Chemistry - 5 Chemical Changes - 5.7 Neutralisation and the pH Scale & 5.8 Strong and Weak Acids

Alkali

a soluble hydroxide

Base

substance that can neutralise acids

Acid

any compound that forms H⁺ ions in solution

Neutral

neither acidic nor alkaline

Dissolving sodium hydroxide in water

sodium hydroxide (water→) sodium ions(aq) + hydroxide ions(aq)

Indicators [4]:

- litmus paper
- universal indicator
- phenolphthalein
- methyl orange

Litmus paper [3]

acid: red
neutral: no change
basic: blue

Universal indicator

an indicator with a different colour for each pH value.

Phenolphthalein [3]

acid: colourless
neutral: colourless
basic: pink

Methyl orange [3]

acid: red-orange
neutral: yellow
basic: yellow

pH scale

measurement system used to indicate the concentration of hydrogen ions (H+) in solution; ranges from 0 to 14

Neutral solution + universal indicator

green

Acidic solution + universal indicator

red - yellow

Basic solution + universal indicator

blue - purple

pH meter

a device used to measure the pH of a solution

Equivalence point

the point in a titration where the number of moles of hydrogen ions equals the number of moles of hydroxide ions (middle of vertical drop)

Ionise

the reaction of a molecular substance with a solvent to form ions in solution.

Acids are classed as strong or weak depending on how they...

ionise in water

In aqueous solutions, acid molecules...

ionise and release H⁺ ions

Strong acids ... in aqueous solutions

fully ionise

How can we tell if an acid is weak?

it is a reversible reaction

Weak acids ... in aqueous solutions

partially ionise

Carbonic acid

H₂CO₃

Ethanoic acid

CH₃COOH

Citric acid

C₆H₈O₇

As the pH scale decreases by one unit...

the concentration of hydrogen ions increases by ten times

Concentration of acid

number of moles of acid molecules per unit of volume

Concentration vs. strength

concentration is the amount of solute dissolved in a given amount of solution while strength refers to the solute's tendency to form ions in water

Ethanoic acid dissolving in water

CH₃COOH ⇌ CH₃COO⁻ + H⁺