Introduction to Acids and Bases Science 10

Introduction to Acids Do your muscles ever feel sore after a heavy workout? If so, you can blame a buildup of lactic acid in your muscles. Lactic acid is a by- product of cell metabolism when too little oxygen is available. Sore muscles and sour milk are caused by the same acid.

Introduction to Acids (con’t) Lactic acid is also produced by bacteria in milk – this is why milk turns sour. Acids occur naturally in many fruits. Insects such as millipedes, scorpions, and ants use acids to deter predators. A physician may use a solution of boric acid to rinse out your eyes, but a drop or two of many other acids would blind you. Some acids add a tangy, sour flavour to foods and drinks, while others are deadly.

Introduction to Bases Bases are bitter-tasting compounds with a slippery feel. Most soaps and drain and window cleaners are bases.

Introduction to Bases (con’t) Quinine is a base which is responsible for the bitter flavour of tonic water and it occurs naturally in the bark of the cinchona tree. Quinine is also used as a drug to treat malaria. Many other drugs are bases, too. For example, lidocaine, a local anaestheic used by dentists, is a base. Like acids, some bases are harmless while others are very dangerous.

Word Connect The word “acid” comes from the Latin acidus, meaning “sour”. Think of the taste of fruits such as lemons or limes. These fruits contain citric acid. Vinegar, a common household product, contains acetic acid. Chemists sometimes use the term “alkali” for a base that is soluble in water. This word shows the Arabic origin of chemistry. The earliest source of bases was the ash obtained by burning plants. Thus the Arabic word alkali means “ashes of a plant.” What clue does the word “alkali” give you about the kinds of elements that you might find in a basic compound?

Defining Acids Most acids and bases dissolve in water. In 1884 the Swedish chemist Svante Arrhenius defined an acid as a substance that produces hydrogen ions in solution, H + (aq). For example, HCl (aq), hydrochloric acid is an ionic compound that dissolves in water, splitting into H + (aq) and Cl – (aq) ions. The resulting solution is an electrolyte.

Defining Bases Arrhenius also defined a base as a substance that produces hydroxide ions in solution, OH – (aq). An example of a base is sodium hydroxide, NaOH (aq). Sodium hydroxide forms OH – ions in solution. Svante Arrhenius ( )

Defining Indicators Most solutions of acids or bases are clear and colourless. Therefore they cannot be distinguished from ordinary water by appearance alone. The simplest way to distinguish them from water is to use an indicator. An indicator is a chemical that changes colour as the concentration of H + (aq) or OH – (aq) changes.

Defining Indicators (con’t) Two of the most common indicators are phenolphthalein and litmus. Litmus is a compound that is extracted from lichens, a plant-like member of the fungi kingdom. Litmus paper is made by dipping paper in litmus solution.

The Oxides of Elements Oxides of many elements react with water to form an acid or a base. An oxide is a compound that consists of an element combined with only oxygen. For example, CO (carbon monoxide), CO 2 (carbon dioxide), and Al 2 O 3 (aluminum oxide) are oxides. H 2 SO 4 and HNO 3 are not.

Check Your Understanding 1. In your own words, summarize Arrhenius’ theory about acids and bases. 2. Balance each of the following skeleton ion equations. Identify the substance on the left of the equation as an acid or a base. a) H 2 CO 3 (aq) → H + (aq) + CO 3 2- (aq) b) Ca(OH) 2 (aq) → Ca 2+ (aq) + OH- (aq) c) KOH (aq) → K + (aq) + SO 4 2- (aq)

Check Your Understanding (con’t) 3. Chris observes that a drop of an unknown solution on red litmus paper causes no colour change. Chris correctly concludes the solution is either neutral or acidic, but cannot be sure which. Explain. What further test(s) could Chris perform to identify whether the solution is, in fact, neutral or acidic?