acids and bases

Question 1

define an acid

Answer 1

a substance that dissociates to form H⁺ ions in aqueous solutions (when dissolved in water)

Question 2

physical properties of an acid

Answer 2

• have a sour taste
• is corrosive in large amounts or
  with a high concentration
• can conduct electricity when aqueous due to H⁺ ions

Question 3

chemical properties of an acid

Answer 3

• low pH value (<7)
• turn blue litmus paper red
• only shows acidic properties when dissolved in water
• react with reactive metals (e.g. sodium, magnesium)

Question 4

define a base

Answer 4

a metal oxide or hydroxide, must contain the O²⁻ or OH⁻ ions

Question 5

define an alkali

Answer 5

a sub-category of a base that is soluble in water and can completely dissolve

Question 6

physical properties of an alkali

Answer 6

• have a bitter taste
• feel slippery and soapy to touch
• is corrosive in large amounts or with a high concentration
• dissociate in water to conduct electricity due to OH⁻ ions

Question 7

chemical properties of an alkali

Answer 7

• high pH value (>7)
• turn blue litmus paper red
• only shows alkaline properties when dissolved in water
• reacts with acids

Question 8

acid-metal reaction

Answer 8

metal + acid → salt + hydrogen [MASH]

Question 9

acid-base reaction (neutralisation)

Answer 9

acid + alkali / base → salt + water [AASW]

Question 10

acid-carbonate reaction

Answer 10

acid + carbonate → salt + water + carbon dioxide [CASH₂OCO₂)

Question 11

alkali-ammonium salt reaction

Answer 11

alkali + ammonium salt → salt + water + ammonia [AAmSWAm]

Question 12

metals that do not react with any acids

Answer 12

• inert metals such as copper, silver or transition metals
• lead does not react with hydrochloric acid, sulfuric acid because of protective lead (II) layer formed

Question 13

test for carbon dioxide

Answer 13

• bubble gas into limewater using a delivery tube
• if carbon dioxide is present, a white precipitate will be formed

Question 14

test for hydrogen gas

Answer 14

• place a lighted splint at the mouth of the test tube
• if hydrogen gas is present, splint will be extinguished with a soft ‘pop’ sound

Question 15

define strength of acid/alkali

Answer 15

• extent of ionisation of an acid/alkali when dissolved in water
• note: strength does not change with volume or concentration

Question 16

define strong acid/alkali

Answer 16

• acid/alkali that completely ionises in an aqueous solution
• highly corrosive, do not touch
• e.g. hydrochloric acid, sulfuric acid, nitric acid, sodium hydroxide

Question 17

define weak acid/alkali

Answer 17

• acid/alkali that only partially ionises in an aqueous solution
• if it’s edible or a household product, it’s usually weak
• e.g. ethanoic acid, citric acid, lactic acid, ammonia

Question 18

function of indicators

Answer 18

• substance that changes the colour depending on whether an acidic or alkali substance is added to it
• gives different colours according to the pH value

Question 19

universal indicator scale

Answer 19

• acid: red, orange and yellow (pH 0 to 6)
• neutral: green (pH 7)
• alkali: dark green, blue, indigo, violet (pH 8 -14)

Question 20

other common indicators

Answer 20

• methyl orange: turns from red to yellow (pH 3 to 5)
• screened methyl orange: turns from violet to green (pH 3 to 5)
• litmus: turns from red to blue (pH 5 to 8)
• thymolphthalein: turns from colourless to blue (pH 9 to 10.5)

Question 21

hydrogen ion concentration of aqueous solutions

Answer 21

• pH <7: concentration of H⁺ ions is higher than OH⁻ ions (acidic)
• pH 7: concentration of H⁺ ions is equal to OH⁻ ions (neutral)
• pH >7: concentration of H⁺ ions is lower than OH⁻ ions (alkaline)