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 Indicators Indicator Colour (acidic solutions) Colour (alkaline solutions) Methyl orange Red Yellow Phenolphthalein Colourless Pink Litmus 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 Elements Metals Basic oxides Amphoteric oxides Non-metals Acidic oxides Neutral oxides

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 sodium hydroxide (alkali) sodium zincate (salt) zinc oxide ( acid) 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? 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

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.