Lake test of $Al^{3+}$ ion is based on adsorption of blue litmus on solid surface of $Al(OH) _3$. The dye aluminon is adsorbed by the gelatinous $Al(OH) _3$ precipitate to form lake coloured solution. This test is used as a confirmatory test for $Al^{3+}$ after precipitation of $Al(OH) _3$ with aqueous ammonia.

Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid, or dissolved solid to a surface.
Adsorption of gases on solids depends on the following factors:

Adsorption finds extensive applications both in research laboratory and in industry. A few applications are discussed below:
In gas masks:
All gas masks are devices containing suitable adsorbent so that the poisonous gases present in the atmosphere are preferentially adsorbed and the air for breathing is purified.
In heterogeneous catalysis:
The action of certain solids as catalysts is best explained in terms of adsorption. The theory is called adsorption theory. According to this theory, the gaseous reactants are adsorbed on the surface of the solid catalyst. As a result, the concentration of the reactants increases on the surface and hence, the rate of reaction increases. The theory is also able to explain the greater efficiency of the catalyst in the finely divided state, the action of catalyst promoters and poisons.
In softening of hard water (De-ionization of water):
The use of ion exchangers for softening of hard water is based upon the principle of competing adsorption just as in chromatography. Hygroscopic nature of $CaCl _2$ is not related to adsorption.

Silica gel has a very strong affinity for water and will adsorb it in preference to most other substances. It is also a good adsorbent for HCl, gasoline-range hydrocarbons, sulphur and nitrogen compounds, aromatics and many others. Silica Gel is in wide use in many industries for a myriad number of applications where a high capacity desiccant or selective adsorbent is required.

There are several commercial applications based on this mechanism in adsorptive separation processes. One typical example is separating normal paraffin from iso-paraffin and aromatics in an adsorption process using zeolite 5A as an adsorbent.

 Activated carbon filtration is a commonly used technology based on the adsorption of contaminants onto the surface of a filter. This method is effective in removing certain organics (such as unwanted taste and odours, micropollutants), chlorine, fluorine or radon from drinking water or wastewater.

Most strongly adsorbed component forms the topmost band while  the least adsorbed component forms the lower most band on adsorbent media.

Animal charcoal is known for its power to remove substance from solution. It is chiefly known for its decolourising property and widely used in the industry as a decolourising agent. 

Heterogeneous catalysts are materials capable of adsorbing liquids and gasses on their surface. Hence, they are based on the process of adsorption.

True dyes like cosin and fluorescein are used as indicator in titrating bromibe solution with zinc sulphate.

Adsorption Chromatography with alumina as the adsorbent may be used for the separation and purification of the Alkaloids of Atropa Belladonna from the bulk of non- alkaloidal matter present in alumina columns, whereas tincture prepared by the percolation process in adsorption column chromatography.

Silica gel is commonly used as a drying agent as it adsorb moisture on its surface to prevent growth of microorganism and foul smelling.

     Easily liquefiable gases can adsorb more than the other gases.

As $CO _2$ is an easily liquefiable gas, hence it is adsorbed more by charcoal.

Adsorption is the adhesion of atoms, ions, or molecules from a gas, liquid, or dissolved solid to a surface. This process creates a film of the adsorbate on the surface of the adsorbent.
Absorption is a physical or chemical phenomenon or a process in which atoms, molecules or ions enter some bulk phase – gas, liquid or solid material.

NHis absorbed by water but adsorbed by charcoal.

Colloidal solution of palladium can adsorb largest volume of hydrogen gas as the surface area of palladium is maximum in its colloidal solution. Higher is the surface area of catalyst, larger will be the volume of hydrogen adsorbed.

Applications of adsorption are :

1.Charcoal is used as a decoloriser as it adsorbs the coloring matter from the coloured solution of sugar.

2. Silica gel adsorbs moisture from the desiccators.

3. Silica and alumina gels are used as adsorbents for removing moisture and for controlling humidity of rooms.

4. Activated charcoal is used in gas masks as it adsorbs all the toxic gases and vapours and purifies the air for breathing.

5 .Adsorption processes are useful in carrying out heterogeneous catalysis.

Activated charcoal adsorps poisonous gas thus used in the gas masks.

Application of adsorption in pharmacy and allied fields are Decolourising agents, adsorption chromatography.

Decolorization of edible oils and removing bitter substances can be obtained through adsorption.

Ion exchange resin method is used for removing hardness of water.

Adsorption chromatography has many applications. Generally, it is used for determining the concentration of a compound (or its purity), separating out a mixture into individual componentsseperation of isomers, and identifying what is in a mixture and seperate on the basis of solubility also. There are three main types of adsorption chromatography - column, thin layer, and gas-solid.

Column chromatography is used for isolation and purification of vitamins and hormones and for the examination of vegetable oil and pharmaceutical preparations.

Physisorption is Vanderwaals adsorption, it occurs at high pressure and low temperature conditions.