Acid and bases (chapter 14)Indicators, acidity in soil and types of oxides

Prior knowledge

Objectives• Understand the use and characteristics of different common

indicators beside the Universal Indicator.• Understand the importance of controlling the pH in soils and

how the excess acidity can be treated.• classify oxides as acidic, basic, amphoteric or neutral based on

metallic/non-metallic character.

Common IndicatorsIndicator Colour

(acidic solutions)

Colour (alkaline solutions)

Methyl orange Red Yellow

Phenolphthalein Colourless Pink

Litmus Red Blue

Acidity in soil• The pH of soil needs to be controlled as it can affect the

growth and development of plants.

The hydrangea flower is pink in colour when grown in alkaline soil.

The hydrangea flower is blue in colour when grown in acidic soil.

Acidity in soil• Most plants grow better in soil that are neutral or slightly

acidic.• However, some plants may grow best in acidic or alkaline soil.• Question:

what can we do if the soil is acidic?

• Bases such as quicklime (calcium oxide, CaO) or slaked lime (calcium hydroxide, ca(OH)2) are used to neutralise the acidity of the soil to the desired point. This is known as liming the soil.

Types of Oxides• An oxide is a compound of oxygen and another element.• Oxides can be classified into four types:

(a) Acidic oxide

(b) Basic oxide

(c) Amphoteric oxide

(d) Neutral oxide

• Whether the element is a metal or a non-mental has a role to play in determining the acid-base behavior of an oxide.

Overview

Acidic oxides• Acidic oxides are formed from non-metals.• They are usually gases at room temperature and dissolve in

water to form acidic solutions with pH less than 7.• Example:

CO2(g) + H2O(l) H2CO3(aq)

• Acidic oxides react with alkalis to form a salt and water• Example:

CO2(g) + 2NaOH(aq) Na2CO3(aq) + H2O(l)

• Important: Acidic oxides do not react with acids.

Basic oxides • Oxides of metals are called basic oxides• Most basic oxides are insoluble in water.• Certain basic oxides, e.g. sodium oxide (Na2O), potassium

oxide (K2O) and calcium oxide (CaO), dissolve in water to form alkaline solutions.

• Basic oxides are solid at room temperature.• Basic oxides react with acids to form a salt and water

Example:CaO(s) + 2HCl(aq) CaCl2(aq) + H2O(l)calcium oxide

hydrochloric acid

Calcium chloride

Water

Amphoteric oxides• Amphoteric oxides are metallic oxides that react with both

acids and alkali to form salt and water.

Example: zinc oxide reacts with both acids and bases to form salt and water.

ZnO(s) + 2HCl(aq) ZnCl2(aq) + H2O(l)

ZnO(s) + 2NaOH(aq) Na2ZnO2(aq) + H2O(l)

zinc oxide (alkali)

hydrochloric acid (acid)

zinc chloride (salt)

water

zinc oxide( acid)

sodium hydroxide(alkali)

sodium zincate (salt)

water

Neutral oxides• Neutral oxides do not react with acid or alkali.• Neutral oxides are insoluble in water.• Examples:

- water, H2O- carbon monoxide, CO- nitric oxide, NO

Flow chart to determine type of oxide

Oxide X

Soluble in acid?

NoYes

Basic or amphoteric oxide Acidic or neutral oxide

Is the oxide soluble in alkali?

No Yes No Yes

Basic Oxide Amphoteric oxide

Acidic oxide Neutral oxide

What have we learnt?• Some other common indicators such as phenolphthalein and

methyl orange.• The importance of controlling pH in soil.• The chemicals used to control pH in soil.• How to classify oxides as acidic, basic, amphoteric or neutral

based on metallic/non-metallic character.