Testing Acids and Alkalis

  1. Click a liquid to dip one end of both (blue and red) litmus paper into that liquid
  2. Mouseout to dry the litmus paper
  3. Use a NEW set of litmus paper for each liquid
Blue Litmus   Red Litmus
Acids Neutral Alkalis
«   water   »
LITMUS is a substance extracted from organisms called lichens; it is commonly used in chemistry to indicate whether a solution is acidic or alkaline. Litmus paper is blue or red, depending on which form of litmus is present.

When acidic solution is placed on litmus paper, the blue litmus will turn red while the red litmus will stay red. Alkalis turn red litmus blue while the blue litmus stays blue. Distilled water is neutral and will leave the litmus colour unchanged. Litmus paper is a rough acid-alkali indicator and not a precise measurement.

All acids are not of the same strength; some are strong and some are weak. There are also strong and weak alkalis. Scientists use the pH (potential of Hydrogen) scale with a range from 1 to 14 to indicate the strength of an acid or alkali.

1 2 3 4 5 6 7 8 9 10 11 12 13 14
«« increasing acidic   increasing alkaline »»

Strong acids are pH 1 and strong alkalis are pH 14. Distilled water is pH 7 or pH neutral ... neither acidic or alkaline.

Universal Indicator is a pH indicator that has different colours to indicate the range of the pH of a solution. Strong acids turn the universal indicator red while weak acids turn it to orange or yellow. Water turns the universal indicator green. Strong alkalis turn it purple.

Hydrochloric acid in your stomach helps to digest your food; however you can get indigestion pains when there is too much acid in your stomach. Taking antacid (which contains a base1) helps relieve your digestive discomfort. This is because when an alkali and an acid are combined, they neutralize one another. This neutralization reaction is generally very fast and produces a salt and water (H2O).

A simple and early theory explains the neutralization reaction in this way:

Acids and bases are electrolytes i.e., when they are dissolved in water, they produce a solution that contains charged particles, or ions2. Acids produce excess hydrogen ions, H+ while bases produce excess hydroxyl ions, OH- when dissolved in water. Neutralization occurs when

H+ + OH- --» H2O
Applications of neutralization

  • Relieving indigestion (see above)

  • Making fertilizers by neutralizing concentrated ammonia with acids

  • Relieving pain from insect bites and stings

    - Use calamine lotion (which contains a base) for bee stings and ant bites
    - Use vinegar for wasp stings (which are alkaline)

  • Preventing tooth decay

    The action of bacteria on food particles trapped between our teeth produces an acid. Toothbase contains a base which neutralizes the acid on the teeth.

  • Treating acidic industrial waste products with quicklime or slaked lime
1 A base that dissolves in water is called an alkali.
2 An ion is an atom or group of atoms that carries a positive or negative charge as a result of having lost or gained one or more electrons.

Classifying Acids, Bases & Alkalis   |   Interactive Science Worksheets © by Alan & Hui Meng