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Friday, December 9, 2011

A Simple Background to Acids and Alkalis Essential for KS3 Science.                                                                                                                                    
One important group of chemicals is acids.
Some common acids are – Hydrochloric Acid
                                                 Sulphuric Acid
                                     Nitric Acid
                                     Ethanoic Acid
Acids are always used in the liquid form in the laboratory.
There are many acids present in our everyday lives. Lemon juice contains citric acid, ant and nettle stings contain methanoic acid, tea contains tannic acid and vinegar contains ethanoic acid.
You can easily tell if a substance is an acid or not by its’ effect on litmus, litmus is a purple dye which comes in the form of a solution or as strips of red and blue paper. Blue litmus paper is used for testing acids, if the paper or the solution turn red then the substance is an acid. Red litmus paper cannot be used as neutral solutions would keep the paper red aswell as acidic solutions.
Acids have many properties that distinguish them from other substances.
These are:
  1. Acids have a sour taste, e.g. the taste of vinegar ( ethanoic acid)
  2. Acids turn litmus solution red and turn blue litmus paper red also
  3. Acids have pH numbers less than 7
  4. Acids react with metals, forming hydrogen and a salt
  5. Acids react with carbonates, forming a salt, water and carbon dioxide
  6. Acids react with alkalis, forming a salt and water
  7. Acids react with metal oxides, forming salt and water
Remember that:
  • The salts of sulphuric acid are known as sulphates
  • The salts of hydrochloric acid are known as chlorides
  • The salts of nitric acid are known as nitrates
There are different strengths of acids; some strong acids are hydrochloric acid, sulphuric acid and nitric acid. Some weak acids are ethanoic acid, citric acid and carbonic acid. This is all to do with the number of H+ ions formed in the solution.
Another important group of chemicals are alkalis.
Some common alkalis are – Sodium Hydroxide
                                     Potassium Hydroxide
                                     Calcium Hydroxide
These are usually solid s but are used in aqueous solutions in the laboratory.
Alkalis are present in many cleaning substances in use in our homes today, especially in kitchen cleaners like oven spray, floor cleaners and creams for sinks. Kitchen cleaners are alkaline because they contain ammonia or sodium hydroxide, which attack grease.
You can easily figure out if a substance is alkaline by its’ effect on litmus solution or red litmus paper. Alkalis turn litmus solution blue and red litmus paper blue.
Alkalis have many characteristics, which separate them from other chemicals.
These are:
  1. Alkalis feel soapy to touch
  2. Alkalis turn litmus solution blue or turn red litmus paper blue
  3. Alkali solutions have pH numbers greater than 7
  4. All alkalis ( except ammonia) will react with ammonium compounds
  5. All alkalis react with acids
There are different strengths of alkalis; this is to do with the formation of OH- ions in the solution. Some strong alkalis are calcium hydroxide, sodium hydroxide and potassium hydroxide. The most recognisable and common weak alkali is ammonia.
The pH of Substances, Universal Indicator and Neutral Solutions
There are many different strengths of acids and alkalis. The strengths of these are judged using the pH scale. The numbers range from 0 – 14.
Click here to see a diagram:
An acidic solution has a pH number less than 7, the lower the number the stronger the acid. An alkaline solution has a pH number greater than 7, the higher the number the stronger the alkali. A neutral solution has a pH number of exactly 7.
You can find the pH of any solution by using universal indicator. Universal Indicator is a mixture of dyes, like litmus it can be used as a solution or as universal indicator paper. It goes a different colour at different pH values. It ranges from red (a strong acid) to a deep violet (a strong alkali). Universal indicator turns neutral solutions green.
Neutral substances are those that are neither acid nor alkali. Some common neutral substances are:
Sodium Chloride (common salt)
Sugar Solution
An alkali can neutralise  an acid and destroy its’ acidity. In fact an acid can be neutralised by a number of compounds called bases. These are a few examples:
Metal Oxides
            Metal Hydroxides
            Metal Carbonates
            Metal Hydrogen Carbonates
Ammonia Solution
Any compound that can neutralise an acid is called a base. Alkalis are soluble bases. Neutralisation always produces a salt.
Acid + Metal Oxide – Metal Salt + Water
Acid + Metal Hydroxide – Metal Salt + Water
Acid + Metal Carbonate – Metal Salt + Water + Carbon Dioxide
Acid + Metal Hydrogen Carbonate – Metal Salt + Water + Carbon Dioxide
Acid + Ammonia Solution – Ammonium Salt + Water
Applications of Neutralisation
Insect Stings: Bee stings are acidic and can be neutralised with baking soda (bicarbonate of soda). Wasp stings are alkaline and can be treated with vinegar to neutralise.
*Remember – Bee Bycarb
                        Vee Vasp*
Indigestion: Our stomach carries around hydrochloric acid. Too much of this leads to indigestion. Too cure indigestion, you can neutralise the excess acid with baking soda or specialised indigestion tablets.
Soil Treatment: When soils are too acidic (often as a result of acid rain) they can be treated with slaked lime, chalk or quicklime, all alkalis. Plants and crops grow best in neutral soils.
Factory Waste: Liquid waste from factories is often acidic. If it reaches a river it will destroy and kill sea life of many forms. Neutralising the waste with slaked lime can prevent this.
If you have any comments or queries on this subject, please contact me at:
 Junior Science Department: Loreto College, Coleraine

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