The stronger parent wins
A salt's pH depends on how strong its acid and base were.
A salt from two strong parents
Common salt, NaCl, is made from hydrochloric acid (a strong acid) and sodium hydroxide (a strong base).
Dissolve NaCl in water and dip universal indicator. What colour?
What this lesson covers
The idea
Salts of a strong acid and a strong base are neutral (pH 7); of a strong acid and a weak base, acidic (pH below 7); of a strong base and a weak acid, basic (pH above 7).
A salt from two strong parents
Common salt, NaCl, is made from hydrochloric acid (a strong acid) and sodium hydroxide (a strong base).
Dissolve NaCl in water and dip universal indicator. What colour?
- Green, so neutral
- Red, so acidic
- Blue, so basic
Test four salt solutions
Dissolve each salt in water. Dip each indicator. Read the parents of each salt.
Spot the pattern. Sort the parent pairs by the salt they make.
The stronger parent decides
Salts of a strong acid and a strong base are neutral, pH 7. Salts of a strong acid and a weak base are acidic, pH below 7. Salts of a strong base and a weak acid are basic, pH above 7.
Think of a tug of war: the stronger parent pulls the salt to its side. A tie gives a neutral salt.
- Parents | Salt solution | Example
- strong acid + strong base | neutral, pH 7 | NaCl
- strong acid + weak base | acidic, pH below 7 | NH₄Cl
- weak acid + strong base | basic, pH above 7 | CH₃COONa
Notes
Salts of a strong acid and a strong base are neutral (pH 7); of a strong acid and a weak base, acidic (pH below 7); of a strong base and a weak acid, basic (pH above 7).
Check yourself
A salt from a strong acid and a strong base has pH …
NH₄Cl is made from HCl (strong acid) and NH₄OH (weak base). Its solution is …
CH₃COONa is made from CH₃COOH (weak acid) and NaOH (strong base). Its solution is …
KNO₃ is made from HNO₃ (strong acid) and KOH (strong base). Its pH is …
- less than 7. That would need a weak base.
- more than 7. That would need a weak acid.
- 7 — correct. Yes! It is neutral.
- acidic — correct. Yes! The strong acid wins.
- neutral. The parents are not equally strong.
- basic. The base is the weak one. The acid wins.
- acidic. The acid is the weak one. The base wins.
- basic — correct. Yes! The strong base wins.
- neutral. The parents are not equally strong.
- less than 7. Both parents are strong, so neither wins.
- more than 7. Both parents are strong, so neither wins.
- 7 — correct. Yes! Two strong parents give a neutral salt.