Reactions in Solutions: A solution is a homogeneous substance consisting of a solute and a solvent. Mostly, water act as a universal solvent and forms an aqueous solution. The term ‘aqua’ means water. Thus, as the name signifies, an aqueous solution is a type of solution in which the solvent is water. In chemical equations, an aqueous solution of any compound is represented as (aq). Hydrophobic (water-repelling) substances do not dissolve well in water, whereas Hydrophilic (water-loving) substances get completely dissolved in water.

Based on the fact whether the substance can match or exceed the strong, attractive forces that water molecules generate between themselves, their ability to dissolve is determined. If the substance is not able to dissolve in water, the molecules form a precipitate. Aqueous solutions undergo metathesis reactions or double-displacement reactions. Aqueous solutions possess electrolytes and can conduct electric current efficiently. In this article, we will discuss the aqueous solution and the reactions that take place in it.

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Chemical Reactions in Aqueous Solutions

Different chemical substances interact differently. But certain chemical reactions exhibit common patterns of reactivity. In chemistry, we come across a vast number of chemical reactions. Thus, classification of these reactions has become necessary based on the observed patterns of interaction of molecules.

In many chemical reactions, water is used because water is a good solvent that can dissolve many substances, and such reactions are called aqueous reactions. The aqueous reactions are classified into three major types: precipitation, acid-base, and oxidation-reduction.

Types of Chemical Reactions in Aqueous Solutions

1. Precipitation Reactions

The reaction that occurs between two aqueous reactants, i.e., one solid and one liquid, is known as the precipitation reaction. They react to form an insoluble product which is known as a precipitate.

For example, silver nitrate \(\left( {{\text{AgN}}{{\text{O}}_3}} \right)\) react with sodium chloride \(\left( {{\text{NaCl}}} \right)\) to form sodium nitrate \(\left( {{\text{NaN}}{{\text{O}}_3}} \right)\) and solid precipitate of silver chloride \(\left( {{\text{AgCl}}} \right)\).

\({\text{AgN}}{{\text{O}}_3}\, + \,{\text{NaCl}}\, \to \,{\text{NaN}}{{\text{O}}_3}\,{\text{ + }}\,{\text{AgCl}}\, \downarrow \)

2. Acid-Base Reaction

Arrhenius defined acids and bases with reference to the aqueous solution. An acid contains positively charged hydrogen ions in its aqueous solution. On the other hand, a base is a substance that accepts hydrogen ions and produces negative hydroxyl ions in an aqueous solution. When acid and base react, a neutralisation reaction occurs in which salt and water are formed.

For example, a neutralisation reaction occurs when hydrochloric acid \(\left( {{\text{HCl}}} \right)\) reacts with \(\left( {{\text{NaOH}}} \right)\) to produce water and salt – sodium chloride \(\left( {{\text{NaCl}}} \right)\). The chemical equation involved is:

\({\text{HCl}}\, + \,{\text{NaOH}}\, \to \,{\text{NaCl}}\, + \,{{\text{H}}_2}{\text{O}}\)

3. Oxidation-Reduction Reactions

A special type of chemical reaction in which oxidation, as well as reduction, takes place mostly in an aqueous medium is known as an Oxidation-Reduction reaction. Oxidation is the process in which a chemical becomes more positive by losing electrons. On the other hand, the reduction is the opposite process in which a chemical gains electrons and as a result becomes more negative.

An oxidation-reduction reaction takes place between a metal and a non-metal—for example, the formation of sodium chloride when sodium reacts with chlorine.

\(2{\text{Na}}\,{\text{ + }}\,{\text{C}}{{\text{l}}_2}\,\, \to \,2{\text{NaCl}}\,\)

Ionic Reactions in Aqueous Solutions

In a chemical reaction, molecules may actually exist in solution as ions. Though molecular equation is useful because it gives us the information about which solutions have been added and what products have been formed. However, the molecular equation does not tell us that the reaction actually involves ions in the solution.

Let us take an example of the reaction between calcium hydroxide and sodium carbonate to form calcium carbonate and sodium hydroxide with the help of an ionic reaction equation. Solid Calcium hydroxide dissolves in water and get dissociated as \({\text{C}}{{\text{a}}^{2 + }}\) and \({\text{O}}{{\text{H}}^{ – \,}}\). We can say that each formula unit of calcium hydroxide when dissolved in water yields one \({\text{C}}{{\text{a}}^{2 + }}\) and two \({\text{O}}{{\text{H}}^{ – \,}}\) ions.

The ionic equation for a reaction involves ions in a solution in which soluble substances are represented by the formulas of the predominant species present in that solution. In the ionic equation, some ions appear on both sides of the arrow, these are known as spectator ions; they are ions present in an ionic equation that does not take part in the reaction. The ionic reaction equation is given below:

\({\text{C}}{{\text{a}}^{2 + }}\left( {{\text{aq}}} \right)\,{\text{ + }}\,{\text{2O}}{{\text{H}}^{ – \,}}\left( {{\text{aq}}} \right)\, + \,2{\text{N}}{{\text{a}}^ + }\left( {{\text{aq}}} \right)\, + \,{\text{CO}}_3^{2 – }\left( {{\text{aq}}} \right)\, \to {\text{CaC}}{{\text{O}}_3}\left( {\text{s}} \right)\, + \,2{\text{N}}{{\text{a}}^ + }\left( {{\text{aq}}} \right)\, + 2{\text{O}}{{\text{H}}^ – }\left( {{\text{aq}}} \right)\)

How To Calculate Mass of Solute in Solution?

Step 1: Mass of a solute is calculated by using molarity. It is most often method used in chemistry. The equation for calculating molarity is:

\({\text{Molarity}}\,{\text{ = }}\frac{{{\text{Moles}}\,{\text{of}}\,{\text{solute}}}}{{{\text{Litres}}\,{\text{of}}\,{\text{solution}}}}\)

With the help of the above formula, if we know the molarity (concentration) of a solution and the total volume of the solution in litres, we can determine the number of moles of solute. Then the mole of solute is converted to grams.

Step 2: Another method of determining the mass or amount of solute in a solution is by calculating percent by mass. The equation used for this mass percent is:

\({\text{Mass}}\,{\text{percent}}\,{\text{ = }}\frac{{{\text{Mass}}\,{\text{of}}\,{\text{solute}}}}{{{\text{Mass}}\,{\text{of}}\,{\text{solution}}}}\, \times \,100\% \)

Step 3: One more method of determining the amount of solute in each solution is per cent by volume. This is calculated by the given formula:

\(\,{\text{Percent by}}\,{\text{Volume}}\,{\text{ = }}\frac{{{\text{Volume}}\,{\text{of}}\,{\text{solute}}}}{{{\text{Volume}}\,{\text{of}}\,{\text{solution}}}}\, \times \,100\% \)

Aqueous is a term that is used to define a system that involves water. Aqueous is the word that is applicable to describe a solution or mixture in which water is the solvent, and the formation of an aqueous solution depends on the nature of its chemical bonds. When a substance dissolves in water, this is denoted by writing (aq) after its chemical name in a chemical equation.

Hydrophilic (water-loving species) and many other ionic compounds dissolve in water. Aqueous solutions often allow the conduct of electricity. Solutions such as seawater contain strong electrolytes and tend to be very good electrical conductors. Conversely, solutions that contain weak electrolytes are known to be poor conductors, such as tap water.

It is due to the reason that strong electrolytes completely dissociate into ions in water, whereas weak electrolytes dissociate incompletely. Most of the chemical reactions that occur between species in an aqueous solution are double replacement reactions; they are mostly precipitation reactions along with acid-base reactions and oxidation-reduction reactions.

Frequently Asked Questions on Reactions in Solutions

Q.1. What are the \(6\) types of reactions?
Ans. The six types of chemical reactions are:

(i) Combination reaction is a chemical reaction that involves combining two or more reactants to form a single new product.

(ii) Decomposition reaction is the chemical reaction in which a complex compound is broken up into two or more simple substances.

(iii) Displacement reaction is the chemical reaction in which a more reactive element displaces some other less reactive elements from a compound.

(iv) Double displacement reaction involves two compounds that react together and form two new compounds by exchanging one of their ions.

(v) Oxidation and Reduction,

(vi) Precipitation reaction.

Q.2. What are the three types of solution reactions?
Ans. The three types of solution reactions are:

(i). Precipitation Reactions: The reaction that occurs between two aqueous reactants, i.e., one solid and one liquid, is known as the precipitation reaction.

(ii). Acid-Base Reaction: Arrhenius defined acids and bases with reference to an aqueous solution. An acid contains positively charged hydrogen ions in its aqueous solution. On the other hand, a base accepts hydrogen ions and produces negative hydroxyl ions in an aqueous solution.

(iii). Oxidation-Reduction Reactions: A special type of chemical reaction in which oxidation and reduction take place mostly in an aqueous medium is known as an Oxidation-Reduction reaction.

Q.3. Why does a reaction only occur in a solution?
Ans. A chemical reaction occurs only in a solution because the chemical compounds need to be dissociated into ions to undergo a chemical reaction.

Q.4. How do you identify a reaction type?
Ans. By looking at the reactants and the type of products in a chemical reaction, we can identify a type of chemical reaction. For example, if two reactants combine to form a single product, then the reaction type is a combination reaction. If a more reactive metal displaces a less reactive metal, the reaction type is displacement reaction.

Q.5. What are combustion reactions?
Ans. A combustion reaction is an exothermic reaction. This means it releases heat at the end of the reaction, but sometimes the reaction proceeds so slowly that the change in temperature is not noticeable. A combustion reaction occurs between a carbon compound and oxygen to produce carbon dioxide gas, water vapour, along with heat and light.

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