Indicators and pH

Most solutions of acids and alkalis are colourless.

How to tell if a solution is acidic or alkaline?

An indicator is a substance that has different colours in acidic and alkaline solutions.

Common examples: Litmus Phenolphthalein Methyl Orange

Grown in acidic soil – they are blue.

Grown in alkaline soil – they are pink/red

A set of numbers used to indicate whether a solution is acidic, neutral or alkaline.

A measure of the acidity or alkalinity of a solution is known as pH.

The pH of a solution is calculated based on the concentration of H+ or OH- present in a solution.

More acidic More alkaline Neutral

pH < 7: Acidic Contains H+

The smaller the pH, the more acidic the solution, the more H+ it contains

pH = 7: Neutral. This is the pH of pure water.

pH > 7: Alkaline Contains OH-

The greater the pH, the more alkaline the solution, the more OH- it contains

Manufacturers have come up with lipsticks that change colour when you put it on. Many believe that the colour of the lipstick depends on the wearer’s mood.

The colour of the lipstick depends on the pH of our skin (which is affected by diet, stress and physical activity level)

1. Universal Indicator A mixture of indicators Gives different colours at different pH values Approximate pH values

2. pH meter Accurate pH values pH probe is dipped into solution and pH is

shown on the meter either on a scale or digitally

3. pH sensor and computer A pH sensor connected to a computer through

an interface can be used to measure the pH of a solution

pH and the body Substances in your body have different pH

values.▪ Stomach (pH ~1.5)▪ Small intestine (pH ~8.4)▪ Blood (pH ~7.4)

E.g. If the pH of the blood is changed by one unit during an injection, the person will die!

pH and food preservation Fresh food go bad due to microorganisms (bacteria)

present. Microorganisms do not grow well in solutions of low

pH.

E.g. Acids can be used to preserve food. ▪ Ethanoic acid (vinegar)▪ Benzoic acid▪ Citric acid

pH and hair Shampoos are alkaline – to dissolve

grease. Alkalis damage hair – we must

always rinse away shampoo after use.

Hair conditioners – contain weak acids, to restore pH of hair to its normal value (weakly acidic: pH ~5)

pH and agriculture* The pH of the soil is important for good plant

growth. Most plants grow best when the pH of the soil

~5.6 (weakly acidic). Most plants will not grow well in soils that are

too acidic/alkaline.

pH and agriculture* Soils become acidic from:

▪ Extensive use of chemical fertilisers▪ Acid rain

Excess acidity can be neutralised by adding calcium hydroxide (‘slaked lime’) – ‘liming the soil’.

Bases are the oxides or hydroxides of metals. Contains either oxide ions (O2-) or hydroxide ions (OH-)

Base Formula Ions presentSodium oxide Na2O

Zinc oxide ZnO

Copper(II) oxide CuO

Magnesium hydroxide Mg(OH)2

Aluminium hydroxide Al(OH)3

Na+ ; O2-

Zn2+ ; O2-

Cu2+ ; O2-

Mg2+ ; OH-

Al3+ ; OH-

Oxides of non-metals (non-metal + oxygen) React with water to form acids React with alkalis to form salt and water

Acidic OxideAcidic Oxide FormulaFormula Acid Produced with WaterAcid Produced with Water

sulphur trioxide SO3 sulphuric acid, H2SO4

sulphur dioxide SO2 sulphurous acid, H2SO3

carbon dioxide CO2 carbonic acid, H2CO3

phosphorous(V) oxide

P4O10 phosphoric acid, H3PO4

Oxides of metals (metal + oxygen) React with acids to form salt and water

Basic OxideBasic Oxide FormulaFormula

magnesium oxide MgO

sodium oxide Na2O

calcium oxide CaO

copper(II) oxide CuO

Oxides of metals Behave as acidic oxide or basic oxide React with acids and alkalis to form salts

Amphoteric OxideAmphoteric Oxide FormulaFormula

aluminium oxide Al2O3

lead(II) oxide PbO

zinc oxide ZnO

Do not react with acids or bases Do not form salts

Examples: Water, H2O Carbon monoxide, CO Nitrogen monoxide, NO