Understanding Acids and Bases – A Guide for Grade 12 Learners

Dear Grade 12 learner

If you’ve ever looked at acids and bases and felt confused, that’s completely okay.

These are tricky concepts at first — but with the right approach, they can actually become one of the most rewarding parts of Physical Sciences.

Let’s take this one step at a time.

You’ve come this far — and you’ve got what it takes. 💛

🧪 What Are Acids and Bases?

There are two main definitions you need to know:

1. Arrhenius Definition

• An acid produces H⁺ ions in water.

• A base produces OH⁻ ions in water.

  
  

Example:

• HCl (hydrochloric acid) → H⁺ + Cl⁻

• NaOH (sodium hydroxide) → Na⁺ + OH⁻

2. Brønsted-Lowry Definition

• An acid is a proton (H⁺) donor

• A base is a proton (H⁺) acceptor

This definition is broader and helps us understand how acids and bases behave during reactions.

🌡️ Strong vs. Weak Acids and Bases

Type	Description	Example
Strong Acid	Fully ionises in water	HCl, HNO₃, H₂SO₄
Weak Acid	Partially ionises in water	CH₃COOH (vinegar)
Strong Base	Fully dissociates into OH⁻	NaOH, KOH
Weak Base	Partially dissociates	NH₃ (ammonia)

Tip: Strong ≠ Concentrated. Strength is about ionisation, not amount.

🎨 Indicators – How to Tell if a Solution is Acidic or Basic

Indicators are substances that change colour depending on the pH of the solution.

Indicator	Acid Colour	Base Colour	pH Range
Litmus	Red	Blue	~4.5 – 8.3
Methyl orange	Red	Yellow	~3.1 – 4.4
Phenolphthalein	Colourless	Pink	~8.3 – 10
Universal	Red to blue	Full range	0 – 14

Use these in titrations to find the equivalence point — where acid and base exactly neutralise each other.

🔢 pH and pOH – Measuring Acidity and Basicity

The pH scale ranges from 0 to 14:

• pH < 7 = Acid

• pH = 7 = Neutral

• pH > 7 = Base

  
  

📘 pH Formula:

pH=−log⁡[H+]\text{pH} = -\log[\text{H}^+]pH=−log[H+]

📘 pOH Formula:

pOH=−log⁡[OH−]\text{pOH} = -\log[\text{OH}^-]pOH=−log[OH−]

And the relationship between the two:

pH+pOH=14\text{pH} + \text{pOH} = 14pH+pOH=14

🧮 Example 1 – Finding the pH

You are given:

[H+]=1×10−3 mol/L[\text{H}^+] = 1 \times 10^{-3} \text{ mol/L}

[H+]=1×10−3 mol/LpH=−log⁡(1×10−3)=3\text{pH} = -\log(1 \times 10^{-3})

= 3pH=−log(1×10−3)=3

✔️ This is an acidic solution.

🧮 Example 2 – Finding pH from OH⁻

[OH−]=1×10−5[\text{OH}^-] = 1 \times 10^{-5}[OH−]=1×10−5

Step 1:

pOH=−log⁡(1×10−5)=5\text{pOH} = -\log(1 \times 10^{-5}) = 5pOH=−log(1×10−5)=5

Step 2:

pH=14−pOH=14−5=9\text{pH} = 14 - \text{pOH} = 14 - 5 = 9pH=14−pOH=14−5=9

✔️ This is a basic solution.

🎓 Quick Recap Table

Concept	Key Idea
Acid	Donates H⁺ (proton donor)
Base	Accepts H⁺ or gives OH⁻ ions
Strong vs Weak	Depends on how much it ionises in water
pH Scale	Measures how acidic or basic a solution is
pH + pOH	Always = 14 at 25°C
Indicators	Show pH range using colour change

💬 A Gentle Word of Encouragement

This topic might feel like a lot — and that’s completely valid.

But remember: each formula, each colour change, and each reaction tells a story of how the tiniest particles interact in balance.

You don’t have to master it in one day.

You just need to keep showing up and trying — even a little bit each time makes a difference.

Believe in yourself.

Physical Sciences-Acids and Bases.pdf

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You are capable of so much more than you think. 💛