NCERT Class 10 Science Chapter 2 Notes Acids, Bases, and Salts- Download PDF Notes

Have you ever wondered why lemon juice is sour or why soap feels so slippery? It is all about acids, bases, and salts. Acids, such as those found in lemons, taste sour and cause blue litmus paper to become red. Bases, like baking soda, taste harsh and turn red litmus paper blue. A combination of acids and bases forms salts, and they include table salt and industrial chemicals. Learning about these chemicals allows us to understand their significance in academic and practical contexts.

In this chapter, topics like the behaviour of acids, bases and salts, their properties, and their reactions with different compounds are discussed in depth. The NCERT Notes for Class 10 Science Chapter 2 will provide a precise and structured revision of the entire chapter that will eliminate the need for students to search for additional resources. These NCERT notes cover simple examples like how acids help our stomachs in digesting food and how bases are used in cleaning products. In this article, we have provided a step-by-step analysis of the topic by including diagrams and reactions that will help you get clarity on the topics and to make the revision effective.

Also read

• NCERT Solution for Class 10 Science Chapter 2
• NCERT Exemplar Solution for Class 10 Science Chapter 2

NCERT Class 10 Science Chapter 2 Notes: Download PDF

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NCERT Class 10 Science Chapter 2 Notes

The NCERT notes are a key guide for students preparing for boards. Students can use these notes for quick revision of the concepts that will in turn help them to tackle the questions. Scroll down to know more!

2.1 Understanding the Chemical Properties of Acids and Bases

One hundred and fifteen different chemical elements are known to us at present. These elements combine to form a large number of compounds. On the basis of their chemical properties, all the compounds can be classified into three groups :

1. Acids

2. Bases

3. Salts

In order to know whether a substance is an acid or a base, we should first know the meaning of the term ‘acid-base indicator’ or just ‘indicator’. This is discussed below.

2.1.1 Acids and Bases in the Laboratory

Indicators for Testing Acids and Bases

An indicator is a ‘dye’ that changes colour when it is put into an acid or a base. An indicator gives different colours in acid and base. Thus, an indicator tells us whether the substance we are testing is an acid or a base by change in its colour. In other words, an indicator tells us whether the substance we are testing is acidic or basic by change in its colour. The three most common indicators to test for acids and bases are : Litmus, Methyl orange and Phenolphthalein.

The most common indicator used for testing acids and bases in the laboratory is litmus. Litmus can be used in the form of litmus solution or in the form of litmus paper. It is of two types : Blue litmus and Red litmus.

Concept of Acids

If we cut a lemon with a knife and taste it, the lemon appears to have a sour taste. The sour taste of lemon is due to the presence of an acid in it. The acid present in lemon which gives it a sour taste is citric acid. Thus, acids are those chemical substances that have a sour taste. Some of the common fruits such as raw mango, raw grapes, lemon, orange, and tamarind, etc., are sour in taste due to the presence of acids in them. Soured milk (or curd) also contains acid in it.

Change in colour: Acids change their colour from blue to red while performing a litmus test.

Concept of Bases

The solutions of substances like caustic soda, lime, and washing soda are bitter in taste, and soapy to touch (slippery to touch). They are called bases. Thus, bases are those chemical substances that have a bitter taste.

Change in colour: Bases change their colour from red to blue while performing a litmus test.

Litmus: This can be extracted from lichen and is a purple dye solution.

It is the fact that if the litmus is neither acidic nor basic in nature, then it shows a purple colour.

Indication of acid and base inside any material:

Natural materials like:

• Red cabbage leaves

• Turmeric

• Coloured petals of flowers such as Petunia, Geranium, etc.

Properties of Acids and Bases

Acids

• The word is derived from the Latin word called “acidus”.
• When dissolved in an aqueous medium, it gives hydrogen ions to the solution.
• Concentrated acids are mixtures of a larger amount of acid dissolved in a smaller amount of water.
• Dilute acids are mixtures of a larger amount of water dissolved in a smaller amount of acids.
• Examples of strong acids: are HCl, H₂SO₄, HNO₃, etc.
• Examples of weak acids: are CH₃COOH, Lactic acid, oxalic acid, etc.

Bases

• The substances which are basic in nature are soapy in touch.
• When dissolved in an aqueous medium, it gives hydroxide ions in the solution.
• Alkali is the base that is soluble in water. Examples of alkali are NaOH, KOH, etc.
• Examples of strong bases: NaOH, KOH, Ca(OH)₂
• Example of weak bases: NH₄OH

Indicators

Indicators are defined as changes in the colour or smell of a substance in different types of acids and bases.

Types of indicators are as follows:

1. Natural indicators

2. Synthetic Indicators

3. Olfactory indicators

2.1.2 How do Acids and Bases React with Metals

The reaction of Acids with Metals produces salt and hydrogen gas.

Acid + metal → salt + Hydrogen gas

Example of such reaction: Zn + 2HCl → ZnCl₂ + H₂

The reaction of Bases when reacting with Metals produces salt and hydrogen gas.

The base, which is reacting with metal, has to be more reactive than the base. But all the metals do not have the nature to react with bases.

Base + metal → salt + Hydrogen gas

Example of such reaction: 2NaOH + Zn → Na₂ZnO₂ + H₂

The evolved hydrogen gas burned with a pop sound when it came closer to the evolved gas.

2.1.3 How do Metal Carbonates and Metal Hydrogencarbonates React with Acids?

Reaction of acid with metal caronates nad hydrogen carbonates

Acid + Metal carbonates or hydrogen carbonates $\longrightarrow$ Salt +Water +Carbon dioxide

$\mathrm{HCl}+\mathrm{NaHCO}_3 \longrightarrow \mathrm{NaCl}+\mathrm{H}_2 \mathrm{O}+\mathrm{CO}_2$

The produced carbon dioxide will be further tested and passed through lime water solution, which turns the solution milky. But it should be noticeable that the amount of carbon dioxide is certain, if it passes in excess amount then the milkiness will disappear.

Reaction when the amount of carbon dioxide is certain.

Milky lime water

$
\mathrm{Ca}(\mathrm{OH})_2+\mathrm{CO}_2 \longrightarrow \mathrm{CaCO}_3+\mathrm{H}_2 \mathrm{O}
$

Reaction when amount of carbon dioxide is in excess.

$\mathrm{CaCO}_3+\mathrm{CO}_2+\mathrm{H}_2 \mathrm{O} \longrightarrow \mathrm{Ca}(\mathrm{HCO})_3$

The reaction of bases when reacting with carbonates and hydrogen carbonates

No such reaction will proceed when the base reacts with metal carbonate and metal hydrogen carbonates.

2.1.4 How do Acids and Bases React with each other?

When acids and bases react with each other they produce salt and water, such reaction is termed a

termed a neutralisation reaction.

$\begin{aligned} & \text { Acid }+ \text { Base } \rightarrow \text { Salt }+ \text { Water } \\ & \mathrm{HCl}+\mathrm{NaOH} \rightarrow \mathrm{NaCl}+\mathrm{H}_2 \mathrm{O}\end{aligned}$

• Strong acid+ weak base gives acidic salt and water.

• Weak acid+ strong base gives acidic salt and water.

• Strong acid+ strong base gives neutral salt and water.

• Weak acid+ weak base gives neutral salt and water.

2.1.5 Reaction of Metallic Oxides with Acids

The nature of metallic oxides is basic.

Metallic oxide + acid $\longrightarrow$ salt + water

$
\mathrm{CaO}+2 \mathrm{HCl} \longrightarrow \mathrm{CaCl}_2+\mathrm{H}_2 \mathrm{O}
$

2.1.6 Reaction of a Non-metallic Oxide with Base

The nature of metallic oxides are acidic.

Non-metallic oxide + Base $\rightarrow$ Salt + Water

$
\mathrm{CO}_2+\mathrm{Ca}(\mathrm{OH})_2 \rightarrow \mathrm{CaCO}_3+\mathrm{H}_2 \mathrm{O}
$

2.2 What do all acids and all bases have in common?

Acids and bases have some similarities in between them and they are listed below:

• Acids give hydrogen ions when dissolve in any solution, so the common ion is hydrogen ion which makes any mixture acidic in nature.

• Bases give hydroxide ions when dissolved in any solution, so the common ion is hydroxide ion which makes any mixture basic in nature.

Water solution with acids or bases-

When acids dissolve in water, they produce hydrogen ions but hydrogen ion cannot exist alone they exist in the form of hydronium ion. When bases dissolve in water they give hydroxide ions.

Bases that are soluble in water are termed alkali.

Point to remember: When we are diluting any acid to water, it should be kept in mind that acid must be added to water, not the water to acid, as the process of dissolving any acid or base is highly exothermic in nature.

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

This can be evaluated by an indicator known as a universal indicator.

Universal indicator: This indicator is a mixture of different indicators which shows colour when any solution concentration has hydrogen ion or hydroxide ion in it. Since the pH of a solution depends on the hydrogen ion concentration, so we can also say that the universal indicator shows different colours at different concentrations of hydrogen ions in the solution. When an acid or base solution is added to the universal indicator, the indicator produces a new color.

The colour produced by universal indicator is used to find the pH value of the acid or base solution by matching the colour with the colours on pH colour chart. And knowing the pH value, we can make out whether the given solution is a strong acid, weak acid, strong base or a weak base.

2.3 How strong are acid or base solutions?

pH scale: For measuring the concentration of hydrogen ion in any solution, that how acidic or basic the solution, we must require a scale that scale is termed as pH scale. p in pH denotes potenz which in general means power.

2.3.1 Importance of pH in everyday Life

pH plays a very important role in our everyday life.

In our digestive system- Hydrochloric acid produced in our stomach helps the digestion of food without causing any harm to the stomach. But when the amount of the acid goes beyond a certain limit due to indigestion, pain and irritation are created in the stomach. So, in order to neutralize the effect of excess acid, a mild base called antacid is usually taken. Magnesium hydroxide (milk of magnesia) is a mild base which is usually used as an antacid.

Acids cause tooth decay- When we eat sugary food, it gets degraded by bacteria present in the mouth and an acid is formed. When the pH becomes lower than 5.5, tooth enamel gets corroded. Saliva, which is slightly alkaline, produced in the mouth neutralizes some acid, but excess acid remains unaffected. The excess acid can be removed only by the use of toothpaste which is alkaline. Neem stick contains alkaline juice. So, the cleaning of tooth by Neem stick also helps to reduce tooth decay.

Acid is produced in fatigued muscle- As a result of physical exercise, stiffness and pain in the muscle starts due to the formation of lactic acid. The supply of oxygen in the muscle is reduced. This causes difficulty in the release of energy leading to increase in the rate of anaerobic metabolism. As a result, lactic acid gets accumulated in the muscles.

Some animals and plants contain acids- Honey-bee injects an acid through its stings which causes pain and irritation. Hence, a mild base like baking soda is applied to treat the wound. Similarly, nettle leaves, which have stinging hairs, when touched inject formic acid in our body. This causes a burning pain.

The brilliance of a tarnished copper vessel can be restored by using acid- You know, lemon juice contains an acid. In order to clean a copper vessel, we rub it with the piece of a lemon. The tarnish on the vessel is caused by the formation of a layer of basic copper oxide. Since lemon juice contains citric acid, it reacts with the copper oxide to form copper citrate and is washed away. The vessel then regains its shining appearance.

pH of soil- Soils are generally acidic. Plants require definite pH range for their proper growth. They do not grow in alkaline soil. Many plants do not grow properly in highly acidic or highly alkaline soil. So, highly acidic soil is treated by spreading quicklime, slaked lime or calcium carbonate to lower its acidity.

2.4 More about salts

A salt is a compound formed from an acid by the replacement of the hydrogen in the acid by a metal. Here is an example. Hydrochloric acid is HCl. Now, if we replace the hydrogen (H) of this acid by a metal atom, say a sodium atom (Na), then we will get a salt NaCl. This is called sodium chloride. It is a salt. In some salts, however, the hydrogen of acid is replaced by an ammonium group (NH₄) as in the case of ammonium chloride, NH₄Cl. The best known salt is sodium chloride (NaCl) which is usually known as common salt.

Salts are formed when acids react with bases. In a way, we can say that a salt has two parents : an acid and a base. So, the name of a salt consists of two parts : the first part of the name of salt is derived from the name of base, and the second part of the name of the salt comes from the name of acid. For example, the name of a salt called ‘sodium chloride’ comes from sodium hydroxide base and hydrochloric acid.

Properties of Salts

1. Salts are mostly solids.

2. They have high melting points and boiling points.

3. Salts are usually soluble in water. Just like acids and bases, solutions of salts in water conduct electricity. That is, salts are electrolytes.

4. Salt solutions conduct electricity due to the presence of ions in them. Salts are ionic compounds. Every salt consists of a positively charged ion (cation) and a negatively charged ion (called anion).

The salts that are produced by strong acid and strong bases are neutral in nature. The pH value of such a salt is 7.

Formation of Sodium Chloride

Seawater contains many salts dissolved in it. Sodium chloride is separated from these salts. Deposits of solid salt are also found in several parts of the world. These large crystals are often brown due to impurities. This is called rock salt.

Formation of 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. The chlorine gas is given off at the anode, and hydrogen gas at the cathode. Sodium hydroxide solution is formed near the cathode.

$2 \mathrm{NaCl}+2 \mathrm{H}_2 \mathrm{O} \longrightarrow 2 \mathrm{NaOH}+\mathrm{Cl}_2+\mathrm{H}_2$

Formation of Bleaching Powder

Bleaching powder is produced by the action of chlorine on dry slaked lime [Ca(OH)₂]. Bleaching powder is represented as Ca(ClO)₂, though the actual composition is quite complex.

$\underset{\substack{\mathrm{Ca}(\mathrm{OH})_2 \\ \text { slaked lime }}}{\mathrm{Cl}} \mathrm{Cl}_2 \longrightarrow \underset{\text { Bleaching powder }}{\mathrm{CaOCl}_2}+\mathrm{H}_2 \mathrm{O}$

Bleaching powder is used-
(i) for bleaching cotton and linen in the textile industry, for bleaching wood pulp in paper factories and for bleaching washed clothes in laundry.
(ii) as an oxidising agent in many chemical industries.
(iii) to make drinking water free from germs.

Formation of 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.

$
\mathrm{NaCl}+\mathrm{H}_2 \mathrm{O}+\mathrm{CO}_2+\mathrm{NH}_3 \rightarrow \underset{\substack{\text { (Ammonium } \\ \text { chloride) }}}{\mathrm{NH}_4 \mathrm{Cl}}+\underset{\text { (Sodium bicarbonate }}{\mathrm{NaHCO}_3}
$

Formation of Baking Powder

For making baking powder, which is a mixture of baking soda (sodium bicarbonate) and a mild edible acid such as tartaric acid. When baking powder is heated or mixed in water, the following reaction takes place -

$
\mathrm{NaHCO}_3+\mathrm{H}^{+} \rightarrow \mathrm{CO}_2+\mathrm{H}_2 \mathrm{O}+\text { Sodium salt of acid }
$

(From any acid)

$\underset{\text { Baking soda }}{\mathrm{NaHCO}_3}+\underset{\text { tartric acid }}{\mathrm{CHO}} \longrightarrow \mathrm{CO}_2+\mathrm{H}_2 \mathrm{O} {+} \mathrm{Na}_2 \mathrm{C}_4 \mathrm{H}_4 \mathrm{O}_6$

Washing soda

Washing soda is the common name for sodium carbonate decahydrate, used as a cleaning agent and water softener.
$
\mathrm{Na}_2 \mathrm{CO}_3+10 \mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{Na}_2 \mathrm{CO}_3 \cdot 10 \mathrm{H}_2 \mathrm{O}
$

2.4.4 Are the Crystals of Salts really Dry?

Copper sulphate crystals, which seem to be dry, contain water of crystallisation. When we heat the crystals, this water is removed and the salt turns white.

Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt. Five water molecules are present in one formula unit of copper sulphate. The chemical formula for hydrated copper sulphate is CuSO₄. 5H₂O.

Plaster of Paris

Plaster of Paris is a white powdery substance made by heating gypsum ($\mathrm{CaSO}_4 \cdot 2 \mathrm{H}_2 \mathrm{O}$); it sets into a hard mass when mixed with water.
$
\mathrm{CaSO}_4 \cdot 2 \mathrm{H}_2 \mathrm{O} \xrightarrow{\text { Heat }} \mathrm{CaSO}_4 \cdot \frac{1}{2} \mathrm{H}_2 \mathrm{O}+\frac{3}{2} \mathrm{H}_2 \mathrm{O}
$

Previous Year Questions of Class 10 Science Chapter 2

Question: A white shirt has a yellow stain of curry. When soap is rubbed on this shirt during washing, the yellow stain turns reddish-brown. On rinsing the shirt with plenty of water, the reddish-brown stain turns yellow again. Name the natural indicator present in the curry stain.

(1) Baking soda

(2) Common salt

(3) Citric acid

(4) Turmeric

Answer: The yellow stain contains turmeric and as turmeric is a natural indicator, it turns red when soap is rubbed because soap is basic. When the shirt is rinsed off, the soap also gets rinsed off and hence again the colour turns to yellow.

Hence, the correct answer is option (4).

Question: Acids do not show acidic behaviour in the absence of water because

(1) Dissociation of hydrogen ions occurs in water only

(2) They release OH^- ions in the absence of water

(3) Acids release heat after addition to water

(4) None of the above

Answer: Acids do not show acidic behaviour in the absence of water because the dissociation of hydrogen ions from an acid occurs in the presence of water only. It is the hydrogen ions that are responsible for the acidic behaviour.

Hence, the correct answer is option (1).

Question: Which of the given option represents a family of salts?

(1) $\mathrm{NaCl}, \mathrm{Na}_2 \mathrm{SO}_4, \mathrm{CaSO}_4$

(2) $\mathrm{K}_2 \mathrm{SO}_4, \mathrm{Na}_2 \mathrm{SO}_4, \mathrm{CaSO}_4$

(3) $\mathrm{NaNO}_3, \mathrm{CaCO}_3, \mathrm{Na}_2 \mathrm{CO}_3$

(4) $\mathrm{MgSO}_4, \mathrm{CuSO}_4, \mathrm{MgCl}_2$

Answer:

According to the given options, the second one represents the family of salts .
$\mathrm{K}_2 \mathrm{SO}_4$ : Potassium sulfate (salt of potassium and sulfate ion)
$\mathrm{Na}_2 \mathrm{SO}_4$ : Sodium sulfate (salt of sodium and sulfate ion)
$\mathrm{CaSO}_4$ : Calcium sulfate (salt of calcium and sulfate ion)

These salts have the same anion: sulfate ion $(\mathrm{SO}_4{ }^{2-})$, but different cations (potassium, sodium, calcium). This option represents a family of salts with a common anion ( $\mathrm{SO}_4{ }^{2-}$ ).

Hence, the correct answer is option (2).

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