Class 10 Science Chapter 2 Notes

Acids Bases And Salts Class 10 Science Chapter 2 Notes are available here. These notes provide a detailed overview of Chapter 2 Acids Bases And Salts from the Science textbook, making it easier for students to understand the chapter. The notes are created by subject experts of our website CBSE Wale. Our goal is to make learning easier and more convenient. The notes are trustworthy and provide students with a reliable resource to support their studies.

Acids Bases And Salts Class 10 Science Chapter 2 Notes

Introduction

• Sour and bitter tastes in food are due to acids and bases present in them.
• Remedy for acidity after overeating: lemon juice, vinegar, or baking soda solution.
• Choosing the remedy involves considering the ability of acids and bases to neutralise each other’s effects.
• Testing sour and bitter substances without tasting them is possible using indicators like litmus and turmeric.
• Acids are sour, turn blue litmus paper red, while bases are bitter and turn red litmus paper blue.
• Soap, which is an alkali, can change the color of curry marks on white clothing from yellow to reddish-brown.
• Synthetic indicators like methyl orange and phenolphthalein can be used to test for acids and bases.
• The chapter will cover reactions of acids and bases, how they neutralise each other, and their applications in daily life.
• Litmus solution is a purple dye extracted from lichen and commonly used as an indicator.
• Litmus solution is purple when it is neither acidic nor basic.
• Other natural materials like red cabbage leaves, turmeric, and coloured petals of flowers such as Hydrangea, Petunia, and Geranium can also indicate the presence of acid or base in a solution.
• These natural materials are known as acid-base indicators or simply indicators.

Understanding The Chemical Properties Of Acids And Bases

Acids and Bases in the Laboratory

• Indicators can inform us about the acidity or basicity of a substance by observing changes in colour.
• Olfactory indicators are substances that change their odour in acidic or basic environments.
• Vanilla, onion, and clove are examples of olfactory indicators.
• Vanilla, onion, and clove are all olfactory indicators. When they are mixed with an acidic solution, their smell does not change. However, when they are mixed with a basic solution, their smell disappears.

How do Acids and Bases React with Metals? 

• The chemical reaction between dilute sulphuric acid (H₂SO₄) and zinc (Zn) can be represented by the following equation:
• H₂SO₄ + Zn → ZnSO₄ + H₂
• In this reaction, the zinc displaces the hydrogen atoms from the sulphuric acid, resulting in the formation of zinc sulphate (ZnSO4) and the liberation of hydrogen gas (H2).
• In the reactions described above, the metal displaces hydrogen atoms from the acids, resulting in the formation of hydrogen gas.
• Additionally, a compound called a salt is formed as a product of the reaction.
• The overall reaction of a metal with an acid can be summarised as follows: 
• Acid + Metal → Salt + Hydrogen gas.
• 2NaOH(aq) + Zn(s) → Na₂ZnO₂(s) + H₂(g)
• It is important to note that not all metals will react with acids to produce hydrogen gas. Some metals, like zinc in this case, have a higher reactivity and are capable of displacing hydrogen from the acid. However, less reactive metals may not undergo such reactions with acids. The reactivity of metals with acids depends on factors such as their position in the reactivity series.

How do Metal Carbonates and Metal Hydrogen Carbonates React with Acids?

• The reaction between a metal carbonate or metal hydrogen carbonate and an acid can be written as:
• Metal carbonate/Metal hydrogen carbonate + Acid → Salt + Carbon dioxide + Water.
• Example:
• 1. Na₂CO₃ + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)
• 2. NaHCO₃ + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂(g)
• When the carbon dioxide gas evolved is passed through lime water, a white precipitate of calcium carbonate (CaCO3) is formed.
• CaCO₃(s) + CO₂(g) + H₂O(l) → Ca(HCO₃)₂(aq)
• In this reaction, carbon dioxide dissolves in water to form carbonic acid (H2CO3), which then reacts with calcium carbonate to produce soluble calcium hydrogen carbonate.
• On passing excess carbon dioxide, the soluble calcium hydrogen carbonate [Ca(HCO₃)₂] is formed.
• Limestone, chalk, and marble are different forms of calcium carbonate.
• All metal carbonates and hydrogen carbonates react with acids to produce a corresponding salt, carbon dioxide, and water.

How do Acids and Bases React with each other?

• The reaction that takes place between sodium hydroxide (NaOH), a base, and hydrochloric acid (HCl), an acid, is represented by the equation:
• NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l).
• This reaction is an example of a neutralisation reaction, where an acid and a base react to produce salt and water.
• In general, a neutralisation reaction can be written as: 
• Acid + Base → Salt + Water.
• In this type of reaction, the acidic and basic properties of the reactants cancel each other out, resulting in the formation of a salt (the combination of a positive ion from the base and a negative ion from the acid) and water.
• Neutralisation reactions are important in various applications, such as in the preparation of salts, pH regulation, and the treatment of acid indigestion.

Reaction of Metallic Oxides with Acids

• When copper oxide reacts with hydrochloric acid, the colour of the solution changes to blue-green, indicating the formation of copper(II) chloride.
• CuO(s) + 2HCl(aq) → CuCl₂(aq) + H₂O(l)
• The formation of copper(II) chloride in the reaction explains the blue-green colour of the solution.
• The reaction between a metal oxide and an acid can be represented by the general equation:
• Metal oxide + Acid → Salt + Water.
• Metal oxides are considered basic oxides because they react with acids to form salts and water, similar to the reaction between a base and an acid.
• Basic oxides have properties that are opposite to acidic oxides, as they neutralise acids and exhibit basic characteristics.

Reaction of a Non-metallic Oxide with Base

• Calcium hydroxide, being a base, reacts with carbon dioxide as an acid to form salt and water.
• The reaction between calcium hydroxide and carbon dioxide is similar to the reaction between a base and an acid.
• Based on this similarity, we can conclude that non-metallic oxides are acidic in nature.
• Non-metallic oxides exhibit acidic properties as they react with bases to form salts and water.

What Do All Acids And All Bases Have In Common?

• All acids and bases have certain common characteristics or properties.
• One common property of acids is their ability to release hydrogen ions (H+) when dissolved in water.
• Acids are proton donors, meaning they can donate hydrogen ions to other substances.
• Bases, on the other hand, are substances that can accept hydrogen ions (H+) or donate hydroxide ions (OH-) when dissolved in water.
• Bases are proton acceptors and can neutralise acids.
• Both acids and bases can conduct electricity when dissolved in water, as they produce ions that allow the flow of electric current.
• Acids and bases can react with each other in a process called neutralisation, where they combine to form salt and water.
• Both acids and bases can change the colour of certain indicators, such as litmus paper or phenolphthalein, providing a way to identify their presence.

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

• It is important to note that hydrogen ions (H+) in hydrochloric acid (HCl) are produced in the presence of water. The dissociation of H+ ions from HCl molecules requires the presence of water.
• The reaction between HCl and water can be represented as:
• HCl + H₂O → (H₃O⁺) + Cl-
• Hydrogen ions cannot exist on their own but exist by combining with water molecules, forming hydronium ions (H3O+). Therefore, hydrogen ions are represented as H+ (aq) or H3O+.
• The reaction of hydrogen ions with water can be represented as:
• (H+) + H₂O → H3O+
• Acids release H3O+ or H+ (aq) ions when dissolved in water.
• When solid bases such as sodium hydroxide (NaOH), potassium hydroxide (KOH), and magnesium hydroxide (Mg(OH)2) are dissolved in water, they generate hydroxide (OH-) ions.
• Bases that are soluble in water are called alkalis.
• The reaction between hydrogen ions (H+) and hydroxide ions (OH-) results in the formation of water:
• H+ (aq) + OH- (aq) → H₂O (l)
• Dissolving an acid or a base in water is an exothermic process, meaning it releases heat. Therefore, caution must be exercised when mixing concentrated acids like nitric acid or sulphuric acid with water.
• When diluting acids, it is important to add the acid slowly to water while stirring constantly. This prevents excessive heat generation, splashing, and potential burns.
• Adding water to concentrated acids is not recommended as it can cause a rapid release of heat, leading to splashing and container breakage.
• Concentrated sulphuric acid and sodium hydroxide pellets are labelled with warning signs to alert users of the potential hazards associated with their handling and dilution.
• Mixing an acid or base with water results in a decrease in the concentration of ions, specifically hydronium ions (H3O+) or hydroxide ions (OH-) per unit volume.
• This process of decreasing concentration by adding water is called dilution.
• Dilution allows for safer handling and manipulation of acids and bases while adjusting their concentration to desired levels.

How Strong Are Acid Or Base Solutions?

• Acid-base indicators can distinguish between acids and bases based on their colour changes.
• Dilution of a solution results in a decrease in the concentration of H+ or OH- ions.
• To quantitatively measure the amount of these ions and determine the strength of an acid or base, a universal indicator is used. It shows different colours at different concentrations of hydrogen ions.
• The pH scale, derived from the German word “potenz,” meaning power, is used to measure hydrogen ion concentration in a solution. The scale ranges from 0 (very acidic) to 14 (very alkaline).
• A neutral solution has a pH of 7. Values below 7 indicate acidity, while values above 7 indicate alkalinity.
• The pH value reflects the hydronium ion concentration in the solution: higher concentration corresponds to lower pH values.
• The strength of an acid or base depends on the number of H+ or OH- ions it produces, respectively.
• Strong acids produce more hydrogen ions, while weak acids produce fewer hydrogen ions.
• Similarly, strong bases produce more hydroxide ions, while weak bases produce fewer hydroxide ions.
• The distinction between weak and strong bases is based on their ability to release hydroxide ions into a solution.

Table 2.2

Importance of pH in Everyday Life

• The pH range of 7.0 to 7.8 is crucial for the proper functioning of our body, as living organisms can only survive within a narrow pH range.
• Acid rain, with a pH less than 5.6, can be harmful to aquatic life when it flows into rivers, lowering the pH of the water.
• Plants require a specific pH range for healthy growth, and soil pH can vary in different regions. Checking soil pH and observing the plant growth can provide insights into the pH requirements of plants.
• The stomach produces hydrochloric acid for food digestion, and excessive acid production during indigestion can cause pain and irritation. Antacids, which are bases, are used to neutralise excess stomach acid and relieve discomfort.
• Tooth decay occurs when the mouth’s pH drops below 5.5, leading to the corrosion of tooth enamel. Bacteria in the mouth produce acids by breaking down sugars and food particles, and cleaning the mouth after eating can help prevent tooth decay. Basic toothpaste can neutralise excess acid and protect against tooth decay.
• Bee stings and nettle leaf stings cause pain and irritation due to the presence of acids. Applying a mild base like baking soda can provide relief from the burning sensation caused by the acids.

More About Salts

• Salts are formed by combining an acid and a base through processes like neutralisation reactions.
• Salts have unique characteristics. They typically have a crystalline structure and are solid at room temperature. Their solubility in water varies based on their composition. Salts exhibit specific chemical and physical properties like melting point, boiling point, and conductivity.

pH of Salts

• When a salt is formed from a strong acid and a strong base, it is considered neutral and has a pH value of 7. This means that the solution of such a salt is neither acidic nor basic.
• However, when a salt is formed from a strong acid and a weak base, the resulting salt solution is acidic in nature. The pH value of this solution will be less than 7, indicating its acidity. The presence of the weak base in the salt contributes to the acidity of the solution.
• Conversely, salts formed from a strong base and a weak acid are considered basic. The pH value of the solution of such salts will be greater than 7, indicating its basic nature. The weak acid component in the salt contributes to the basicity of the solution.

Chemicals from Common Salt

• Sodium chloride, also known as common salt, is formed by the combination of hydrochloric acid and sodium hydroxide. It is the salt commonly used in food and is considered a neutral salt.
• Seawater contains various dissolved salts, including sodium chloride. Sodium chloride can be separated from other salts present in seawater. Solid salt deposits, known as rock salt, can also be found in different parts of the world. These crystals are often brown in colour due to impurities and are obtained through mining, similar to coal mining.

Common salt — A raw material for chemicals

• Common salt is a valuable raw material used in the production of several everyday items.
• It is a key ingredient in the manufacturing process of sodium hydroxide, baking soda, washing soda, bleaching powder, and other substances.
• Despite being derived from common salt, each of these materials serves different purposes and has distinct properties.

Sodium hydroxide

• Passing electricity through a solution of sodium chloride (brine) leads to the decomposition of the salt.
• This process is known as the chlor-alkali process and results in the formation of sodium hydroxide, chlorine gas, and hydrogen gas.
• Chlorine gas is released at the anode, while hydrogen gas is produced at the cathode.
• Sodium hydroxide solution is formed near the cathode and is collected for use.
• Each of the three products obtained from this process has practical applications in various industries and daily life.

Bleaching powder 

• Chlorine gas, obtained from the electrolysis of aqueous sodium chloride, is used in the production of bleaching powder.
• Bleaching powder is created by reacting chlorine with dry slaked lime (calcium hydroxide).
• The chemical equation for the formation of bleaching powder is 
• Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O
• Bleaching powder has multiple applications:
• (i) It is used for bleaching cotton, linen, wood pulp, and washed clothes in industries such as textiles, paper, and laundry.
• (ii) It serves as an oxidising agent in various chemical industries.
• (iii) It is employed for purifying drinking water by eliminating germs.

Baking soda

• Baking soda, or sodium hydrogen carbonate (NaHCO₃), is commonly used in the kitchen for making crispy pakoras and for faster cooking.
• Sodium hydrogencarbonate is produced using sodium chloride as one of the raw materials.
• Reaction of baking soda formation:
• NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃
• Baking soda is a mild, non-corrosive basic salt that can neutralise acids.
• When heated during cooking, baking soda undergoes a reaction and transforms into sodium carbonate (Na2CO3), water (H2O), and carbon dioxide (CO2).
• Baking soda has various household uses:
• (i) It is used for making baking powder, which is a mixture of baking soda and a mild edible acid like tartaric acid. When baking powder is heated or mixed with water, it produces carbon dioxide, causing bread or cake to rise and become soft and spongy.
• (ii) Baking soda is an ingredient in antacids and can neutralise excess stomach acid, providing relief.
• (iii) It is used in soda-acid fire extinguishers.

Washing soda

• Sodium carbonate (washing soda) is a chemical that can be obtained from sodium chloride (common salt).
• Washing soda is obtained by heating baking soda and recrystallizing sodium carbonate.
• Washing soda is a basic salt and has various industrial and domestic uses.
• The formula Na₂CO₃.10H₂O signifies that washing soda contains ten water molecules.
• The presence of water molecules does not make Na₂CO₃ wet, but it indicates that water is chemically bound within the crystal structure.
• Washing soda finds applications in industries such as glass, soap, and paper manufacturing.
• It is used in the production of other sodium compounds like borax.
• Sodium carbonate can be used as a cleaning agent for household purposes.
• It is effective in removing permanent hardness from water.

Are The Crystals Of Salts Really Dry?

• Copper sulphate crystals contain water of crystallisation, which gives them a blue colour.
• Heating the crystals removes the water of crystallisation, causing the salt to turn white.
• When the white crystals are moistened with water, the blue colour reappears.
• Water of crystallisation refers to a fixed number of water molecules present in one formula unit of a salt.
• Copper sulphate has five water molecules in its formula unit, and its chemical formula is CuSO₄.5H₂O.
• The presence of water of crystallisation does not mean that the molecule is wet but indicates that water is chemically bound within the crystal structure.
• Gypsum is another salt that contains water of crystallisation.
• Gypsum has two water molecules as water of crystallisation, and its chemical formula is CaSO₄.2H₂O.

Plaster of Paris

• Heating gypsum at 373 K causes it to lose water molecules and transform into calcium sulphate hemihydrate (CaSO₄.1/2H₂O).
• Calcium sulphate hemihydrate is commonly known as Plaster of Paris.
• Plaster of Paris is used by doctors as a plaster to support fractured bones.
• Plaster of Paris is a white powder that, when mixed with water, reverts back to gypsum, forming a hard solid mass.
• The chemical equation for the reaction is: CaSO₄.1/2H₂O + 3/2 H₂O → CaSO₄.2H₂O
• The notation “1/2H₂O” signifies that half a water molecule is attached as water of crystallisation. This is because two formula units of CaSO4 share one molecule of water.
• Plaster of Paris is used in various applications such as making toys, decorative materials, and achieving smooth surfaces.
• The reason why calcium sulphate hemihydrate is called “Plaster of Paris” can be explored further to understand its historical or traditional association with the city of Paris.