why is sodium thiosulfate readily soluble in water? how can a large amount of it be dissolved into only 3 ml of water?

First, most sodium salts are soluble in aqueous solution. Second, you don't say if you are using Na2S2O3 or Na2S2O3.5H2O NOR what you mean by "large amount."

I am using Na2S2O3.5H2O. By large amount i mean about 15 grams

Sodium thiosulfate (Na2S2O3) is readily soluble in water due to its ionic nature and the strong electrostatic attraction between the sodium (Na+) and thiosulfate (S2O3^-) ions. When sodium thiosulfate is added to water, the water molecules surround the individual ions, separating them from each other and pulling them into solution.

To understand how a large amount of sodium thiosulfate can dissolve into a small volume of water, we need to consider the concept of solubility and concentration. Solubility refers to the maximum amount of solute (in this case, sodium thiosulfate) that can dissolve in a given amount of solvent (water) at a specified temperature. Concentration, on the other hand, is the amount of solute present in a specific volume of solvent.

In the case of your question, dissolving a large amount of sodium thiosulfate into only 3 ml of water is possible because the solubility of sodium thiosulfate in water is relatively high. It means that under certain conditions, a significant amount of sodium thiosulfate can dissolve in a small volume of water before reaching its maximum solubility limit.

To further increase the solubility of sodium thiosulfate in water, you can apply a few methods:

1. Stirring/agitation: Increasing the contact and movement between the solid sodium thiosulfate and water helps speed up the dissolution process.

2. Heating: Generally, increasing the temperature also increases the solubility of most solids in liquids, including sodium thiosulfate. However, it's important to note that this may have certain limitations based on the substance and its solubility curve.

By applying these methods, you can maximize the amount of sodium thiosulfate that can dissolve into the given volume of water, such as 3 ml. Nonetheless, it's important to consider the solubility limit and avoid exceeding it, as additional solute will likely precipitate out of the solution.