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How to test for Acids or Alkalis?How to test for Acids or Alkalis?

Use an indicator.Use an indicator.

An indicator is a substance that An indicator is a substance that will change colour in will change colour in acidicacidic or or alkalinealkaline solution. solution.

✽ Litmus paper

✽ Universal indicator solution & paper

✽ Methyl orange

✽ Phenolphthalein

Indicators

Methyl orangeMethyl orange

below pH 3.1 above pH 4.4

3.1 ↔ 4.4

phenolphthaleinphenolphthalein

below pH 8.2 above pH 10.0

colorless ⇌ pink

Litmus paperLitmus paper

below pH 4.5 above pH 8.3

4.5 ↔ 8.3

IndicatorIndicator Colour in acidic

solution

pH range which

indicator changes colour

Colour in alkaline solution

Methyl orangeMethyl orange Red 3-5 yellow

**Screened **Screened methyl orangemethyl orange

Violet 3-5 Green

LitmusLitmus Red 5-8 Blue

phenolpthaleinphenolpthalein colourless 8-10 Pink

Indicator

Colour in acidic solution

Colour in alkaline solution

Blue litmus paper

Red litmus paper

Methyl orange

Phenolphthalein

Red

Red

Red

Colourless

Blue

Blue

Yellow

Pink

Colour chart of indicators

Inquiry time!

A solution is tested with a piece of red litmus paper and the paper stays red. Sue says that the solution is acidic. Marcus says he is not sure and that the solution should also be tested with a piece of blue litmus paper.

Who is correct?

Universal indicator

pure water

rain water

blood

some toothpaste

dilute sodium

hydroxide

vinegar

dilute hydrochloric

acid

detergentscanned fizzy drink

lemon juice

Universal indicator

Universal indicator

a a pH indicator that measures the degree of that measures the degree of acidity or alkalinity of solutions acidity or alkalinity of solutions

Very acidic – RedAcidic - Orange/Yellow

Neutral - Green

Alkaline - BlueVery Alkaline – Purple

A pH meter…….

a device which can accurately measure the acidity and alkalinity of solutions

Indicator

Natural Indicator

- Red Cabbage

Approximate

pH2 4 6 8 10 12

Colour Red Purple Violet BlueBlue-green

Green

URL:http://www.doscience.com/act_archive/home_activities/cabbage_juice/cabbage_juice2.html

Natural Indicator (Hydrangea flower)

Flowers are blue when grown in acidic soil

Flowers are pink/red when grown in alkaline soil

Summary Reaction with Indicators

Indicator Colour in acidic solution

Colour in alkaline solution

Blue litmus paper

Red litmus paper

Methyl orange

Phenolphthalein

Red

Red

Red

Colourless

Blue

Blue

Yellow

Pink