Lemon juice and vinegar taste sour, while soapy water feels slippery. This difference is due to the acidity and basicity of solutions; chemically, the amounts of H+ and OH- are the key.
Basic Knowledge
A substance that produces hydrogen ions (H+) in an aqueous solution is called an acid. A substance that produces hydroxide ions (OH-) is called a base (alkali).
Representative acids: hydrochloric acid (HCl)・sulfuric acid (H2SO4)・nitric acid (HNO3)・acetic acid (CH3COOH)・carbonic acid (H2CO3)
Representative bases: sodium hydroxide (NaOH)・calcium hydroxide (Ca(OH)2)・ammonia (NH3)
The acidity, neutrality, or basicity of a solution is expressed by its pH (hydrogen-ion index). pH 7 is neutral, pH < 7 is acidic, and pH > 7 is basic.
Acid (a substance that produces H+ when dissolved in water. Hydrochloric acid, sulfuric acid, acetic acid)
Base (alkali) (a substance that produces OH- when dissolved in water. NaOH・Ca(OH)2)
pH (the strength of acidity or basicity. 0〜14. 7 is neutral; the smaller the value, the stronger the acidity)
Strong acid・weak acid (a strong acid ionizes completely. HCl, H2SO4. A weak acid ionizes only partly. CH3COOH)
Strong base・weak base (strong bases: NaOH, KOH. Weak base: NH3)
Indicator (a reagent that changes color according to pH. Litmus paper・BTB solution・phenolphthalein)
Further Study
Litmus paper turns red in an acidic solution and blue in a basic solution (“blue to red means acidic” is a useful way to remember this). BTB solution is yellow in an acidic solution, green in a neutral solution, and blue in a basic solution. Phenolphthalein solution turns reddish purple only in a basic solution.
The strength of an acid or base is determined by how many H+ or OH- ions it produces. Hydrochloric acid ionizes completely (strong acid), whereas acetic acid ionizes only partly (weak acid). Their pH values differ even at the same concentration.
- Acid = produces H+(HCl, H2SO4, CH3COOH)
- Base = produces OH-(NaOH, Ca(OH)2, NH3)
- pH < 7 acidic, pH = 7 neutral, pH > 7 basic
- Litmus: acidic → red, basic → blue
- BTB: acidic → yellow, neutral → green, basic → blue
- Phenolphthalein: reddish purple only in a basic solution
- Strong acids: HCl, H2SO4 / weak acids: CH3COOH, H2CO3
Points to Note
① The smaller the pH, the stronger the acidity (pH 1 is more acidic than pH 3). ② Strong and weak acids are different from concentrated and dilute solutions. Dilute hydrochloric acid is still a strong acid. ③ Ammonia is a base, but rather than containing OH- directly, NH3 reacts with water to produce OH- (it is a weak base).
Practice
- Define an acid and a base, and give one representative example of each using a chemical formula.
- What color does BTB solution become when added to a basic aqueous solution?
- Are aqueous solutions with pH 3 and pH 9 acidic, neutral, or basic?