Acidic and alkaline solutions
Acids
Substance producing more hydrogen ions than hydroxide ions when dissolved in water. form Having a pH lower than 7.Mixture formed by a solute and a solvent. in water. Acids are sources of hydrogen Electrically charged particle, formed when an atom or molecule gains or loses electrons., H+. For example, dilute hydrochloric acid forms hydrogen ions:
HCl(aq) → H+(aq) + Cl-(aq)
Alkalis
Substance producing more hydroxide ions than hydrogen ions when dissolved in water. form Having a pH greater than 7. solutions in water. These contain hydroxide ions, OH-. For example, dilute sodium hydroxide solution contains hydroxide ions:
NaOH(aq) → Na+(aq) + OH-(aq)
The pH scale
The Scale of acidity or alkalinity. A pH (power of hydrogen) value below 7 is acidic, a pH value above 7 is alkaline. scale measures the acidity or alkalinity of a solution.
| pH | Meaning |
| < 7 | acidic |
| = 7 | neutral |
| > 7 | alkaline |
| pH | < 7 |
|---|---|
| Meaning | acidic |
| pH | = 7 |
|---|---|
| Meaning | neutral |
| pH | > 7 |
|---|---|
| Meaning | alkaline |
Learn more on the pH scale in this podcast.
Listen to the full series on BBC Sounds.
Using an indicator
The pH of a solution can be estimated using A chemical solution that produces many different colour changes corresponding to different pH levels. and a colour chart:
- dip a clean glass rod into the solution
- transfer one drop of solution to a piece of universal indicator paper on a white tile
- wait 30 seconds for the colour to develop, then match the colour to the colour chart
Using a pH meter
The pH of a solution can be measured using a pH meter:
- wash the pH probe with distilled water
- put the end of the probe into the solution
- record the reading on the pH meter
The pH meter may need calibrating first. This is done by adjusting the reading to match the pH of a buffer solution, which is a solution with a known pH.