The acid which has a peroxy linkage is (a) Dithionic acid (b) Sulphurous acid (c) Caro's acid (d) Pyrosulphuric acid

Peroxides are a group of compounds where the defining feature is the presence of an oxygen-oxygen single bond, represented by the group -O-O-. This bond is known as a 'peroxy linkage', and it's a bit less stable than the typical oxygen single bonds found in water and most organic compounds. This instability gives peroxides useful and sometimes dangerous reactive properties.

In chemistry, peroxy linkages are important because they can easily break and generate free radicals, which can initiate polymerization reactions or function as bleaching agents. Peroxides are widely used in the formulation of products like hair dyes, detergents, and as antiseptic agents. However, their inherent instability also means they must be handled carefully, as they can be explosive under certain conditions.

The chemical structure of acids, which are substances that can donate a proton (hydrogen ion, H+) to another compound, is fundamental to their behavior and reactivity. An acid's strength, how it interacts with other chemicals, and its physical properties are all connected to its molecular structure.

Most acids contain hydrogen and may also include oxygen along with other elements. While simple acids like hydrochloric acid (HCl) have straightforward structures, more complex acids can incorporate additional functional groups or multiple hydrogen atoms capable of dissociation. The presence of certain groups, such as hydroxyl (–OH) or the peroxy linkage (-O-O-), can significantly affect the acid's properties and its reactivity.

Caro's acid, also known as peroxomonosulfuric acid, is a strong acid that is unique because it contains a peroxy linkage within its structure. The chemical formula for Caro’s acid is H₂SO₅, which includes an extra oxygen when compared to sulfuric acid (H₂SO₄).

The peroxy linkage is what gives Caro's acid its oxidative strength, enabling it to act as a powerful oxidizing agent. It's capable of oxidizing compounds that are unreactive to other types of acids, making it important in various industrial processes such as the manufacture of caprolactam, which is a precursor for producing nylon. Like other peroxides, this reactive nature also means Caro's acid must be stored and handled under controlled conditions to ensure safety.