Chemistry - acids and alkali

Acids and Alkalis

Physical and Chemical Properties

Properties of Acids

1. Acids have sour taste.

2. Acids are corrosive.

3. Acids dissolve in water to form

solutions which conduct electricity.

Acids in Foodo Citric acid (in lemons)o Ethanoic acid (in vinegar)o Carbonic acid (in soft drinks)o Tartaric acid (in grapes)o Malic acid (in apples)o Lactic acid (in yoghurt)

Weak organic acids found in food

An acid is a substance which produces hydrogen ions when it is dissolved in

water.

The hydrogen ions give an acid its acidic properties.

Acids only behave as acids when they are dissolved in water.

What is an Acid?

TAKE NOTE

NOT all substances that contain hydrogen are acids.

For example, •NH3 (ammonia) contains hydrogen but it is not an acid. •H2 (hydrogen) is also not an acid.

Properties of Alkalis

1. Alkalis have bitter taste.

2. Alkalis have soapy feel.

3. Alkalis dissolve in water to form

solutions which conduct electricity.

• An alkali is a substance which produces hydroxide ions when it is dissolved in water.

• The hydroxide ions, OH-(aq) give an alkali its alkaline properties.

• Alkalis only behave as alkalis when they are dissolved in water.

What is an Alkali?

An acid is a substance which produces hydrogen ions when it is dissolved in water.

SummaryWhat is an acid?

Hydrochloric acid, sulfuric acid, nitric acid, ethanoic acid, citric acid …

Examples of acids?

An alkali is a substance which produces hydroxide ions when it is dissolved in water.

SummaryWhat is an alkali?

Sodium hydroxide, potassium hydroxide, calcium hydroxide, aqueous ammonia

Examples of alkalis?

Indicatorso An indicator is a substance which changes

colour depending on whether the solution tested is acidic or alkaline.

o There are many types of indicators, including the universal indicator and litmus indicator.

Litmus Indicator• Acids turn blue litmus paper red.o Litmus paper exists in two colours, red and

blue. • Alkalis turn red litmus paper blue.

Acids + Metals

Acids react with some metals to form a salt and hydrogen gas.

hydrochloric acid magnesiu

m

magnesium chloride

hydrogen gas+ +

sulfuric acid magnesium

magnesium sulfate

hydrogen gas+ +

1

Acid + Metal Salt + Hydrogen gas

Acids + Metals

The type and name of salt depends on the acid used.

Reaction with

• Hydrochloric acid

• Sulfuric acid

• Nitric acid

Chloride salts

Sulfate salts

Nitrate salts

Acids + Metals

Acids react with some metals to form a salt and hydrogen gas.

Some metals which can react with acids are magnesium, aluminium, zinc and iron.

Explosive reactions occur when using reactive metals like lithium, sodium and potassium.

Exception:

Unreactive metals such as copper and silver do not react with dilute acids.

Acids + Metals

How to test for hydrogen gas? Place a lighted splint in the presence of the gas.

If hydrogen is present, the lighted splint will be extinguished with a ‘pop’ sound.

Acids + Carbonates

Acids react with carbonates to form a salt, carbon dioxide gas and water.

2

hydrochloric acid

sodium carbonate

sodium chloride

carbon dioxide

gas+ +

nitric acid calcium carbonate

calcium nitrate

+

water+

carbon dioxide

gas+ water+

Acid + Carbonate Salt + Carbon dioxide gas + Water

Acids + Carbonates

How to test for carbon dioxide gas? Bubble the gas through limewater.

Carbon dioxide forms a white precipitate with limewater.

Acids + Carbonates

How to test for carbon dioxide gas? Bubble the gas through limewater.

Carbon dioxide forms a white precipitate with limewater.

Limewater

Bubble carbon

dioxide gasWhite

precipitate formed

Acids + Alkalis

Acids react with alkalis to form a salt and water only.

The reaction between acids and alkalis is called

neutralisation.

3

Acid + Alkali Salt + Water

Acids + Alkalis

Examples:

hydrochloric acid

sodium hydroxide

sodium chloride

Water+ +

sulfuric acidpotassium hydroxide

potassium sulfate

Water+ +