Acid, Base and Salt.
1. Acid
- An acid is a compound, produce hydrogen ions, H+(aq), in solution, which is responsible for their acidic properties.
- Hydrogen ions cannot exist alone, but they exist after combining with water molecules.so, on dissolving in water yields hydronium ions (H3O+) as the only positive ions.
- Acids are compounds that contain Hydrogen (Hydrochloric, HCl; Sulphuric, H₂SO₄; Nitric, HNO₃).
- Acids, sour in taste are usually compounds of non-metals with Hydrogen and sometimes Oxygen.
- The strength of an acid depends on the concentration of the hydronium ions present in a solution.
- However, some acids do not dissociate to any appreciable extent in water such as carbonic acid. Therefore, these acids will have a low concentration of hydronium ions.
- Acids are corrosive and dissolve metal.
- Strong Acid: Hydrochloric acid, sulphuric acid, nitric acid etc.
- Weak Acid: acetic acid, formic acid, carbonic acid etc.
2. Base and Alkali
- A Base is a substance that gives OH-ions when dissolved in water which is responsible for their basic property.
- Bases are usually metal hydroxides.
- Water soluble bases are alkali.
- The strength of a base depends on the concentration of the hydroxyl ions when it is dissolved in water.
- Strong Base: Sodium hydroxide (caustic soda) (NaOH), Potassium Hydroxide (caustic potash) (KOH), Calcium hydroxide Ca(OH)₂.
- Weak Base: Magnesium hydroxide: Mg(OH)₂, Ammonium Hydroxide: NH₄OH.
- Bases are bitter to taste. They are soapy to touch bitter and corrosive.
3. Salt
Important salts used in everyday life and industrial applications are Sodium chloride (NaCl), Sodium carbonate, (Na₂CO₃), Sodium Bicarbonate, (NaHCO₃), Sodium Hydroxide (NaOH).
4. Indicator
- They are used to test whether a substance is acidic or basic. Turmeric, litmus, China rose petals (Gudhal), etc., are some of the naturally occurring indicators.
- Litmus is extracted from lichens, a plant belonging to the division Thallophyta. It has a purple colour in distilled water. When added to an acidic solution, it turns red and when added to a basic solution, it turns blue.
- Olfactory indicators – There are some substances whose odour changes in acidic or basic media.
- The solutions which do not change the colour of either red or blue litmus are known as neutral solutions. These substances are neither acidic nor basic.
Dilution of Acid or Base:
- The process of dissolving an acid or a base in water is a highly exothermic one.
- The acid must always be added slowly to water with constant stirring. If water is added to a concentrated acid, the heat generated may cause the mixture to splash out and cause burns.
- Mixing an acid or base with water results in a decrease in the concentration of ions (H3O+/OH–) per unit volume.
