ACID AND ALKALIS

ACIDS• The common acids found in laboratory or in fruits/foods

are as follows:

Strong acid Weak acids

Hydrochloric acid Citric acid

Sulfuric acid Ethanoic acid

Nitric acid Malic acid

PROPERTIES OF ACIDS• All acids, either strong or weak, have the following

properties:

- Sour taste

- Able to conduct electricity

- Corrosive if concentrated

- Able to turn blue litmus paper red

- Able to react with metals, carbonate compounds and alkalis.

REACTION WITH METALS• The reaction between metals and acids produce salts and

hydrogen gas.

• Acid + Metal Salt + Hydrogen

• A lighted splint is used to test for the presence of hydrogen gas. It extinguishes the splint and gives a “pop” sound.

REACTION WITH CARBONATES• Acid react with carbonate compounds to produce salts,

carbon dioxide and water.

• Acid + Carbonate Salt + Carbon Dioxide + Water

• Limewater is used to test for the presence of carbon dioxide gas. The limewater turns chalky when carbon dioxide is bubbled into it.

REACTION WITH ALKALIS• Acid react with alkalis to produce salts and water only

• The reaction between acids and alkalis is called the NEUTRALISATION reaction.

• Acid + Alkalis Salt + Water

• Universal indicator can be used to show that the solution has become neutral after the reaction. It gives a pale green color in neutral solutions.

USES OF ACIDSAcids Uses

Sulfuric acid Used in car batteries

Hydrochloric acid Removes rust from iron and steel

Ethanoic acid Used in vinegars to preserve food

Nitric acid Used in the production of fertilisers

PROPERTIES OF ALKALIS• When dissolved in water, alkalis exhibit the following

properties:

- Soapy feeling

- Turning red litmus paper blue

- Able to conduct electricity

- Corrosive if concentrated

- Reaction with acids to form salt and water.

• The common alkalis used in the school lab are sodium hydroxide, potassium hydroxide and ammonia.

USES OF ALKALISAlkalis Uses

Calcium hydroxide They neutralize excess acid in soil

Aluminium hydroxide Used in antacid(or indigestion tablet)

Magnesium hydroxide Used in antacid and toothpaste

Sodium hydroxide Used in making soaps and detergents

PH SCALE• The pH scale is a measure of the relative acidity or

alkalinity of a solution.

• Strong acid weak acid Weak alkali strong alkali

EXAMPLES• Examples of acidic and alkaline solutions in our daily

lives:

INDICATOR• An indicator is a mixture of substances which shows

different color in different pH values of solutions.

• Universal indicator is an indicator which contains a mixture of substances that are able to give different colors corresponding to different pH values.

• It can be used to determine the strength of acids and alkalis by observing the change of its color.

• The table below shows the colors of Universal Indicator corresponding to the pH scale:

pH scale 0-2 3-4 4-6 7 8-11 12-14

Color of Universal Indicator

Red Orange Yellow Pale green

Green-blue

Violet

INDICATOR• Other types of indicator commonly used in the lab:

Types of indicator

Acidic Neutral Alkaline

Litmus Red - Blue

Phenolphtlein Colourless Colourless Pink ( > pH 9)

Methyl Orange Red (< pH 3.5) Yellow Yellow