We know that an acidic solution contains hydrogen ions, that is, H+ ions and a basic solution contains hydroxyl ions, that is, OH- ions. Another important fact to remember is that a solution with more H+ ions is more acidic, that is, a strong acid while the one with more OH- ions is more basic, that is, a strong base.

To determine how strong a given acid or a base is, we make use of a universal indicator which is a mixture of several indicators. This universal indicator gives different colors at different concentrations of hydrogen ions in a solution.

We use a specific scale for measuring the hydrogen ion concentration, known as the pH scale. The pH of a solution is a number that indicates the acidic or basic nature of a solution. It ranges from '0' to '14'. The pH of a neutral solution is 7. Values less than 7 represent acidic solutions while those greater than 7 represent the basic nature of solutions.

If the acidic nature increases, that is, the hydrogen or hydronium ion concentration increases, the pH goes down. On the other hand, if the basic nature increases, the pH of the solution goes up. Generally, paper impregnated with the universal indicator is used for measuring pH. Such paper is known as the pH paper.

Strength of Acids and Bases

The strength of acids and bases depends on the number of H+ ions and OH- ions produced, respectively. Let us understand this with the help of an example. If we take hydrogen chloride and acetic acid of the same concentration, say one molar, then these produce different amounts of H+ ions. Acids that give rise to more H+ ions are said to be strong acids while the ones that give less H+ ions are said to be weak acids. Similarly, if we take two bases of the same concentration, say 1 Molar NaOH and 1 Molar NH4OH, then the base that gives more OH- ions is a strong base and the one that produces less OH- ions is considered a weak base. It is because of the different strengths of the solutions that differentiate them into strong and weak acids and bases.

Acid-Base Reactions

How do Acid and Base react with metals?

Generally, acids react with metals to form salt and release hydrogen gas. For example, when dilute sulphuric acid reacts with zinc granules, zinc sulfate is formed and hydrogen gas is released. Metal displaces hydrogen from the acid.

Similarly strong base like sodium hydroxide reacts with zinc granules to form sodium zincate along with the evolution of hydrogen gas. This reaction is not possible with all the metals.

Here in this module, we will study about reactions of acids and bases with each other.

1. Take about 2 ml of dilute NaOH solution in a test tube and add two drops of phenolphthalein solution.
2. You will observe that The color of the solution becomes pink.
3. Now add dilute HCl solution to this solution drop by drop.
4. The Colour of the solution now changes to colorless from pink.
5. Now again add NaOH to the mixture drop by drop. The pink color of phenolphthalein reappears.

The effect of base (NaOH) is nullified by the acid (HCl) but with further addition of base (NaOH), the color of phenolphthalein reappears. In fact, the reaction between an acid and a base gives salt and water and this reaction is called a Neutralization reaction.

How do metal carbonates and metal hydro carbonates react with acids?

When metal carbonate like sodium carbonate i.e. Na₂CO₃ reacts with an acid like hydrochloric acid, salt is formed along with carbon dioxide and water. The gas evolved can be identified by passing it through lime water i.e. Ca(OH)₂. The gas turns the lime water milky and therefore we can conclude the gas to be carbon dioxide.

On passing excess of carbon dioxide, water-soluble calcium bicarbonate is formed, which dissolves in water, so the milkiness disappears. Similarly, when metal hydrogen carbonate like NaHCO₃ reacts with dilute hydrochloric acid, salt and water are formed with the evolution of carbon dioxide.

Reaction of Metallic Oxide with Acids and Non-Metallic Oxide with base

Let's learn about the reaction of metallic oxide with an acid and of non metallic oxide with base

Take a small amount of copper oxide in a beaker and add dilute hydrochloric acid slowly while stirring.

The color of the solution becomes blue-green and copper oxide dissolves. The blue-green color of the solution is due to the formation of copper chloride. In fact, the reaction of metallic oxides with acids results in the formation of salt and water.

Similarly when CO₂ is passed through lime water Ca(OH)₂, which is a base, is formed. Lime water turns milky because of the formation of calcium carbonate. Generally, non-metallic oxides react with bases to form salt and water.

What Happens to an Acid or a Base in a Water Solution?

Do acids produce ions only in an aqueous solution? To find the answer Let us do some experiments:

1. Take about 1g solid NaCl in a clean and dry test tube and set up the apparatus as shown
2. Add some concentrated sulphuric acid to the test tube.

What do you observe? Is there a gas coming out of the delivery tube? Test the gas evolved successively with dry and wet blue litmus paper. In which case does the litmus paper change color? On the basis of the above Activity, what do you infer? This experiment suggests that hydrogen ions in HCl are produced in the presence of water.

The separation of H+ ions from HCl molecules does not occur in the absence of water. The reaction being HCL reacts with water to give hydronium ions that are H3O⁺ and chlorine ions. It is very important to understand that Hydrogen ions cannot exist alone, but they exist after combining with water molecules. Thus hydrogen ions must always be shown as H+ ions or hydronium ions (H3O+ ions). It can be represented by the reaction as Hydrogen ions react with water to form hydronium ion H3O+. Now here we have seen that acids give H3O+ or H+ (aq) ions in water.

Let us see what happens when a base is dissolved in water.

The reaction will be:

NaOH(s) is dissolved in water it gives Na plus ions and (aq)+OH minus (aq)ions

Similarly, if KOH(s) is dissolved in water, it gives K plus ions and (aq)+ OH (aq)ions. Also if MgOH2 (s) is dissolved in water it gives Mg plus 2 ions and (aq)+2 OH (aq)ions.

So we can conclude that Bases generate hydroxide (OH ) ions in water.

Bases that are soluble in water are called alkalis. But do you know? All bases do not dissolve in water. An alkali is a base that dissolves in water. They are soapy to the touch, bitter, and corrosive.

So to conclude, all acids generate H+ (aq) and all bases generate OH (aq),

We can view the neutralization reaction as follows: Acid + Base = Salt + Water

OR

H⁺+ X^- + M⁺+ OH^- ? MX + H2O

Where H ions from the acid reacts with OH minus ions from the base to form water.

Let us see what is involved when water is mixed with an acid or a base.

1. Take 10 mL water in a beaker.
2. Add a few drops of concentrated H2SO4 to it and swirl the beaker slowly.
3. Touch the base of the beaker. Is there a change in temperature? Is this an exothermic or endothermic process?

Repeat the above Activity with sodium hydroxide pellets and record your observations as shown here! From the experiment, we would understand that the process of dissolving an acid or a base in water is a highly exothermic one.

Care must be taken while mixing concentrated nitric acid or sulphuric acid with water. The acid must always be added slowly to water with constant stirring.

If water is added to a concentrated acid, the heat generated may cause the mixture to splash out and cause burns. The glass container may also break due to excessive local heating. You will generally find warning signs on the can of concentrated sulphuric acid and on the bottle of sodium hydroxide pellets. It is important to understand that Mixing an acid or base with water results in a decrease in the concentration of ions (H3O+ /OH) per unit volume.

Such a process is called dilution and the acid or the base is said to be diluted.

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Read More: Salts- Family of Salts And Its Uses | Acids, Bases, and Salts