What is an acid?
An acid is a substance that produces hydrogen ions, H+, in aqueous solution. The table below shows some common acids and their formulae.
Properties of Acids
1. Acids have a sour taste. 2. Acids dissolve in water to form solutions which conduct electricity. 3. Acids turn blue litmus paper red.
Reaction with Metals
Acids react with reactive metals to form hydrogen gas and a salt. The general equation for the reaction is: metal + acid → salt + hydrogen We can test for hydrogen gas by placing a lighted splint at the mouth of the test tube. Hydrogen gas extinguishes the lighted splint with a ’pop’ sound. NOTE: Not all metals react with acids to give a salt and hydrogen gas. - Unreactive metals like copper and silver do not react with acids. - Lead appears not to react with dilute hydrochloric acid and dilute sulfuric acid. An insoluble layer of lead(II) chloride or lead(II) sulfate produced forms a coating around the metal, protecting it from further attack by the acid. The video below demonstrates the reaction between metals and acids.
Reaction with Carbonates
Acids react with carbonates and hydrogen carbonates to form a salt, water and carbon dioxide. The general equation for the reaction between acids and carbonates is: carbonate + acid → salt +water + carbon dioxide The general equation for the reaction between acids and hydrogen carbonates is: hydrogen carbonate + acid → salt + water + carbon dioxide We can test for carbon dioxide gas by bubbling the gas through limewater. Carbon dioxide forms a white precipitate with limewater. Precipitate refers to insoluble solid particles produced in a liquid by chemical reactions. The video below demonstrates the reaction between an acid and a carbonate.
Reaction with Metal Oxides and Hydroxides
Acids react with metal oxides and hydroxides to form a salt and water only. The general equation for the reaction of metal oxides with acid is: metal oxide + acid → salt + water The general equation for the reaction of metal hydroxides with acid is: metal hydroxide + acid → salt + water The video below shows how metal oxides react with acids to form a salt and water.
Role of Water in Acids
Acids only display their properties when dissolved in water, as they only produce hydrogen ions (H+) in water. The hydrogen ions give acids their acidic properties.
Uses of Acids
Sulfuric Acid - manufacture of fertilisers - manufacture of detergents - battery acid in cars Hydrochloric Acid - cleaning impurities (e.g. rust from metals) - leather processing Ethanoic Acid - vinegar (as food preservative and flavour enhancer) - manufacture of adhesives Phosphoric Acid - flavouring in food and beverages
Bases
A base is any metal oxide or hydroxide that reacts with an acid to produce a salt and water only. A base contains either oxide ions (O2-) or hydroxide ions (OH-).
Akalis
An alkali is a base that is soluble in water. Sodium hydroxide and potassium hydroxide are examples of alkalis.
Properties of Alkalis
1. Alkalis have a bitter taste and feel soapy. 2. Alkalis turn red litmus paper blue.
Reaction with Acids
Alkalis react with acids to form a salt and water only. The general equation for the reaction is: alkali + acid → salt + water In a neutralisation reaction, the hydrogen ions, H+, from the acid and the hydroxide ions, OH-, from the alkali react to form water. H+(aq) + OH-(aq) → H2O(I) Refer to the video below for a clearer idea of neutralisation reactions.
Reaction with Ammonium Salts
Alkalis, when heated with ammonium salts, give off ammonia gas. The general equation for this reaction is: alkali + ammonium salt → salt + water + ammonia Ammonia gas can be recognised by its characteristic pungent smell. We can test for ammonia gas with a piece of moist red litmus paper. Ammonia gas turns the moist red litmus paper blue. The video below shows the reaction between an alkali (sodium hydroxide) and an ammonium salt (ammonium chloride).
Reaction with Metal Salts
Alkalis react with a solution of one metal salt to give another metal salt and metal hydroxide. The general equation for the reaction is: alkali (containing metal A) + salt (of metal B) → salt (of metal A) +metal hydroxide (of metal B) The resulting metal hydroxide appears as a precipitate if it is insoluble in water. The video below shows the reaction between copper sulfate (a metal salt) and sodium hydroxide (an alkali).
Uses of Bases and Alkalis
Magnesium oxide - used as antacid for relieving gastric pain - used in making refractory bricks Sodium hydroxide - used in making soaps and detergents Calcium hydroxide - used to reduce acidity in soil Calcium oxide - used to neutralise acidic soil - used to make iron, concrete and cement Ammonia solution - used in window cleaning solutions - used to make fertilisers
Strong and Weak Acids
Strength refers to how easily an acid or alkali ionises when dissolved in water. A strong acid is an acid that is completely ionised in aqueous solution. A weak acid is an acid that is only partially ionised in aqueous solution. NOTE: Do not confuse the strength of an acid with its concentration. - ’Strong’ and ’weak’ refer to the extent of ionisation of an acid. - ’Concentrated’ and ’dilute’ give an idea of how much of an acid is dissolved in the solution. The strength of an acid is not affected by its concentration. - dilute acid solution ≠ weak acid solution - concentrated acid solution ≠ strong acid solution
pH Scale
The pH scale is a set of numbers from 0 to 14 which is used to indicate whether a solution is acidic, neutral or alkaline. - Acids have pH values less than 7. - Alkalis have pH values greater than 7. - A neutral solution has a pH value of exactly 7.
pH and Concentration of Hydrogen Ions
The pH of a solution is related to the concentration of hydrogen ions or hydroxide ions present in a solution.
How can we measure the pH of a solution?
The pH of a given solution can be determined by using - an indicator - pH sensor attached to a data logger - pH meter
Universal Indicator
The pH value of a solution can be determined by using Universal Indicator. It comes in the form of a solution or pH paper and gives different colours in solutind of different pH. The video below shows how the pH of an unknown solution can be determined using the Universal Indicator.
Other Indicators
Each indicator has one colour in an acidic solution and another colour in an alkaline solution. It will change colour at a certain pH range. The table below shows the colour changes of some indicators.
Why is soil pH important?
It is important to control the pH of soil as it affects the growth and development of plants. Most plants grow best when the pH is around 7.
How can we treat excess acidity?
Quicklime (calcium oxide) and slaked lime (calcium hydroxide) are often used to reduce the acidity of the soil. This is known as ’liming’ the soil.
