Describe how to compare hydrogen ion concentration, neutrality, relative acidity and relative alkalinity in terms of colour and $mathrm{pH}$ using universal indicator paper

Acids, Bases and Salts – The Characteristic Properties of Acids and Bases

Objective: Describe how to compare hydrogen ion concentration, neutrality, relative acidity and relative alkalinity in terms of colour and pH using universal indicator paper.

1. What Are Acids and Bases?

Acids are substances that release hydrogen ions (\$\mathrm{H^+}\$) when dissolved in water. Bases, on the other hand, either accept those hydrogen ions or release hydroxide ions (\$\mathrm{OH^-}\$). Think of acids as a battery that can give out protons like a phone charger giving out power, while bases are like a sponge that soaks up those protons.

2. Hydrogen Ion Concentration (\$[H^+]\$)

The concentration of \$\mathrm{H^+}\$ ions determines how acidic a solution is. The higher the \$[H^+]\$, the lower the pH and the stronger the acid.

1. Measure the \$[H^+]\$ using a pH meter or indicator paper.
2. Convert to pH using the formula: \$\displaystyle \text{pH} = -\log[H^+]\$.
3. Compare values: pH 1 is 10¹³ times more acidic than pH 4.

3. Neutrality – The Sweet Spot at pH 7

Neutral solutions have a pH of 7, meaning \$[H^+] = [OH^-] = 1.0 \times 10^{-7}\,\text{mol/L}\$. Water is the classic example: it’s neither acidic nor basic.

4. Relative Acidity and Alkalinity Using Universal Indicator Paper

Universal indicator paper changes colour across the pH spectrum. Below is a handy reference:

🔍 How to use the paper:

• Dip a strip into the solution.
• Wait 10–15 seconds for the colour to stabilise.
• Match the colour to the table above.
• Write down the corresponding pH range and note the relative acidity or alkalinity.

5. Comparing Solutions – A Practical Example

Suppose you have three solutions: HCl, CH₃COOH, and NH₃. Using universal indicator paper:

1. HCl turns the paper red → pH 0–1 → very acidic.
2. CH₃COOH turns the paper yellow → pH 4–5 → weak acid.
3. NH₃ turns the paper blue → pH 8–9 → weak base.

From this, you can see that HCl is the most acidic, CH₃COOH is less so, and NH₃ is on the opposite side of the scale, being basic.

6. Key Takeaways for the IGCSE

• Higher \$[H^+]\$ → lower pH → stronger acid.
• Neutral solutions have pH 7.
• Universal indicator paper gives a quick visual cue: red = very acidic, violet = very basic.
• Use the colour–pH table to rank relative acidity/alkalinity of unknown solutions.
• Remember the analogy: acids are like a battery giving out protons; bases are like a sponge soaking them up.

💡 Tip for exam day: When you see a colour change, jot down the colour first, then match it to the pH range. This helps avoid confusion and ensures you remember the relative strengths of the solutions.