Acids, Bases and Alkalis Done by: Glenn Tan (26) Science Ace Term 2
acids • An acid is a substance that produces hydrogen ions, , when it is dissolved in water. • These hydrogen ions are responsible for the properties of acids
Properties of acids • Acids have a sour taste • Acids conduct electricity in aqueous solution • Acids are hazardous • Dilute acids are irritants; they cause skin irritation • Acids change the colour of indicators • Acids turn blue litmus paper red • Turn Universal Indicator orange or red
Properties of acids • Acids react with reactive metals to form hydrogen and a salt. • React with carbonates to form salt, carbon dioxide and water.
Properties of acids • Acids react with metal oxides and hydroxides to form a salt and water only. This reaction is called neutralization reaction. (reaction with metal oxide)
Uses of acids • Manufacture agricultural fertilisers • Manufacture detergents, paints, dyes, artificial fibres and plastics • Used in batteries for road vehicles • Removal of rust • Preservation of foods
Different types of acids • Hydrochloric acid ( Digestive juice in stomach, used for removal of rust. • Nitric acid ( Used in fertilisers and explosives. • Sulfuric acid ( Making plastics, fertilisers, fire extinguishers • Ethanoic acid () Found in vinegar. Used for cooking, treating wasp stings, preservation of vegetables.
Role of water in acidity • Acids are covalent compounds. Solid citric acid or hydrogen chloride dissolved in organic solvent has no reaction with metals, bases and carbonates. • Acids only show their properties when they are dissolved in water. This is because acids dissociate in water to produce ions which are responsible for the acidic properties. In alcohol, exists as covalent molecules. Hence, in alcohol is a non-conductor of electricity. In water, dissociates to form and ions. Hence in water is a conductor of electricity.
Basicity of an acid • Some acids form just one hydrogen ion when dissolved in water whereas others form more than one. • The maximum number of hydrogen ions produced by a molecule of an acid is called the basicity of the acid.
Strength of acids • A strong acid is one that completely ionises in water. (Hydrocholric acid) • The higher the amount of ions that dissociate in water, the stronger is the acid. • A weak acid is one that partially ionises in water. • The lower the amount of ions that dissociate in water, the weaker is the acid. (Ethanoic acid)
bases • A compound that reacts with acids to produce a salt and water • Bases are either metal oxides or metal hydroxides • Bases that are soluble are called alkalis • Bases that are insoluble in water are still called bases Bases Alkalis Eg. Sodium hydroxide Eg. Copper (II) oxide
alkalis • A base that is soluble in water
Properties of alkalis • Taste bitter • Feels soapy • Turns red litmus paper blue • Turns Universal Indicator green or violet • React with acids to form a salt and water • This reaction is also known as neutralisation. • Alkalis are hazardous • Concentrated solutions of sodium hydroxide and potassium hydroxide are corrosive and burn skin.
Properties of alkalis • Alkalis react with ammonium compounds Reaction of ammonium chloride with sodium hydroxide • Alkalis react with solutions of metal ions Reaction of copper(II) sulfate solution with dilute sodium hydroxide solution. The solid that is formed in this reaction is called a precipitate and the reaction is called a precipitation. This reaction is used to as a test to identify metal ions.
Strength of alkalis • A strong alkali is one that fully ionises in water. • The higher the number of ions that dissociate in water, the stronger the alkali. • A weak alkali is one that partially-ionises in water. • The lower the number of ions that dissociate in water, the weaker the alkali
Different types of alkalis and bases • Ammonia solution ( Used in window cleaning solutions and fertilisers • Calcium oxide () Used in neutralising acidic soil, and to make iron, concrete and cement • Magnesium hydroxide ( Used in toothpaste to neutralise acid on teeth, and in antacids, to relieve indigestion • Sodium hydroxide ( Used in making soaps and detergents, and in industrial cleaning detergents
Concentration and strength • Concentration tells us how much a substance is dissolved in of the solution • Strength refers to how easily an acid or an alkali dissociates when dissolved in water. * Concentration of an acid or alkali can be changed. However, the strength of an acid or an alkali cannot be changed. • The concentration of an acid or alkali is determined by titration using a suitable indicator such as methyl orange or phenolphthalein.
Titration between acids and alkalis • Titration between an acid and alkali is also known as neutralisation • Enables volume ratios of acid-alkali needed for neutralisation to be found. By knowing one concentration, the other can be calculated. • In a titration experiment, a known volume of acid is titrated against an unknown concentration of alkali. By making use of the end point, it is possible to perform stoichiometric calculation to find the alkali concentration.
titration volume of alkali used alkali Known acid volume / concentration phenolphthalein is added Phenolphthalein changed colour in alkaline pH
Indicators and ph • To test for acids and alkalis, we use indicators as most solutions of acids and alkalis are colourless • An indicator is a substance that has different colours in acidic and alkaline solutions • Simple indicators can only tell whether a solution is an acid or an alkali. Other indicators not only indicate an acid or an alkali, but also how acidic or alkaline a solution is. • A measure of the acidity or alkalinity of a solution is known as pH.
Ph scale • The pH scale is a set of numbers used to show whether a solution is acidic, neutral or alkaline. • The pH scale ranges from 0 to 14. • A pH of 7 is neutral. This is the pH of pure water. • A solution with pH less than 7 is acidic. The solution contains hydrogen ions. The smaller the pH, the more acidic the solution is and the more hydrogen ions it contains. • A solution with pH greater than 7 is alkaline. The solution contains hydrogen ions. The bigger the pH, the more alkaline the solution is and the more hydroxide ions it contains. A solution with a pH of 12 has more hydroxide ions than a solution with a pH of 9.
Ph scale More acidic More alkaline neutral
Universal indicator • Universal indicator is a mixture of indicators. It is used either in form of a solution or a paper. The indicator gives different colours at different pH values. • The pH of a solution can be measure by dipping a piece of Universal paper in the solution. The pH is then found by comparing the colour obtained with a colour chart.
Universal indicator Blue Red Green Purple Bluish-green Yellow Bluish -purple Orange More alkali More acidic Neutral
Acidity in soil • It is important for us to control the pH of soil because this affects the growth of and development of plants • Most plants grow best when the soil is neutral or slightly acidic • However, some plants grow better in a more acidic soil while others grow better in a weakly alkaline soil. • Chemicals are often added to adjust its pH • Bases such as quicklime (CaO) or slaked lime () are added to control the acidity of the soil for healthy plant growth. This is known as liming the soil.
Acidity in soil • The bases react with the acids in the soil and raise the pH so that plants can grow healthily. Adding too much base will make the soil to alkaline and unsuitable for plant growth. Here are a few videos of controlling acidity in soil: http://www.youtube.com/watch?v=ru6vaDC1_tk http://www.youtube.com/watch?v=io10Y-dA13w
Food preservation • Microorganisms, such as bacteria, which are present in food, make the fresh food go bad quickly. Microorganisms do not grow well in solutions of low pH. • Ethanoic acid (vinegar) is used to preserve vegetables, such as onions • Benzoic acid is commonly used in fruit juices, oyster sauce and jams • Citric acid is used in foods as both a preservative and flavouring.
Hair • Normal hair is weakly acidic, where the hair is smooth, strong and healthy. • Alkaline solutions make the hair curly. Which will result in the hair being more dull, weaker and easier to break. • Perming solutions are always alkaline in order to change the shape of hair. • Most shampoos are alkaline because alkaline dissolve grease present in the hair. Hair conditioners contain weak acids which will neutralise any excess alkali left in the hair after shampooing.
THAT’S ALL FOLKS！ Cheers!