Chemical Dissolution & Solubility | Definition

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Key points.

  • Chemical dissolution (Dissolver) is when one or more substances are dispersed in a solvent in a molecular or ionic state.
  • The solution does not have to be a liquid but can also be a gas or a solid.
  • Solutes are divided into polar and non-polar substances.
  • Solvents can be classified as polar, semi-polar, or non-polar according to their polarity (dielectric constant ε).
Chemical Dissolution

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Chemical Dissolution Definition

Chemical dissolution is when one or more substances (solid, liquid, or gas) are dispersed in a solvent (dispersion medium) in a molecular or ionic state. Dissolution also known as Dissolving. The substance to be dispersed is called the solute, and the dispersion medium is called the solvent.

The general rule of chemical dissolution is that like dissolves meaning that solutes and solvents of similar polarity are soluble.

Classification of Solvents and Solutes


Solutes are divided into polar and non-polar substances.

Polar substances

A polar substance is a molecule with a dipole. When the electronegativity of the two bonded atoms is different, there is an uneven charge distribution, the positive and negative centers cannot coincide, and the bond has a polarity.

However, the dipole of the whole molecule is a vector sum of the individual polar bond dipoles, not a simple summation. This means that a molecule’s polarity is related to the presence or absence of polar bonds and the spatial configuration of the molecule. In the case of carbon tetrachloride, for example, each carbon-chlorine bond is polar, but their dipoles cancel each other out, and the whole molecule is non-polar.

Non-polar substances

Non-polar substances are the opposite of polar substances.


According to the size of the polarity (dielectric constant ε), solvents can be classified as polar (ε = 30 – 80), semi-polar (ε = 5 – 30), non-polar (ε = 0 – 5) .

Polar solvents

Commonly used polar solvents are water, glycerol, dimethyl sulfoxide, etc. The most commonly used solvent is water, a strong polar solvent that can dissolve electrolytes and polar compounds.

The dielectric constant of polar solvents is relatively large. It can weaken the attraction between oppositely charged ions in the electrolyte, resulting in “ion-dipole bonding,” causing the ions to be solvents (or hydrated) and dispersed into the solvent. Water dissolves organic acids, sugars, lower alcohols, aldehydes, lower ketones, amides, etc., by forming hydrogen bonds between the polar groups of these substances and water, i.e., hydration, forming hydrated ions and dissolving them in water.

Non-polar solvents

Commonly used non-polar solvents include chloroform, benzene, liquid paraffin, vegetable oil, ether, etc.

Non-polar solvents have a very low dielectric constant and cannot weaken the gravitational force of electrolyte ions or form hydrogen bonds with other polar molecules. Dissolving non-polar substances by non-polar solvents results from van der Waals forces between the solute and solvent molecules. The instantaneous dipole inside the solvent molecule overcomes the cohesion between the non-polar solute molecules. It leads to dissolution, while ionic or polar substances are insoluble or only slightly soluble in non-polar solvents.

Semi-polar solvents

Some solvents with a certain degree of polarities, such as ethanol, propylene glycol, polyethylene glycol, and acetone, can induce a certain degree of polarity in some non-polar molecules and dissolve them. These solvents are called semi-polar solvents.

Semi-polar solvents can be used as intermediate solvents to make polar and non-polar solvents miscible or to increase the solubility of non-polar drugs in polar solvents (water). For example, acetone increases the solubility of ether in water, and ethanol increases the solubility of hydrocortisone in water, etc.

The ability of a solute to dissolve in a solvent, in addition to the polarity of the two, is more influenced by the ability to form hydrogen bonds between the solute and the solvent than by polarity for polar solvents.

Chemical Solubility

Chemical solubility is the amount of solute dissolved in a saturated solution of a given amount of solvent at a given temperature (gas at a given pressure). Chemical solubility is generally expressed as one part of solute (1 g or 1 ml) dissolved in some ml of solvent. The approximate solubility of a drug is expressed in the following terms.

  • Extremely soluble: means that 1g (ml) of solute can be dissolved in less than 1ml of solvent.
  • Soluble: means that 1g (ml) of solute can be dissolved in 1 – 10ml of solvent.
  • Soluble: 1g (ml) of solute dissolves in 10 – 30ml of solvent.
  • Slightly soluble: 1g (ml) of solute dissolves in 30 – 100ml of solvent.
  • Slightly soluble: 1g (ml) of solute dissolves in 100 – 1000ml of solvent.
  • Very slightly soluble: 1g (ml) of solute can be dissolved in 1000 – 10000ml of solvent.

Almost insoluble or insoluble means that 1g (ml) of solute cannot be completely dissolved in 10,000ml of solvent. The chemical dissolution process of a drug is the process of dissolution and diffusion; once diffusion reaches equilibrium, chemical dissolution cannot occur.


A solution is not necessarily a liquid but can also be a gas or a solid. For example, air and alloys are solutions.

Solutions are transparent but not necessarily colorless. For example, copper sulfate solution, iron chloride solution, etc.

When two liquids dissolve, the one with the greater amount is usually called the solvent, and the one with the lesser amount is called the solute.

The solute in a solution can be one or several, while the solvent is only one.

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