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, hydrochloric acid produces hydrogen ions:
HCl(aq) → H+(aq) + Cl-(aq)
Acidic solutions have Scale of acidity or alkalinity. A pH (power of hydrogen) value below 7 is acidic, a pH value above 7 is alkaline. values less than 7.
Alkalis
Substance producing more hydroxide ions than hydrogen ions when dissolved in water. form Having a pH greater than 7. solutions in water. Alkalis are sources of hydroxide ions, OH-. For example, sodium hydroxide produces hydroxide ions:
NaOH(aq) → Na+(aq) + OH-(aq)
Alkaline solutions have pH values greater than 7.
Learn more on the pH scale in this podcast.
Listen to the full series on BBC Sounds.
Neutral solutions
A When a substance is neither acidic nor alkaline, and has a pH of 7. solution is neither acidic nor alkaline. A neutral solution has a pH value of 7.
Indicators
Acid-alkali A substance that has different colours, depending upon the pH of the solution it is in. can show whether a solution is acidic, neutral (pH 7) or alkaline. The table shows the colours for two different common indicator solutions.
| Indicator | Acidic | Neutral | Alkaline |
| Litmus paper | Red | Purple | Blue |
| Methyl orange | Red | Yellow | Yellow |
| Phenolphthalein | Colourless | Colourless | Pink |
| Indicator | Litmus paper |
|---|---|
| Acidic | Red |
| Neutral | Purple |
| Alkaline | Blue |
| Indicator | Methyl orange |
|---|---|
| Acidic | Red |
| Neutral | Yellow |
| Alkaline | Yellow |
| Indicator | Phenolphthalein |
|---|---|
| Acidic | Colourless |
| Neutral | Colourless |
| Alkaline | Pink |
Question
Describe the limitations of methyl orange when determining if a solution is alkaline.
Methyl orange is yellow in neutral solutions as well as in alkaline solutions, so you can only be sure that a solution is acidic if methyl orange turns it red.
Question
Describe the limitations of phenolphthalein when determining if a solution is acidic.
Phenolphthalein is colourless in neutral solutions and in acidic solutions, so so you can only be sure that a solution is alkaline if phenolphthalein turns it pink.
The pH scale
The pH scale measures the acidity or alkalinity of a solution. The pH of a solution can be measured using a pH probe, or estimated using A chemical solution that produces many different colour changes corresponding to different pH levels. and a colour chart.
