal indicator.

IndicatorsIndicators

Best indicator-universal indicatoruniversal indicator, why?

Universal indicatorUniversal indicator- a mixture of many indicators- shows not only whether a substance is acidic acidic (yellow, orange or red)(yellow, orange or red) or alkalinealkaline (blue, indigoblue, indigo or violetor violet) but also if it is a strong acid or a weakstrong acid or a weak acidacid, a strong strong alkali or a weak alkali.alkali or a weak alkali.

Indicators

1. The different colours of the universal indicator colours of the universal indicator is associated with different pH numbersis associated with different pH numbers

2.2. Used in the form of paper or liquidUsed in the form of paper or liquid

Universal Indicator / pH Universal Indicator / pH indicatorindicatorUniversal Indicator / pH Universal Indicator / pH indicatorindicator

- Dip universal - Dip universal indicator indicator paper into solution paper into solution

- Observe colour - Observe colour changechange

- Compare colour with - Compare colour with colour chart colour chart

=> pH of solution=> pH of solution

- Dip universal - Dip universal indicator indicator paper into solution paper into solution

- Observe colour - Observe colour changechange

- Compare colour with - Compare colour with colour chart colour chart

=> pH of solution=> pH of solution

pH scale The pH scale is a range of numbers from 0 to 14, which indicates the strength of an acid or alkali (how acidic or alkaline a substance is).

pH scale

Strong Strong acidic acidic

solutionsolution

Strong Strong acidic acidic

solutionsolution

Weak Weak acidic acidic

solutionsolution

Weak Weak acidic acidic

solutionsolution

Strong Strong alkalinealkalinesolutionsolution

Strong Strong alkalinealkalinesolutionsolution

Weak Weak alkalinealkalinesolutionsolution

Weak Weak alkalinealkalinesolutionsolution

1-21-21-21-2 4 - 64 - 64 - 64 - 6 8-108-108-108-10 12-1412-1412-1412-14

GenerallyGenerally

Acid Rain - pH 2 to 5Acid Rain - pH 2 to 5

Normal Rain - pH 5 to 6Normal Rain - pH 5 to 6

Why is rain water slightly acidic?Why is rain water slightly acidic?

pH pH rangerange

Type of Solution

pH meter• an electronic device used to measure the

acidity or the alkalinity of solutions very effectively, more accurate than indicators

• pH electrode is placed into the solution pH electrode is placed into the solution and the reading is given on the digital and the reading is given on the digital displaydisplay

pH meter

A pH meter is an electronic device that can be used to measure the acidity or the alkalinity of solutions very effectively.

(a) pH meter is used to measure the pH of water in swimming pool

(b) pH meter is used to measure pH of the soil

pH and AgriculturepH and Agriculture• plants grow well in soil about plants grow well in soil about pH 6.5 pH 6.5 (about neutral) (about neutral)

• soil becomes soil becomes acidicacidic fromfrom

To To reduce acidity of soilreduce acidity of soil

-> acid rain-> acid rain-> extensive use of chemical fertilisers -> extensive use of chemical fertilisers such as ammonium sulphatesuch as ammonium sulphate

- add calcium hydroxide (slaked - add calcium hydroxide (slaked lime),lime), a cheap alkali a cheap alkali- spread calcium hydroxide powder - spread calcium hydroxide powder

over fields => over fields => liming the soilliming the soil