Acids, Bases and Salts • Topic 2 of 3

pH Scale & its Importance

Some acids and bases are far stronger than others. To measure how acidic or basic a solution is, chemists use the pH scale, a number line that usually runs from 0 to 14. The 'p' stands for 'potenz' (power); pH is a measure of the hydrogen-ion concentration in a solution. The more H+ ions a solution has, the lower its pH and the more acidic it is.

Reading the scale

  • pH = 7 is neutral (pure water).
  • pH less than 7 is acidic; the lower the value, the stronger the acid (lemon juice about 2, gastric acid about 1.5).
  • pH more than 7 is basic (alkaline); the higher the value, the stronger the base (oven cleaner near 13).

Strong versus weak

A strong acid (HCl, H2SO4, HNO3) ionises completely in water and gives many H+ ions, so it has a very low pH. A weak acid (acetic acid in vinegar, carbonic acid) ionises only partly, gives fewer H+ ions and has a higher pH. The same idea applies to bases: NaOH and KOH are strong alkalis, while NH4OH (ammonium hydroxide) is a weak base. Note that 'strong' is not the same as 'concentrated' — strength depends on the degree of ionisation, not on the amount dissolved.

Indicators

An indicator changes colour with pH. Litmus is red in acid and blue in base; methyl orange turns red in acid and yellow in base; phenolphthalein stays colourless in acid and turns pink in base. A universal indicator shows a full range of colours and lets us read the approximate pH directly. Some plants act as olfactory indicators, changing smell rather than colour.

pH in everyday life

  • Digestion: the stomach makes HCl (pH about 1.5–2) to digest food; an excess causes acidity, relieved by an antacid such as milk of magnesia, Mg(OH)2.
  • Tooth decay: bacteria turn sugar into acid; below pH 5.5 the enamel (calcium phosphate) dissolves. Brushing with a basic toothpaste neutralises the acid.
  • Soil and agriculture: most crops grow best near neutral soil; acidic soil is treated with lime (CaO) or chalk and alkaline soil with gypsum or organic matter.
  • Self-defence in nature: a bee or nettle sting injects an acid (relieved by baking soda), while a wasp sting is basic.
The pH Scale (0 = strong acid, 7 = neutral, 14 = strong base)
pHNatureExample
1–2strongly acidicgastric acid, lemon juice
4–6weakly acidicvinegar, black coffee
7neutralpure water
8–9weakly basicbaking soda, blood
12–14strongly basicNaOH, oven cleaner
Solved Examples
1
Worked Example
A solution has a pH of 3. Is it acidic, basic or neutral, and roughly how does it compare with a solution of pH 6?
Solution
  1. pH less than 7 means the solution is acidic.
  2. Lower pH means more H+ ions, so pH 3 is more acidic than pH 6.
  3. Each unit of pH is a tenfold change, so pH 3 has about 1000 times more H+ ions than pH 6.

Answer: It is acidic and about 1000 times more acidic than a pH 6 solution.

2
Worked Example
Two acids of the same concentration, HCl and acetic acid, are tested. Which has the lower pH and why?
Solution
  1. HCl is a strong acid and ionises completely, giving many H+ ions.
  2. Acetic acid is a weak acid and ionises only partly, giving fewer H+ ions.
  3. More H+ ions means a lower pH.

Answer: HCl has the lower pH because, being a strong acid, it produces more H+ ions.

3
Worked Example
Why does tooth decay begin when the pH in the mouth falls below 5.5?
Solution
  1. Bacteria in the mouth break down trapped sugar into acids.
  2. When the pH drops below 5.5, the acid is strong enough to attack tooth enamel (calcium phosphate).
  3. Brushing with a basic toothpaste neutralises the acid and raises the pH.

Answer: Below pH 5.5 the acid dissolves the enamel, causing decay; a basic toothpaste neutralises it.

4
Worked Example
A farmer's field has acidic soil and the crop is growing poorly. Suggest a treatment and explain it.
Solution
  1. Most crops need soil close to neutral (around pH 7).
  2. To raise the pH of acidic soil, a base is added.
  3. Quicklime (CaO), slaked lime or chalk neutralises the excess acid in the soil.

Answer: Add lime (CaO) or chalk to neutralise the soil acid and bring the pH near neutral.

5
Worked Example
A person has acidity (too much acid in the stomach). Which substance gives relief and why?
Solution
  1. Excess stomach acid (HCl) lowers the pH and causes pain.
  2. A mild base neutralises the extra acid.
  3. Milk of magnesia, Mg(OH)2, an antacid, raises the pH back towards normal.

Answer: An antacid such as milk of magnesia, Mg(OH)2, neutralises the excess HCl and relieves acidity.

6
Worked Example
Name the colour change of phenolphthalein and methyl orange when added to a basic solution.
Solution
  1. Phenolphthalein is colourless in acid; in a base it turns pink.
  2. Methyl orange is red in acid; in a base it turns yellow.
  3. Both confirm the solution is basic.

Answer: Phenolphthalein turns pink and methyl orange turns yellow in a basic solution.

Key Points

  • The pH scale runs from 0 to 14: pH 7 is neutral, below 7 acidic and above 7 basic; lower pH means more H+ ions.
  • Strong acids/bases ionise fully (HCl, NaOH); weak ones ionise only partly (acetic acid, NH4OH); strength is not the same as concentration.
  • Indicators show pH by colour: litmus (red/blue), methyl orange (red/yellow), phenolphthalein (colourless/pink); a universal indicator gives a colour for each pH.
  • Stomach uses HCl for digestion (pH ~1.5-2); excess acidity is treated with an antacid like milk of magnesia, Mg(OH)2.
  • Tooth enamel dissolves below pH 5.5; acidic soil is treated with lime; bee/nettle stings (acidic) are relieved by baking soda.
Quick Check — 4 MCQs
Tap an option to check your answer0 / 4
Q1.The pH of a neutral solution such as pure water is:
Explanation: A neutral solution has equal H+ and OH- ions and a pH of 7.
Q2.A solution with pH = 2 is:
Explanation: The lower the pH below 7, the stronger the acid; pH 2 is strongly acidic.
Q3.Tooth decay starts when the pH in the mouth falls below:
Explanation: Below pH 5.5 the acid produced by bacteria dissolves tooth enamel.
Q4.Which is used to treat the acidity caused by excess stomach acid?
Explanation: Milk of magnesia, Mg(OH)2, is a mild base that neutralises excess HCl in the stomach.
Practice more MCQs → 📝 Assignment · 30 marks