Acids, Bases and Salts — Definition

Definition

Imagine a world where everything is either sour, slippery, or salty. While an oversimplification, this gives us a basic intuition for acids, bases, and salts. In chemistry, these terms are much more precisely defined, and understanding them is crucial for NEET. Let's break it down.

Acids: Think of lemon juice or vinegar – they taste sour. In chemistry, acids are substances that, when dissolved in water, increase the concentration of hydrogen ions ($H^+$) or hydronium ions ($H_3O^+$). There are three main theories to define acids:

1
Arrhenius Acids: — These are substances that produce $H^+$ ions (or $H_3O^+$ ions) when dissolved in water. Examples: Hydrochloric acid ($HCl$), sulfuric acid ($H_2SO_4$).
2
Brønsted-Lowry Acids: — These are proton ($H^+$) donors. This definition is broader than Arrhenius, as it doesn't require water as a solvent. Example: $HCl$ donates a proton to water to form $H_3O^+$. $NH_4^+$ can also donate a proton.
3
Lewis Acids: — These are electron-pair acceptors. This is the broadest definition, encompassing species that don't even have hydrogen, like boron trifluoride ($BF_3$) or aluminum chloride ($AlCl_3$).

Bases: Think of baking soda or soap – they often feel slippery and taste bitter. Bases are substances that, when dissolved in water, increase the concentration of hydroxide ions ($OH^-$). Similar to acids, there are three main theories:

1
Arrhenius Bases: — These are substances that produce $OH^-$ ions when dissolved in water. Examples: Sodium hydroxide ($NaOH$), potassium hydroxide ($KOH$).
2
Brønsted-Lowry Bases: — These are proton ($H^+$) acceptors. Example: Ammonia ($NH_3$) accepts a proton from water to form $NH_4^+$ and $OH^-$. $OH^-$ itself is a Brønsted-Lowry base.
3
Lewis Bases: — These are electron-pair donors. Example: Ammonia ($NH_3$) with its lone pair of electrons can donate them to a Lewis acid.

Salts: When an acid and a base react, they neutralize each other, typically forming water and a salt. A salt is an ionic compound composed of a cation (positively charged ion) from a base and an anion (negatively charged ion) from an acid.

For instance, when hydrochloric acid ($HCl$) reacts with sodium hydroxide ($NaOH$), they form water ($H_2O$) and sodium chloride ($NaCl$), which is a common salt. Salts can be neutral, acidic, or basic depending on the strength of the parent acid and base from which they are formed.

They are crucial in many biological and industrial processes.