7D: Acids & Alkalis

What do you need to know?

  • Be able to place acids and alkalis on the pH scale.
  • Explain what we mean by neutralisation.
  • Suggest evidence that a chemical reaction has happened.
  • State the difference between an element and a compound. Be able to identify reactants and products in a reaction.
  • Be able to write simple word equations (in particular metal + acid reaction, and metal carbonate + acid reactions, naming salts).
  • Explain some effects of acid rain.
  • Be able to explain that gas tests for Hydrogen, Oxygen and Carbon dioxide.

Recap on elements & compounds

Acids and Alkalis

Indicators and the pH scale

When an acid is dissolved in water we get an acidic solution, and alkalis make alkaline solutions. If a solution is neither acidic nor alkaline we call itneutral. Pure water is neutral, and so is paraffin.

Indicators are substances that change colour when they are added to acidic or alkaline solutions. You can prepare homemade indicators from red cabbage or beetroot juice - these will help you see if a solution is acidic or alkaline.

Litmus and universal indicator are two indicators that are commonly used in the laboratory.

Litmus Paper

Universal Indicator

These are the important points about the pH scale:

  • neutral solutions are pH 7 exactly
  • acidic solutions have pH values less than 7
  • alkaline solutions have pH values more than 7
  • the closer to pH 0 you go, the more strongly acidic a solution is
  • the closer to pH 14 you go, the more strongly alkaline a solution is

Neutralisation

Metal oxides and metal hydroxides

Metal oxides and metal hydroxides are two types of bases. For example copper oxide and sodium hydroxide.

Here are general word equations for what happens in their neutralisation reactions with acids.

metal oxide + acida salt + water
metal hydroxide + acida salt + water

Notice that a salt and water are always produced. The mixture usually warms up a little during the reaction, too. The exact salt made depends upon which acid and base were used.

Why is neutralisation useful?

  • Farmers use lime (calcium oxide) to neutralise acid soils.
  • Your stomach contains hydrochloric acid, and too much of this causes indigestion. Antacid tablets contain bases such as magnesium hydroxide and magnesium carbonate to neutralise the extra acid.
  • Bee stings are acidic. They can be neutralised using baking powder, which contains sodium hydrogen carbonate.

Reactions of acids with metals

This is the general word equation for the reaction:

metal + acidsalt + hydrogen

It doesn't matter which metal or acid is used, if there is a reaction we always get hydrogen gas as well as the salt.

Carbonates

Carbonates and hydrogen carbonates are two other types of base. They also make a salt and water when we neutralise them with acid. But this time we get carbon dioxide gas too.

The reaction fizzes as bubbles of carbon dioxide are given off. This is easy to remember because we see the word 'carbonate' in the chemical names.

These are the general word equations for what happens:

acid + metal carbonatea salt + water + carbon dioxide
acid + metal hydrogen carbonatea salt + water + carbon dioxide

The Periodic Table

Evidence of a checmical reaction

  • Temperature change
  • Gas given off
  • Colour change
  • Formation of new substance

Testing for gases

  • Oxygen - Relights a glowing splint
  • Hydrogen - Squeaky pop!
  • Carbon Dioxide - Turns limewater milky

Acid Rain

Acidic gases such as sulphur dioxide are produced when fossil fuels like coal, oil and gas burn. Sulphur dioxide dissolves in the clouds and causes acid rain. This damages buildings, trees and harms life in rivers and lakes. It also causes chemical weathering of rocks to happen much faster than normal.