Preparation of Salt - Methods with Detailed Explanation for JEE

VSAT 2022

Salts - A Brief introduction

In chemistry, salt is an ionic compound that consists of cation (the positively charged ion) and anion (negatively charged ion) in equal amounts. The major factor to understand the method of preparation of salts is the solubility of salt in water. If the salt is not soluble in water, it can be prepared using double decomposition. Precipitation is another method in which the insoluble salt will be precipitated, and thus the formed salt can be washed and dried. If the salt is soluble, it can be synthesised using a neutralisation reaction between acid and alkali. Neutralisation is accurately done by the titration reaction. Salt can be produced by a direct union of metal and a nonmetal, the example of this is the preparation of a well-known example of sodium chloride, the common salt. When sodium and chloride are reacted together, we get sodium chloride. Similarly, FeS (iron sulphide can be produced by heating sulphur with iron filings. 


Method of Preparation of Salt

There are various methods for the preparation of salt. It can be prepared naturally as well as in the laboratory. The natural method for the preparation of salt is by seawater. This can be done by the process of evaporation. In laboratory preparation of salt, generally employed methods are neutralisation, precipitation reaction, the reaction between metal and acid, and so on. Let us discuss which method is used in preparation of salts!


We mainly emphasise some important methods of laboratory preparation of salt and preparation of salt by seawater.


Laboratory Preparation of Salt

1. By The Action of an Acid on a Metal to Produce Salts

Soluble salts are made using this method. For example, when granulated zinc is added to a dilute solution of tetraoxosulphate (VI) acid, hydrogen effervescence occurs.

$\mathrm{H}_{2} \mathrm{SO}_{4}(\mathrm{aq})+\mathrm{Zn}(\mathrm{s}) \rightarrow \mathrm{ZnSO}_{4}(\mathrm{aq})+\mathrm{H}_{2}(\mathrm{~g})$


The solution is filtered, leaving on the filter paper insoluble impurities such as excess zinc and carbon particles. The solution of ZnSO4 is then crystallised.


It should be noted that any excess H2SO4 in the solution is removed by adding more zinc; otherwise, the acid will become concentrated during the crystallisation process.


2. Salt Preparation via Double Decomposition

Insoluble salts are made using this method. It is the reaction of two soluble compounds that results in one soluble and one insoluble product. The most important product is the insoluble salt, which is obtained as it precipitates out of the solution. The reaction of barium chloride and sodium sulphate to form barium sulphate and sodium chloride is an example:

$\mathrm{Na}_{2} \mathrm{SO}_{4}(\mathrm{aq})+\mathrm{BaCl}_{2}(\mathrm{aq}) \quad \rightarrow \quad \mathrm{BaSO}_{4}(\mathrm{~s})+2 \mathrm{NaCl}(\mathrm{aq}),$

Ionic bonds are found in all of the reactants and products. As a solid, barium sulphate precipitates from the solution.


3. Neutralisation Salt Preparation

Remember that neutralisation is the reaction between acids and bases that only produces salts and water. It would depend on whether the base is soluble (i.e. alkali) or insoluble in water to use this method.

$\begin{align} &\mathrm{HCl}(\mathrm{aq})+\mathrm{NaOH}(\mathrm{aq}) \quad \rightarrow \mathrm{NaCl}(\mathrm{s})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \\ &\mathrm{HNO}_{3}(\mathrm{aq})+\mathrm{KOH}(\mathrm{aq}) \quad \rightarrow \mathrm{KNO}_{3}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(1) \end{align}$


Salt can be produced by the neutralisation reaction of acid and base, we get the salt along with water. This is the most common method used in laboratories to prepare salts. Titration is used to precisely neutralise the acid and alkali so that there is no further need for separation. There will be an indicator used to find the endpoint so that there is no further addition of the reactant and we get the salt. The salt thus obtained will be dried and evaporated.


Preparation of Salt from Sea Water

Seawater is a naturally occurring and easily accessible source of salt. It contains an average of 35 parts per thousand of salt. The dissolved salt content of seawater is approximately 3.5 percent. Evaporation ponds are used in the process of extracting salt from seawater, which is an ancient technology. Shallow, water-proof ponds are excavated and linked to the sea by a short canal. With a large surface area and shallow depth, a given volume of water can absorb more sunlight. The canal is closed after the pond is flooded. The water evaporates as a result of the sun's rays. As the water evaporates, the salt remains, resulting in increasingly saline water. Enough of the water evaporates to leave a layer of sea salt crystals behind that can be harvested.


In modern sea salt extraction operations, a number of these ponds are typically concentrated in one location and separated by levees. When evaporating sea water to extract salt, time is usually of the essence because unexpected rain can ruin days of evaporation. As a result, some salt evaporation operations use very large indoor ponds called pans that are weather-protected.


Conclusion 

Salt can be made in a variety of ways. The method chosen is determined by whether the salts are soluble or not. Soluble salts are made using crystallisation methods, whereas insoluble salts are made using precipitation methods. Acids are neutralised using soluble or insoluble bases to form salts and water. When the salt is soluble it is prepared using the precipitation method and if the salt is soluble, then it is prepared using the crystallisation method. There are various methods to prepare salt from seawater. One among them is evaporation, the sea water is collected and trapped in big shallow pits and allowed to stand there. This helps in the evaporation process by sunlight.

FAQs on Preparation of Salt - Methods with Detailed Explanation for JEE

1. What is the state of matter of salt?

Salt is solid and they don't melt until 800oC, due to the strong electrostatic interaction between the positively charged sodium ion and negatively charged chloride ion. Solids can be shaped or cut. Each particle of salt, for example, retains its shape and volume. Powdered and granulated solids, such as sugar or salt crystals, may flow like liquids, but they are distinctively solid under a microscope. The smallest pieces of a solid are very close together and do not move very much. One example of this type of solid is table salt.

2. What is the solubility rule for hydroxide,  carbonate, and sulphate salts?

Solubility rule for hydroxide,  carbonate and sulphate salts are as follow:  Except for sodium, potassium, and ammonium, all-metal carbonates, phosphates, borates, sulphates, chromates, and arsenates are insoluble in water but soluble in dilute acids. hydroxides of sodium and potassium are soluble, whereas calcium hydroxides are slightly soluble. All other metals' oxides and hydroxides are insoluble. Except for lead, mercury (I), barium, and calcium, all-metal sulphates are soluble in water. Silver sulphate is only marginally soluble. Sulphates that are insoluble in water are also insoluble in dilute acids.

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