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Acids, Bases and Salts are the three kinds of substances that produce different results in different types of solutions.
- Acid is a substance whose water solution is sour in taste, turns blue litmus red and neutralizes bases.
- Bases are aqueous solutions which are bitter in taste, turn red litmus blue or neutralize acids.
- Salts are neutral substances whose aqueous solution does not affect litmus.
Several acids and bases can be found naturally in nature. For example, citric acid can be found in fruits like lemon, tangerine and oranges. While, tartaric acid can be found in tamarind and lactic acid can be naturally found in milk and its products. Apart from these, substances like apples contain malic acid. In the same way, lime water is an example of bases which occur naturally.
A neutralisation reaction can be found to take place when an acid reacts with a base. At the end of this reaction, salt and water are seen to be the end products. An acid-base neutralisation reaction is a double-replacement reaction in this approach.
Read More: Classification of Oxides
Key Terms: Acid, Base, Salts, pH, Neutrons, Protons, Valence Electrons, Ionization, Dissociation, Bronsted Lowry Concept, Arrhenius concept, Lewis Concept
Acids, Bases and Salts
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Acid is a substance that donates protons or accepts a pair of valence electrons to form a bond. A base is a substance that can take protons or donate a pair of valence electrons to form a bond. Bases can be thought of as the chemical opposite of acids.
Acids
Acid is a term derived from the Latin word “Acidus” or “Acere”, meaning sour.
- One of the most common characteristics of acids is their sour taste.
- Acid is a substance which yields ionizable ion (H3O+) in its aqueous solution.
- Acids turn blue litmus paper into red.
- It dissociates in their aqueous solution to form constituent ions.
The same can be shown by the following examples:
- HCl (aq) → H+ + Cl-
- H2SO4 → 2H+ + SO4-2
- CH3CO2H (aq) → H+ + CH3CO2-
Bases
One of the most common characteristic property of bases is their bitter taste and the feel of soap. A base can be defined as a substance that yields hydroxyl ion (OH–) in its aqueous solution. Bases are known to turn red litmus paper into blue.
Salts
Salt can be defined as an ionic compound which results from the neutralization reaction between acids and bases. Salts are composed of positively charged ions, also called cations, and negatively charged ions, which are anions, that are either organic or inorganic in nature. These ions can be found in relative amounts, thus causing the nature of the salt to be neutral.
- HCl + NaOH → NaCl + H2O
- H2SO4 + Ca(OH)2 → CaSO4 + 2H2O
Salts And Their Derivation
Salt is a substance produced by the reaction of an acid with a base. A salt consists of the positive ion (cation) of a base and the negative ion (anion) of an acid. Here are important derivations of some Salts:
| Name of Salt | Formula | Derived from | Derived From | Chemical Reaction |
|---|---|---|---|---|
| Ammonium Chloride | NH4Cl | NH4OH | HCl | NH4OH + HCl → NH4Cl + H2O |
| Sodium Chloride | NaCl | NaOH | HCl | NaOH + HCl → NaCl + H2O |
| Sodium Sulphate | Na2SO4 | NaOH | H2SO4 | NaOH + H2SO4 → Na2SO4 + H2O |
| Potassium Sulphate | K2SO4 | KOH | H2SO4 | KOH + H2SO4 → K2SO4 + H2O |
Ionization and Dissociation
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Dissociation can be defined as the separation of ions from an ionic crystal in case the solid ionic compound is seen to to dissolve in water. Ionization, however, is the method wherein a neutral molecule is seen to break into charged ions when it is dissolved in a solution. The extent of ionization is based on the strength of the bonds between ions and the extent of ion solvation.
Arrhenius Concept
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As per the Arrhenius concept, substances that produce H+ ions when it is dissolved in water are known as acids. The substances thar ionize in water to produce OH– ions are called bases.
- HA → H+ + A– (Acid)
- BOH → B+ + OH– (Base)
Arrhenius once proposed that acid-base reactions are classified by acids in case they dissociate in an aqueous solution to make hydrogen ions (H+) alongside bases in case they form hydroxide (OH–) ions in an aqueous solution.
Limitations of Arrhenius Concept
Some limitations of Arrhenius Concept:
- Water is necessary for acids and bases. Dry HCl is unable to act as an acid. HCl can only act in acid while in water and not in other solvents.
- This concept does not illustrate the acidic and basic characteristics of substances in non-aqueous solvents.
- The neutralization process is deemed possible for reactions that occur in aqueous solutions, even though reactions consisting of salt formation can take place in the absence of a solvent.
Bronsted-Lowry Concept
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Bronsted and Lowry, in the year 1923, proposed certain general definitions of acids and bases. As per their words, an acid is any material containing hydrogen (molecule, anion or cation) that can donate a proton to another substance. In the same way, a base can be defined as a substance (molecule, cation or anion) which can accept a proton from another substance. Thus, acids can be defined as proton donors, while bases are proton acceptors.
Limitations of Bronsted Lowry Concept
Some of the Bronsted Lowry Concept limitations include:
- Bronsted Lowry is unable to illustrate the reaction which occurs in the non-protonic solvent, such as SO2, COCl3, N2O4, and more.
- It is also unable to illustrate the reactions that occur between acidic oxides and basic oxides that can easily occur in the absence of solvent.
Lewis Concept
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As per the Lewis theory of acid-base reactions, bases are known to donate electron pairs, while acids accept electron pairs. Thus, Lewis acid is known to be an electron-pair acceptor. Lewis theory is known to complement the model of oxidation-reduction reactions. Oxidation-reduction reactions occur due to the electron transfer from one atom to another, alongside a net change that can be found in the oxidation number of one or more atoms.
The Lewis theory also claimed that acids are known to react with bases, sharing a pair of electrons. However, it has no change in the oxidation numbers of any atoms. Thus,
- Al(OH)3 + 3H+ → Al3+ + 3H2O (Aluminium hydroxide acts as a base)
- Al(OH)3 + OH– → Al(OH)4- (Aluminium hydroxide acts as an acid)
When Aluminium hydroxide is seen to accept protons, it starts to accept as a base. If it accepts electrons, it starts to accept as an acid. This Lewis acid-base theory defines why non-metal oxides like carbon dioxide dissolve in H2O to compose acids, like carbonic acid H2CO3.
Hence, CO2(g) + H2O(l) → H2CO3(aq)
Limitations of Lewis Concept
Some of the Lewis Concept Limitations include:
- The relative strength of acids and bases cannot be illustrated by Lewis concept.
- Lewis concept is not at par with acid-base reaction concept.
- Lewis concept does not illustrate the behaviour of protonic acids such as HCl.
| Related Articles | ||
|---|---|---|
| Acids Bases and Salt Revision Notes | List of Strong Acids | Acids Bases and Salt Important Questions |
| Weak Base | Acid Test | Acid Strength |
How to Identify Acids and Bases?
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Indicators are used to know the presence of acids and bases. It may be natural or synthetic.
Natural indicators are purple in colour and are extracted from lichen. There are many other natural materials like turmeric, red cabbage leaves, coloured petals of some flowers such as Petunia, Hydrangea, and Geranium, which indicate the presence of acid or base in a solution. These are called acid-base indicators.
| Indicators | Colour In Acidic Solution | Colour in Base Solution |
|---|---|---|
| Blue Litmus | Red | No colour change |
| Red Litmus | No colour change | Blue |
| Methyl Orange | Pink | Yellow |
| Phenolphthalein | Colourless | Pink |
Physical Properties of Acids and Bases
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Physical properties of Acids and Bases are as follows:
| Properties | Acids | Bases |
|---|---|---|
| Taste | Sour | Bitter |
| Color on Litmus Paper | Turns Blue Litmus Red | Turns Red Litmus Blue |
| Ions produced on dissociation | H+ | OH- |
| pH | <7 (less than 7) | >7 (more than 7) |
| Strong Acids | HCl, HNO3, H2SO4 | NaOH, KOH |
| Weak Acids | CH3COOH, H3PO4, H2CO3 | NH4OH |
Chemical Properties of Acids and Bases
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Chemical properties of Acids and Bases are listed below:
| Type of Reaction | Acid | Base |
|---|---|---|
| Reaction with Metals | Acid + Metal → Salt + Hydrogen gas (H2) E.g., Zn(s) + dil. H2SO4 → ZnSO4 (Zinc Sulphate) + H2 | Base + Metal → Salt + Hydrogen gas (H2) E.g., 2NaOH +Zn → Na2ZnO2 (Sodium zincate) + H2 |
| Reaction with hydrogen carbonates (bicarbonate) and carbonates | Metal carbonate/Metal hydrogen carbonate + Acid → Salt + Carbon dioxide + Water E.g., HCl+NaOH → NaCl + H2O Na2CO3+ 2 HCl(aq) →2NaCl(aq)+ H2O(l) + CO2(g) Na2CO3+ 2H2SO4(aq) → 2Na2SO4(aq)+ H2O(l) + CO2(g) NaHCO3+ HCl → NaCl+ H2O+ CO2 | Base+ Carbonate/ bicarbonate → No reaction |
| Neutralisation Reaction | Base + Acid → Salt + Water E.g., NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l) | Base + Acid → Salt + Water E.g., CaO+ HCl (l) → CaCl2 (aq) + H2O (l) |
| Reaction with Oxides | Metal oxide + Acid → Salt + Water E.g., CaO + HCl (l) → CaCl2 (aq) + H2O (l) | Non- Metallic oxide + Base → Salt + Water E.g., Ca(OH)2 + CO2 → CaCO3 + H2O |
| Dissolution in Water | Acid gives H+ ions in water. E.g., HCl → H+ + Cl- HCl + H2O → H3O+ + Cl– | Base gives OH- ions in water. |
What is pH Paper?
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pH paper is a paper which measures the pH variation of pH. The p in pH stands for ‘potenz’ in German, meaning power. On the pH scale we can measure pH from 0 (very acidic) to 14 (very alkaline). concentration, lower is the pH value. The pH of a neutral solution is 7. Values less than 7 on the pH scale represent an acidic solution. As the pH value increases from 7 to 14, it represents an increase in OH–ion concentration in the solution, that is, an increase in the strength of alkali.
Variation of pH with change in concentration
Value of pH
The value of pH can be tabulated as:
| Value of pH | Colour of pH indicator | Nature of solution | H+ ion concentration | OH- ion concentration |
|---|---|---|---|---|
| 0 | Dark red | Highly acidic | Very High | Very Low |
| 4 | Orange or yellow | Acidic | High | Low |
| 7 | Green | Neutral | Equal | Equal |
| 10 | Bluish green or Blue | Alkaline | Low | High |
| 14 | Dark Blue or violet | Highly Basic | Very Low | Very High |
Importance of pH in Everyday Life
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Here are the key importance of pH in Everyday Life:
- Any changes in the pH value of the soil can make it infertile.
- Both plants and animals can survive only with a minimal range of pH changes. This is because both animal and human bodies can survive in a pH range of about 7 to 7.5.
- The pH value is essential for the activation of pepsin, which helps in the digestion of protein.
- The teeth of humans start to decay when the pH value decreases.
- Acidity can be caused if the pH value in our stomach decreases.
Download Importance of pH in everyday life PDF
| Related Articles | ||
|---|---|---|
| Properties of Bases | Difference between Alkali and Base | Acids and Bases |
| Chapter 2 Acids, Bases and Salts | Types of Solutions | Basic Chemistry |
Important Topics for JEE MainAs per JEE Main 2024 Session 1, the important topic included in the chapter Acids, Bases and Salts is as follows:
Some memory based important questions asked in JEE Main 2024 Session 1 include: 1. What is the pH of CH3COONH4+? (At 25°C). Given: Ka of CH3COOH = 1.8 x 10-5, Kp of NH4OH = 1.8 x 10-5. 2. The Ksp of Mg(OH)2 is 1 x 10-12. Find the limiting pH at 25 °C at which 0.01 M Mg2+ ions will precipitate. |
Things to Remember
- Acid can be defined as a type of substance that comes with a water solution sour in taste, responsible to turn blue litmus red and neutralizing bases.
- Bases are known to be aqueous solutions that taste bitter, turning red litmus blue, and neutralizing acids.
- Salts are neutral substances that have an aqueous solution which doesn’t affect litmus.
- pH paper can be defined as a type of paper that is known to measure the pH variation of pH.
- A neutralisation reaction typically occurs when an acid reacts with a base.
Sample Questions
Ques. Give the names and formulae of two
(i) strong monobasic acids
(ii) two weak dibasic acids (2 Marks)
Ans. (i) hydrochloric acid (HCl), nitric acid (HNO3)
(ii) carbonic acid (H 2CO3), oxalic acid (C 2H 2O4)
Ques. Why alkalies like sodium hydroxide and potassium hydroxide should not be left exposed to air? (2 Marks)
Ans. Because they are hygroscopic in nature and absorb moisture from the atmosphere in which they ultimately dissolve.
Ques. Arrange the following in increasing order of their pH values:
NaOH solution, blood, lemon juice (1 Mark)
Ans. Increasing order of pH values is: lemon juice < blood < NaOH solution
Ques. Name the natural source of each of the following:
(a) Citric acid
(b) Oxalic acid
(c) Lactic acid
(d) Tartaric acid (CBSE 2014) (2 Marks)
Ans. (a) Citrus fruits like lemons and oranges
(b) Tomatoes
(c) Milk
(d) Tamarind.
Ques. What happens when a base reacts with a non-metallic oxide. What would you infer about the nature of the non-metallic oxide? (2 Marks)
Ans. A base reacts with a non-metal oxide to form salt and water. The non-metal oxide is, therefore, an acidic oxide. For example,
2 NaOH (aq) CO2 (g) ———> Na2CO3 (aq) + H2O (aq)
Ques. Name the acids and bases from which the following salts may be obtained.
(i) Potassium sulphate
(ii) Calcium chloride (2 Marks)
Ans. (i) Potassium hydroxide (KOH), Sulphuric acid (H2SO4)
(ii) Calcium hydroxide Ca(OH)2, Plydrochioric acid (HCl)
Ques. A sample of bleaching powder was kept in an airtight container. After a month, it lost some of its chlorine content. How will you account for it? (2 Marks)
Ans. Bleaching powder, if kept even in an airtight container, will slowly decompose on its own and form calcium chlorate and calcium chloride. The reaction is called auto-oxidation. This will result in a decrease in its chlorine contents.
Ques. An old person complained of acute pain in the stomach. Doctor gave him a small antacid tablet and he got immediate relief. What actually happened? (2 Marks)
Ans. The old person was suffering from acute acidity. Antacid tablets contain sodium hydrogen carbonate (NaHCO3). It reacts with the acid (HCl) formed because of acidity and neutralizes its effect. That is how the old person got relief.
Ques. (a) A gas is produced when cones. H2SO4 is added to solid sodium chloride taken in a test tube and the tube is heated. The gas coming out through the delivery tube is passed over a dry litmus paper and then over a moist litmus paper. What would you observe? Explain your answer.
(b) Fresh milk has a pH of 6. When it changes to curd (yogurt), will its pH value increase or decrease? Why?
(c) What will be the colour of blue litmus in a solution of sodium carbonate? (4 Marks)
Ans. (a) Upon heating sodium chloride with a cone. H2SO4, hydrogen chloride gas evolves.
NaCl(s) + H2SO4(aq) → NaHSO4 (aq) + HCl (g)
In the gaseous state, the acidic character is not shown because HCl does not release any H+ ions. In the presence of moisture (moist litmus paper), the gas changes to hydrochloric acid i.e., HCl(aq). The acid releases H+ ions and thus, shows acidic character. Therefore, moist blue litmus paper becomes red.
(b) When fresh milk changes to curd, the pH of the solution is likely to decrease. Actually, lactose present in milk gets converted to lactic acid when curd or yogurt is formed from milk. Therefore, the medium becomes more acidic and its pH decreases.
(c) The solution of sodium carbonate (Na2CO3) is of basic nature. Actually, the salt dissolves in water to produce NaOH (strong base) and H2CO3 (weak acid). The blue litmus will not undergo any change in the basic medium. It will remain blue.
Ques.The oxide of a metal M was water soluble. When a blue litmus strip was dipped in this solution, it did not undergo any change in colour. Predict the nature of the oxide. (2 Marks)
Ans. The metal oxide (MO) is of basic nature. It dissolves in water to form metal hydroxide as follows :
MO + H2O → M(OH)2
A blue litmus does not undergo any change in colour in the basic medium.
Ques. (a) A solution has a pH of 7. Explain how you would you :
(i) increase its pH
(ii) decrease its pH
(b) If a solution changes the colour of the litmus from red to blue, what can you say about its pH?
(c) What can you say about the pH of a solution that liberates carbon dioxide from sodium carbonate? (3 Marks)
Ans. (a) The solution with pH 7 is neutral. Its pH can be increased by adding a small amount of base like sodium hydroxide. Basic solutions have pH more than 7. Similarly, pH can be decreased by adding small amounts of acid like hydrochloric acid. Acidic solutions have pH less than 7.
(b) The change in colour of litmus from red to blue indicates that the solution is of basic nature with pH more than 7.
(c) Carbon dioxide can be liberated by reacting sodium carbonate solution with acid like dilute hydrochloric acid. This shows that the solution is of acidic nature with pH less than 7.
Ques. A person is suffering from indigestion due to the intake of hot spicy food. What remedy will you prescribe to the patient? (2 Marks)
Ans. Give the name of a chemical that can give relief to him.
The spicy food has resulted in acidity in the stomach. An antacid is needed to cure it. Magnesium hydroxide Mg(OH) 2 acts as an antacid.
Ques. Explain why :
(i) Common salt becomes sticky during the rainy season
(ii) Blue vitriol changes to white upon heating
(iii) If a bottle full of concentrated sulphuric acid is left open in the atmosphere by accident, the acid starts flowing out of the bottle of its own. (3 Marks)
Ans. (i) Common salt contains the impurity of magnesium chloride (MgCl2) which is of deliquescent nature. When exposed to the atmosphere, it becomes moist. Therefore, common salt becomes sticky during the rainy season.
(ii) Blue vitriol (CUSO 4 . 5H2O) upon heating changes to anhydrous copper sulphate (CUSO4) which is white in colour.
(iii) Concentrated sulphuric acid is highly hygroscopic. It absorbs moisture from air and gets diluted. Since the volume increases, the acid starts flowing out of the bottle.
Ques. Write the chemical name and formula of common salt. List two main sources of common salt in nature. Write any three uses of common salt. How is it connected to our freedom struggle? (5 Marks)
Ans. Sodium chloride (NaCl) also called common salt or table salt is the most essential part of our diet. Chemically it is formed by the reaction between solutions of sodium hydroxide and hydrochloric acid. Sea water is the major source of sodium chloride where it is present in the dissolved form alongwith other soluble salts such as chlorides and sulphates of calcium and magnesium. It is separated by some suitable method. Deposits of the salt are found in different parts of the world and are known as rock salt. The formation of rock salt is due to the slow evaporation of sea water which takes ages. When pure, it is a white crystalline solid. However, it is often brown due to the presence of impurities.
- Sodium Chloride — Essential for Life
Sodium chloride is quite essential for life. Biologically, it has a number of functions to perform such as in muscle contraction, in conduction of nerve impulses in the nervous system and is also converted into hydrochloric acid in the stomach which helps in the digestion of food. When we sweat, there is loss of sodium chloride and some. Other salts along with water. This leads to muscle cramps. The loss has to be compensated suitably by giving certain salt preparations to the patients. These are called electrolytes. Electral powder is a very popular electrolyte.
- Chemicals from Common Salt
Sodium chloride is also a very useful raw material for different chemicals. A few out of these are : hydrochloric acid (HCl), washing soda (Na 2CO 3-10H 2O), baking soda (NaHCO 3) etc. Upon electrolysis, a strong solution of the salt (brine), sodium hydroxide, chlorine and hydrogen are obtained.
Apart from these, it is used in the leather industry for the leather tanning. In severe colds, rock salt is spread on icy roads to melt ice. It is also a fertilizer for sugar beet.
6CaOCl2 (Bleaching powder) → Ca(ClO3)2 (Calcium chlorate) + 5CaCl2 (Calcium chloride)
Ques.(i) What are strong acids and weak acids ? Give an example for each.
(ii) A dry pellet of a common base ‘B’ when kept in the open absorbs moisture and turns sticky. The compound is also formed by the Chloralkali process. Identify B. What type of reaction occurs when B is treated with dilute hydrochloric acid ? Write the chemical equation. (4 Marks)
Ans. (i) The strength of an acid is expressed in terms of its H+ ions releasing tendency in aqueous solution or in terms of its degree of dissociation
(α). Strong acids have a large value of α (close to one) while weak acids have comparatively less value. For example, hydrochloric acid (HCl) is a strong acid while acetic acid (CH 3COOH) is a weak acid.
(ii) The available information suggests that the base ‘B’ present in the pellet is sodium hydroxide (NaOH). It is of deliquescent nature. It absorbs moisture from the atmosphere and becomes sticky. The base reacts with dilute hydrochloric acid to form salt and water. This reaction is known as the neutralisation reaction.
NaOH (aq) + HCl(aq) ———–> NaCl (aq) + H 2O(l)
Ques. How will you test for the gas which is liberated when hydrochloric acid reacts with an active metal? (2012) (1 Mark)
Ans. Bring a burning matchstick near the gas. It burns with a ‘pop’ sound showing that it is hydrogen.
Ques. Name the acid present in the following:
(i) Tomato
(ii) Vinegar
(iii) Tamarind (2015) (1 Mark)
Ans. (i) Oxalic acid (ii) Acetic acid (iii) Tartaric acid
Ques. (i) Give the constituents of baking powder
(ii) Why does cake or bread swell on adding baking powder? Write a chemical equation. (2013) (1 Mark)
Ans. (i) Baking powder contains sodium hydrogen carbonate and tartaric acid.
(ii)It is due to carbon dioxide
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