Making salts

• substances with a pH of less than 7 are acids
• the stronger the acid, the lower the pH number
• acids turn blue litmus paper red
• they turn universal indicator red if they are strong, and orange or yellow if they are weak

• Substances that can react with acids and neutralise them to make a salt and water are called bases.
• They are usually metal oxides or metal hydroxides. For example, copper oxide and sodium hydroxide are bases.

• Bases that dissolve in water are called alkalis.
• Copper oxide is not an alkali because it does not dissolve in water, but sodium hydroxide is an alkali because it does dissolve in water.
• Alkaline solutions have a pH of more than 7.
• The stronger the alkali, the higher the pH number.
• Alkalis turn red litmus paper blue.
• They turn universal indicator dark blue or purple if they are strong, and blue-green if they are weak.

Metal hydroxides, such as sodium hydroxide, usually dissolve in water to form clear, colourless solutions. When an acid reacts with a metal hydroxide, the only products formed are a salt plus water. Here is the general word equation for the reaction:

acid + metal hydroxide    →    a salt + water

You usually observe these things during the reaction:

• there is a temperature rise
• the pH of the reaction mixture changes

Some metal oxides, such as sodium oxide, dissolve in water to form clear, colourless solutions. Many of them are not soluble in water, but they will react with acids. Copper(II) oxide is like this. When an acid reacts with a metal oxide, the only products formed are a salt plus water. Here is the general word equation for the reaction:

acid + metal oxide    →    a salt + water

You usually observe the same things during the reaction that you observe with metal hydroxides.

Although sodium carbonate can dissolve in water, most metal carbonates are not soluble. Calcium carbonate (chalk, limestone and marble) is like this. When an acid reacts with a metal carbonate, the products formed are a salt plus water, but carbon dioxide is also formed. Here is the general word equation for the reaction:

acid + metal carbonate    →    a salt + water + carbon dioxide

You usually observe bubbles of gas being given off during the reaction. You can show that the gas is carbon dioxide by bubbling it through limewater: this turns cloudy white when it reacts with carbon dioxide.

 Here are the word equations and balanced formulae equations for the reactions involving hydrochloric acid:

sodium hydroxide + hydrochloric acid   →   sodium chloride + water

NaOH + HCl   →   NaCl + H₂O

copper oxide + hydrochloric acid   →   copper chloride + water  

CuO + 2HCl   →   CuCl₂ + H₂O

calcium carbonate + hydrochloric acid   →   calcium chloride + water + carbon dioxide  

CaCO₃ + 2HCl   →   CaCl₂ + H₂O + CO₂

Here are the word equations and balanced formulae equations for the reactions involving sulphuric acid:

potassium hydroxide + sulphuric acid   →   potassium sulphate + water

2KOH + H₂SO₄   →   K₂SO₄ + 2H₂O

zinc oxide + sulphuric acid   →   zinc sulphate + water

ZnO + H₂SO₄   →   ZnSO₄ + H₂O

sodium carbonate + sulphuric acid   →   sodium sulphate + water + carbon dioxide

Na₂CO_{3 + H2SO4   →   Na2SO4 + H2O + CO2}