What is pH? Acids and Bases

Building a pH Scale

1. Turn your books to the side

2. Draw a line 28cm long and add a mark every 2 cm

3. Add numbers 0-14 along the scale

4. Using your results from last lesson, add each household item to the scale

5. You may want to add colour to your scale to match the indicator paper

​Research the following compounds and add their pH levels to your scale: blood, urine, stomach acid, sea water, honey, wine, bleach, cooking oil, an energy drink)

​https://www.youtube.com/watch?v=Yrf3Z_TQu_k&list=TLPQMjUwODIwMjUtIxPX0qAQ5w&index=2

What is pH?

pH tells us how acidic or basic (alkaline) a substance is. It is about the ions in water.

We use dyes called indicators to tell us the pH of something.

The scale goes from 0-14 (0 being the most acidic and 14 being the most basic/alkaline). pH 7 is neutral.

​There are a number of different indicators, but you can see that they don't have a detailed range like universal indicator does (that's what we used). Universal indicator isn’t just one chemical – it’s a mixture of several different pH indicators.

Where do the ions come from?

• Acids contain hydrogen atoms that can break off as H⁺ ions when the substance dissolves in water.

  • Example: Hydrochloric acid (HCl) → H⁺ + Cl⁻

  • The H⁺ comes from the acid molecules splitting apart.

• Bases contain parts that can produce OH⁻ ions when dissolved in water.

  • Example: Sodium hydroxide (NaOH) → Na⁺ + OH⁻

  • The OH⁻ comes from the base molecule splitting apart.

Where do they come from? Where do they go?

They come from the substance, not the water itself.

​When H⁺ ions are released from the acid, they don’t just float around by themselves. They attach to water molecules to form hydronium ions (H₃O⁺): H⁺ + H₂O → H₃O⁺


When OH⁻ ions are released from the base, they don’t just float around by themselves. They can bond with H⁺ ions from water to form water molecules (H₂O):

OH⁻ + H⁺ → H₂O

So in water, the OH⁻ ions combine with free H⁺ ions, which lowers the concentration of H⁺ and makes the solution basic.