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

2. Prior knowledge

3. 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.

4. Common Indicators Indicator Colour (acidic solutions)
Colour (alkaline solutions)
Methyl orange
Red
Yellow
Phenolphthalein
Colourless
Pink
Litmus
Blue

5. 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.

6. 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.

7. 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.

8. Overview Elements Metals Basic oxides Amphoteric oxides Non-metals
Acidic oxides
Neutral oxides

9. 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) NaOH(aq)  Na2CO3(aq) H2O(l)
Important: Acidic oxides do not react with acids.

10. 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) HCl(aq)  CaCl2(aq) H2O(l)
calcium oxide
hydrochloric
acid
Calcium chloride
Water

11. 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) HCl(aq)  ZnCl2(aq) + H2O(l)
ZnO(s) NaOH(aq)  Na2ZnO2(aq) H2O(l)
zinc oxide (alkali)
hydrochloric acid (acid)
zinc chloride (salt)
water
sodium hydroxide
(alkali)
sodium zincate (salt)
zinc oxide
( acid)
water

12. 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

13. Flow chart to determine type of oxide
Oxide X
Soluble in acid?
Yes No
Basic or amphoteric oxide
Acidic or neutral oxide
Is the oxide soluble in alkali? No
Yes No
Yes
Basic Oxide
Amphoteric oxide
Neutral oxide
Acidic oxide

14. 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.