Year 10 Acids and Bases

Learning outcomes:

On completion of this unit students should be able to:

  • define acids, list their properties and give examples
  • distinguish between different types of acids
  • define bases and alkalis, list their properties and give examples
  • read and write the formula of common acids and bases
  • describe the action of acids and bases on indicators
  • explain how acidity can be measured
  • test and state the pH of everyday liquids
  • describe the formation of acid rain
  • define neutralisation and explain its action
  • describe the reaction of acids with metals and carbonates
  • design and conduct a fair test (extension)
Time allocation: 5 weeks            Assessment: written test

Acids and bases are:

  • found in food and drink
  • used around the home, garden and farms
  • used in industry to produce other chemicals

What are acids?

Acids are a family of chemicals that release acid particles ( hydrogen ions ) when dissolved in water. Acids have many features or properties in common.Some of these properties are physical ( those that do not involve chemical reactions ) while others are chemical ( how a substance reacts with other chemicals ). From your own experiences you may recognise these properties:

  • Acids have a sour or sharp taste. The word acid comes from the latin word acidus, which means sour.
  • Acids can be solids, liquids or gases. They show their acidic properties only when they are dissolved in water.
  • Acids are corrosive - they burn or irritate your skin and eat away at other substances.
  • Acids react with carbonates and make carbon dioxide gas.
One group of acids are the organic acids. They can be found in plants and animals and also in food and drink. The organic acids include:

  • tartaric acid ( grapes / wine / sparkling grape juice )
  • oxalic acid ( rhubarb leaves are poisonous - do not eat )
  • formic acid ( red ants )
  • citric acid and ascorbic acid ( citrus fruits )
  • lactic acid ( yoghurt )
  • acetic acid ( vinegar )
  • salicylic acid ( aspirin )
  • carbonic acid ( soft drinks )

Another group of acids are the inorganic or mineral acids. These acids are made from the minerals found in rocks and in the Earth's crust. They include the hydrochloric acid, sulfuric acid and nitric acid. These acids are stronger than organic acids so need to be handled carefully.

Acids are made of atoms. Another feature acids have in common can be seen in their formulas. All acids contain hydrogen ( H ), and the organic acids all contain the element carbon ( C ) as well.

Scientists say an acid is a substance that releases hydrogen ions ( H+ ) when it is dissolved in water. It is these hydrogen ions that make a solution acidic.

What are bases?

Bases are another family of chemicals. They are said to be the opposite of acids because they react with acids to neutralise them or decrease their acidity. Bases remove acid particles from a solution.

A base that can be dissolved in water is called an alkali.

Bases share some common physical and chemical features. From your own experiences you may recognise some of these features:

  • Bases / alkalis feel slippery
  • Bases / alkalis taste bitter (don't try this at home!)
  • Alkalis are liquids. Bases can be solids but they show these properties only when they are alkalis.
  • Bases / alkalis are corrosive. They burn or irritate your skin and eat away at other substances.
  • Bases / alkalis cancel out or neutralise the effects of acids but they can also be neutralised by acids.

Bases and alkalis are found around the home as well as in the science lab. 
For example:

Oven cleaner - sodium hydroxide
Quick-eze - calcium hydroxide
Baking soda - calcium bicarbonate
Liquid soap - potassium hydroxide
Handy Andy - ammonium hydroxide
Mylanta - magnesium hydroxide

Many of these bases are hydroxides. This means they contain hydroxide ions (OH-)

Strong bases, such as sodium hydroxide, release many hydroxide ions when they are dissolved in water. They form strong alkaline solutions. Weak bases, such as magnesium hydroxide, release only a few hydroxide ions to form weak alkaline solutions.

When a base is added to an acid, the hydroxide ions from the base combine with the hydrogen ions to from the acid to form water. This reaction neutralises the acid. 

Spot the difference - chemical indicators

Chemical indicators are made from dyes extracted from plants. Some indicators show whether a substance is an acid or an alkali. Others show the strength of the acid or alkali.


Litmus is a dye that dissolves in water. It is extracted from some types of lichen. Soaking absorbent paper in litmus dye and then drying it makes litmus paper, which is then used to see if a solution is and acid or a base.

Blue litmus paper turns red when dipped in acid but stays blue in an alkali.

Red litmus paper turns blue when dipped in alkali but stays red in an acid.

Universal Indicator

Universal indicator is a mixture of several indicators. It can be a green coloured liquid you add to a solution or a pale yellow-orange strip you dip into a solution. The colour the universal indicator turns when it is dipped into a solution can be used to indicate the solution's pH, for example red = pH 1, green = pH 7, purple = pH 14.

The pH scale

The pH scale is a more accurate way of measuring the acidity of a solution. It measures the quantity of hydrogen ions present in a solution. The scale goes from 0 to 14.

pH paper changes colour like universal indicator paper when dipped in acid or alkaline solutions. There are also digital pH meters, which have a probe that is dipped in the solution to be tested and gives a digital reading of the pH.

Many plants are natural indicators of pH. Hydrangeas have different petal colours depending on the acidity of the soil.
In very acid soil, petals are blue. In less acidic soils, the petals are purple or mauve. In neutral soils, the petals are red or pink.

Acids, Bases and the Human Body

Acids, Bases and pH in our Everyday Lives

Acid Rain


Acid-carbonate Reactions

Acid-base Reactions

Acids and Metals

What is a Salt?

Tell me about acids and bases


Ideas or concepts

Science skills

Explain with examples