When excess of carbon dioxide is passed through lime water, the milkiness first formed disappears due to (a) the reversible reaction taking place (b) formation of water soluble calcium bicarbonate (c) huge amount of heat evolved during the reaction (d) formation of water soluble complex of calcium.

Calcium hydroxide, known chemically as Ca(OH)₂, is a base that is highly relevant in many industrial and environmental processes. In the context of our exercise, calcium hydroxide is integral to the initial stage of the reaction we're examining, where it is dissolved in water to create what is commonly known as lime water.

When carbon dioxide (CO₂) is introduced to lime water, the calcium hydroxide reacts with it to form calcium carbonate (CaCO₃). This is a classic chemical reaction demonstrating the properties of calcium hydroxide as a base: it will react with a gas like CO₂ which, in turn, is a weak acid once dissolved in water to form a carbonate compound.

It's important to understand that the solubility of calcium hydroxide in water is relatively low, which explains why an excess of CO₂ can shift the reaction towards the formation of different products. The low solubility limit also means that even if the solution initially turns 'milky' due to the creation of calcium carbonate, further reactions can take place as more CO₂ is added.

Calcium carbonate is a widespread substance found in rock formations all around the world and is the principal component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Chemically represented by CaCO₃, it initially forms when CO₂ reacts with lime water, creating a milky appearance in the solution, indicative of its insolubility.

The production of calcium carbonate is evidence of a chemical change; it's the precipitate that leaves from the solution as a solid. However, the reaction does not stop there, if an excess amount of CO₂ is added. In our exercise, this milky suspension that first indicates the formation of calcium carbonate will then disappear when the compound transforms into another substance which is more soluble in water.

The educational takeaway from observing calcium carbonate in this reaction is understanding its role as an intermediate: it is a pivotal point that can lead to further chemical changes under the appropriate conditions, such as the increase in CO₂ concentration.

In comparison to calcium carbonate, calcium bicarbonate (also known as calcium hydrogen carbonate) does not readily precipitate from water solutions because it is highly soluble. Represented by the formula Ca(HCO₃)₂, this compound arises when an excess amount of carbon dioxide is passed through lime water containing calcium carbonate.

As the CO₂ levels increase, the equilibrium of the earlier reaction shifts to favor the production of calcium bicarbonate. The higher solubility of this new product in water leads to the milkiness of the solution clearing up. This clear solution is the result of calcium bicarbonate completely dissolving, contrasting the previous precipitated calcium carbonate.

To fully comprehend this concept, it helps to recognize that solubility is not a static property but can be influenced by various factors including the presence of additional solutes, temperature, and the chemical composition of the solvent. The calcium bicarbonate solubility aspect in our exercise is fundamental to make sense of why the reaction behaves differently with varying amounts of carbon dioxide.