Introduction to Acids and Bases

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Introduction to Acids and Bases

• Science 10

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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.
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Introduction to Acids (cont)

• 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.

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Introduction to Bases

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

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Introduction to Bases (cont)

• 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.

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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?

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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.

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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 (1859-1927)
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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.

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Defining Indicators (cont)

• 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.

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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), CO2 (carbon dioxide), and
  Al2O3 (aluminum oxide) are oxides. H2SO4 and HNO3
  are not.

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Check Your Understanding

• In your own words, summarize Arrhenius theory
  about acids and bases.
• Balance each of the following skeleton ion
  equations. Identify the substance on the left of
  the equation as an acid or a base.
• H2CO3 (aq) ? H (aq) CO3 2- (aq)
• Ca(OH)2 (aq) ? Ca 2 (aq) OH- (aq)
• KOH (aq) ? K (aq) SO4 2- (aq)

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Check Your Understanding (cont)

• 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?