Topic 3:Acidic, Basic & Neutral Substances

• pH

– Range is 0 to 14

• Work from middle towards the ends

• Ends of the scale are the extremes

– Measure of the % of hydrogen ions (H+) in a solution

– Bi-directional scale

• From 7 left vs. from 7 right

– The difference between one number and the next on the scale represents a 10-fold difference

• pH of 3 is 10X stronger than a pH of 4

• pH of 2 is 100X stronger than a pH of 4

• pH of 10 is 10X stronger than a solution of pH 9

• pH of 11 is 100X stronger than a solution of pH 11

• Acid

– Sharp / sour taste

– Solution with a pH lower than 7

• Base (Alkali)

– Slippery / bitter

– Solution with a pH higher than 7

• Neutral

– Neither an acid or base

– pH of 7.0

– Measuring pH

• pH meter – digital probe

• Indicators

– Chemical compound that will visually show the pH of a solution

–Litmus paper

»Red or blue colored paper that reacts with acids and bases

– Litmus paper

» If the solution is acidic then the litmus paper turns/stays red

» If the solution is basic then the litmus paper turns/stays blue

» If the solution is neutral then red litmus stays red, blue litmus stays blue

• Which of the above liquids are acidic? Basic? Neutral?

• Neutralization

– Reaction between acid + base that produces

water + salt

– After reaction, solution will have a neutral pH

– HCl (aq) + NaOH (aq) → NaCl (s) + H2O (l)acid base salt water

– Examples

• Taking an antacid for an upset stomach

–Antacid = mild base, stomach = acidic

• Acid Rain

– Normal rainwater is naturally slightly acidic

• Raindrops dissolve CO2 from the air to form a very weak acid called carbonic acid

• pH of 5.6

– Industrial processes and fuel combustion produce large quantities of CO2, SO2, and NO gases into the air (pollution)

• pH less than 5.6

– These chemical dissolve in water to form acids

• CO2 → carbonic acids

• SO2 → sulfuric acids

• NO → nitric acids

– Concerns

• Can cause bodies of water to become acidic

– Most organisms can only survive in a narrow pH range

– What would a graphical representation of this look like?

– Acidic lakes can be treated with lime (CaOH = calcium hydroxide)

• CaOH = base

• Results in neutralization

• Ca(OH)2(aq) + H2SO4(aq) → CaSO4(aq) + H2O(l)

base acid salt water

Laboratory Assignment:An Indication of pH

• Online pH Labs

– http://www.bgfl.org/bgfl/custom/resources_ftp/client_ftp/ks3/science/acids/

– http://sv.berkeley.edu/showcase/flash/juicebar.html

– http://www.miamisci.org/ph/phpanel.html