Acids, Bases and Salts-Notes
Chemistry - Notes
Acid
Acid is a substance that contains hydrogen and can donate hydrogen ions (H+) to other substances. Acids have a sour taste, turn blue litmus paper red, and neutralise bases.
Properties of acids
- Acids are hydrogen-containing substances
- Acids taste sour
- Acids turn blue litmus paper red
- Acids neutralize bases
- Acids form salts when they combine with certain metals
- Acids can liberate dihydrogen when they react with some metals
Base
A base is a substance that can accept hydrogen ions (H+) from acids and neutralize them. Bases are also known as alkalis.
Properties of bases
- Bases have a bitter taste
- Bases feel soapy or slippery to the touch
- Bases react with acids to form salts
- Bases change the color of indicators, such as turning red litmus paper blue
- Bases can be strong or weak
Salt
Salt is defined as an ionic compound that is formed when an acid and a base react. Salts are made up of positively charged ions (cations) and negatively charged ions (anions).
Formation of Salt
- Salts are formed when part or all of the hydrogen in an acid is replaced by a metal or another cation.
- Salts are formed through a neutralization reaction between an acid and a base.
- Most salts are soluble in water. When salt is mixed into water, it breaks down and dissolves.
Reaction of Acids and Bases with Metals
Acid + Metal \(\longrightarrow\) Salt + Hydrogen gas
Base + Metal \(\longrightarrow\) Salt + Hydrogen gas
Reaction of Metal Carbonates and Metal Hydrogencarbonates with Acids
Metal carbonate/Metal hydrogencarbonate \(\longrightarrow\) Salt + Carbon dioxide + Water
- \[ \scriptsize \begin{aligned} \mathrm{Na_2CO_3(s) }&\mathrm{+ 2HCl(aq)} \rightarrow \\&\mathrm{2NaCl(aq) + H_2O(l) + CO_2(g)} \end{aligned} \]
- \[ \scriptsize \begin{aligned} \mathrm{NaHCO_3(s)}&\mathrm{ + HCl(aq)} \rightarrow \\& \mathrm{NaCl(aq) + H_2O(l) + CO_2(g)} \end{aligned} \]
On passing the carbon dioxide gas evolved through lime water
\[\scriptsize\begin{aligned}\mathrm{\underset{\text{Lime Water}}{Ca(OH)_2(aq)}}&\mathrm{ + CO_2(g)} \rightarrow\\& \mathrm{\underset{\text{White Precipitate}}{CaCO_3(s)} + H_2O(l) }\end{aligned}\]
On passing excess carbon dioxide the following reaction takes place
\[\scriptsize\begin{aligned}\mathrm{CaCO_3(s)+ H_2O(l)+}&\mathrm{ CO_2(g)} \rightarrow\\&\mathrm{\underset{\text{Soluble in Water}}{Ca(HCO_3)_2(aq)} }\end{aligned}\]
Reaction of Acids and Bases with each other
When an acid reacts with a base, they undergo a chemical reaction called neutralisation. In this process, the acid and base cancel each other's effect, forming salt and water.
Base + Acid \(\rightarrow\) Salt + Water
\(\scriptsize\mathrm{NaOH(aq) + HCl(aq) \rightarrow NaCl(aq) + H_2O(l)}\)
Highlights
- Salt is the compound formed from the acid's anion and the base's cation.
- The mix becomes neutral (pH \(\approx 7\)) if the acid and base are mixed in the right amounts.
- This reaction is important for understanding how to treat acidity (like antacids for the stomach).
Reaction of Metallic Oxides with Acids
Metallic oxides are basic in nature. When metallic oxides react with acids, they behave like bases and neutralise acids to form salt and water.
Metal oxide + Acid \(\longrightarrow\) Salt + Water
\(\scriptsize\mathrm{CuO(s) + 2HCl(aq)\rightarrow CuCl_2(aq)+H_2O}\)
Acid or a Base in a Water Solution
When an acid or base is dissolved in water, it ionizes and releases hydrogen ions \(\mathrm(H^+)\) or hydroxide ions \(\mathrm(OH^-)\). The concentration of these ions determines if the solution is acidic or basic.
Acids in Water:
- Acids release hydrogen ions \(\mathrm{(H^+)}\) when dissolved in water.
- This process is called ionisation.
- The actual hydrogen ions exist as hydronium ions (H₃O⁺) because H⁺ ions combine with water molecules.
Example:
\[\scriptsize\mathrm{HCl\xrightarrow{H_2O}H^+ + Cl^-}\] \[\scriptsize\mathrm{H^+ + H_2O\rightarrow H_3O^+}\]
Bases in Water:
- Bases release hydroxide ions \(\mathrm{(OH^⁻)}\) when dissolved in water.
- This is called dissociation.
Example:
\[\scriptsize\mathrm{NaOH\xrightarrow{H_2O}Na^+ + OH^-}\]
Other effects
- The process of dissolving an acid or a base in water is called ionization.
- The extent of dissociation of an acid or base determines if it is strong or weak
- The concentration of hydronium ions (H3O+) and hydroxide ions (OH-) in a solution is described using pH and pOH.
- Mixing an acid or base with water decreases the concentration of ions per unit volume, which is called dilution.
Note:
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.
Strength of Acids and Bases
The strength of an acid or base solution depends on the concentration of hydrogen ions \(\mathrm{(H^+)}\) in acids or hydroxide ions \(\mathrm{(OH^-)}\) in bases. This strength is measured using pH (potential of Hydrogen) scale.
The pH Scale (0 to 14)
- pH < 7: Solution is acidic (more \(H^+\)).
- pH = 7: Solution is neutral (pure water).
- pH > 7: Solution is basic (more \(OH^-\))
Lower the pH value, stronger the acid.
Higher the pH value (above 7), stronger the base.
Strong Acids and Bases
- Strong acid: Completely dissociates in water to produce a high concentration of \(H^+\) ions (example: \(\mathrm{HCl, H_2SO_4}\)).
- Weak acid: Partially dissociates, producing fewer \(H^+\) ions (example: acetic acid).
- Strong base: Completely dissociates in water to produce a high concentration of \(OH^-\)ions (example: NaOH, KOH).
- Weak base: Partially dissociates (example: ammonium hydroxide).
Importance of pH in Everyday Life
- Our body works within the pH range of 7.0 to 7.8. Living organisms can survive only in a narrow range of pH change
- It is very interesting to note that our stomach produces hydrochloric acid. It helps in the digestion of food without harming the stomach.
- Tooth decay starts when the pH of the mouth is lower than 5.5.
Naturally Occurring Acids
| Acid | Source |
|---|---|
| Citric acid | Found in lemons, limes, oranges, and other citrus fruits. |
| Lactic acid | Produced in muscles during exercise and is also found in sour milk and yogurt. |
| Acetic acid | The main component of vinegar, formed from the fermentation of alcohols. |
| Malic acid | Gives apples and other fruits their tart taste. |
| Ascorbic acid | Commonly known as Vitamin C, found in many fruits and vegetables. |
| Formic acid | Produced by ants and some plants, causing the stinging sensation when they bite or sting. |
| Tartaric acid | Naturally occurs in grapes, tamarind, and bananas. |
| Oxalic acid | Found in plants like spinach, tomato and rhubarb. |
| Hydrochloric acid | Naturally produced in the stomach to aid in digestion. |
| Fatty acids | A large group of acids that are major components of fats and oils in plants and animals. |
| Methanoic acid | Ant sting, Nettle sting. |
More about Salts
Sodium hydroxide
When electricity is passed through an aqueous solution of sodium chloride (called brine), it decomposes to form sodium hydroxide. The process is called the chlor-alkali process because of the products formed– chlor for chlorine and alkali for sodium hydroxide.
\[\tiny\mathrm{2NaCl(aq) + 2H_2O(l) → 2NaOH(aq) + Cl_2(g) + H_2(g)}\]
Chlorine gas is given off at the anode, and hydrogen gas at the cathode. Sodium hydroxide solution is formed near the cathode. The three products produced in this process are all useful.
Bleaching powder
Bleaching powder is produced by the action of chlorine on dry slaked lime \(\mathrm{[Ca(OH)_2]}\). Bleaching powder is represented as \(\mathrm{CaOCl_2}\), though the actual composition is quite complex.
\[\scriptsize\mathrm{Ca(OH)_2 + Cl_2 \rightarrow CaOCl_2 + H_2O}\]
Uses of Bleaching Powder
- for bleaching cotton and linen in the textile industry, for bleaching wood pulp in paper factories and for bleaching washed clothes in the laundry
- as an oxidising agent in many chemical industries; and
- to make drinking water free from germs.
Baking soda
The baking soda is commonly used in the kitchen for making tasty crispy pakoras, etc. Sometimes it is added for faster cooking. The chemical name of the compound is sodium hydrogencarbonate \(\mathrm{(NaHCO_3)}\). It is produced using sodium chloride as one of the raw materials.
\[\tiny \begin{aligned} \mathrm{NaCl} + &\mathrm{H_2O} + \mathrm{CO_2} + \mathrm{NH_3} \rightarrow \\&\underset{\text{Ammonium Chloride}}{\mathrm{NH_4Cl}} +\, \underset{\text{Sodium hydrogencarbonate}}{\mathrm{NaHCO_3}} \end{aligned} \]
Uses of Baking soda
- For making baking powder, which is a mixture of baking soda (sodium
hydrogencarbonate) and a mild edible acid such as tartaric acid. When baking powder is heated or mixed
in water, the following reaction takes place
\[\scriptsize \begin{aligned} \mathrm{NaHCO_3} + \mathrm{H^+} &\xrightarrow{\text{(From any acid)}}\;&\\\mathrm{CO_2} + \mathrm{H_2O} &+\, \text{Sodium salt of acid} \end{aligned} \]
Carbon dioxide produced during the reaction can cause bread or cake to rise, making them soft and spongy. - Sodium hydrogencarbonate is also an ingredient in antacids. Being alkaline, it neutralises excess acid in the stomach and provides relief.
- It is also used in soda-acid fire extinguishers
Washing soda
Another chemical that can be obtained from sodium chloride is \(\scriptsize\mathrm{Na_2CO_3.10H_2O}\) (washing soda). You have seen above that sodium carbonate can be obtained by heating baking soda; recrystallisation of sodium carbonate gives washing soda. It is also a basic salt.
\[\scriptsize\mathrm{\underset{\text{(Sodium carbonate)}}{Na_2CO_3} + 10H_2O \rightarrow Na_2CO_3.10H_2O}\]
Uses of washing soda
- Sodium carbonate (washing soda) is used in glass, soap and paper industries.
- It is used in the manufacture of sodium compounds such as borax.
- It is used in the manufacture of sodium compounds such as borax.
- It is used for removing permanent hardness of water.
Plaster of Paris (PoP)
On heating gypsum at 373 K, it loses water molecules and becomes calcium sulphate hemihydrate \(\scriptsize\mathrm{(CaSO_4 .\frac{1}{2} H_2O)}\). This is called Plaster of Paris, the substance which doctors use as plaster for supporting fractured bones in the right position. Plaster of Paris is a white powder and on mixing with water, it changes to gypsum once again giving a hard solid mass.
\[\scriptsize\mathrm{\underset{(Plaster of Paris)}{CaSO_4.\frac{1}{2} H_2O}+ \frac{1}{2} H_2O \rightarrow \underset{( Gypsum)}{CaSO_4.2H_2O}}\]
Uses of Plaster of Paris
- Used by doctors to set fractured bones in place as casts.
- Making decorative ceiling designs, wall mouldings, and cornices in buildings.
- Creating models, sculptures, and statues for art and education.
- Producing toys, chalk sticks, and drawing boards.
- Acting as a fireproofing material for coating surfaces.
- Making impression materials in dentistry and for fossils/moulds in archaeology.
- Used in making bandages for supporting joints and bones in case of injuries.
- Filling gaps or cracks in walls before painting (as wall putty).
Water of Crystallisation:
- Water of crystallisation is the fixed quantity of water molecules that is chemically bound to a salt within its crystals.
- It gives hydrated salts their crystalline shape and structure.
- These water molecules are not just physically trapped—they are an essential part of the salt's chemical formula.
- When hydrated salts are heated, they lose their water of crystallisation and become anhydrous (dry).
Example:
- Blue crystals of Copper(II) sulphate contain water of crystallisation, written as \( \mathrm{CuSO_4 \cdot 5H_2O} \).
- Plaster of Paris contains water of crystallisation, written as \( \mathrm{CaSO_4 \cdot 0.5H_2O} \).
- On heating, hydrated salts lose water of crystallisation and become anhydrous (dry and often colorless).
Important Points
- Acid-base indicators are dyes or mixtures of dyes which are used to indicate the presence of acids and bases.
- Acidic nature of a substance is due to the formation of \(H^+(aq)\) ions in solution. Formation of \(OH^–(aq)\) ions in solution is responsible for the basic nature of a substance.
- When an acid reacts with a metal, hydrogen gas is evolved and a corresponding salt is formed.
- When a base reacts with a metal, along with the evolution of hydrogen gas a salt is formed which has a negative ion composed of the metal and oxygen.
- When an acid reacts with a metal carbonate or metal hydrogencarbonate, it gives the corresponding salt, carbon dioxide gas and water.
- Acidic and basic solutions in water conduct electricity because they produce hydrogen and hydroxide ions respectively.
- The strength of an acid or an alkali can be tested by using a scale called the pH scale (0-14) which gives the measure of hydrogen ion concentration in a solution.
- A neutral solution has a pH of exactly 7, while an acidic solution has a pH less than 7 and a basic solution a pH more than 7.
- Living beings carry out their metabolic activities within an optimal pH range.
- Mixing concentrated acids or bases with water is a highly exothermic process.
- Acids and bases neutralise each other to form corresponding salts and water.
- Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt.
- Salts have various uses in everyday life and in industries.
