Acids, Bases and Salts – Class 9 Science Question & Answers (Maharashtra Board)

• a) chloride, nitrate, hydride, ammonium

  • Answer Ammonium.

  • Justification: Ammonium ($\text{NH}_{4}^{+}$) is a positively charged ion (cation), whereas all the others (chloride, nitrate, hydride) are negatively charged ions (anions).

• b) hydrogen chloride, sodium hydroxide, calcium oxide, ammonia

  • Answer Hydrogen chloride.

  • Justification: Hydrogen chloride ($\text{HCl}$) is an acid, whereas sodium hydroxide, calcium oxide, and ammonia are basic in nature.

• c) acetic acid, carbonic acid, hydrochloric acid, nitric acid

  • Answer Acetic acid.

  • Justification: Acetic acid is an organic acid found naturally in living things, while carbonic acid, hydrochloric acid, and nitric acid are inorganic (mineral) acids.

• d) ammonium chloride, sodium chloride, potassium nitrate, sodium sulphate

Answer Potassium hydroxide (or depending on typical textbook sets, let's check standard variations like ammonium chloride which is acidic salt).

Justification: Ammonium chloride is an acidic salt formed from a strong acid and a weak base, while sodium chloride, potassium nitrate, and sodium sulphate are neutral salts.

• Answer The deep blue color of the copper sulphate solution becomes lighter (pale blue) due to dilution. This is a physical change where the concentration of solute particles per unit volume decreases.

• Answer When blue litmus paper is dropped into dilute $\text{HCl}$, it turns red because acids are acidic in nature. When concentrated $\text{NaOH}$ is subsequently added and stirred, the solution undergoes neutralization, and the litmus paper turns blue as the basic nature takes over once the acid is completely neutralized.

• Answer Bubbles of gas will form at both electrodes. Hydrogen gas evolves at the cathode (negative electrode) and oxygen gas evolves at the anode (positive electrode) because water molecules undergo electrolysis in the presence of the acid.

• Answer When magnesium oxide (a basic oxide) is added to dilute $\text{HCl}$, it dissolves and forms magnesium chloride and water through a neutralization reaction. However, when added to dilute $\text{NaOH}$, no reaction takes place because both substances are basic in nature and bases do not react with other bases.

• Answer Zinc oxide is an amphoteric oxide, meaning it reacts with both acids and bases. When added to dilute $\text{HCl}$, it forms zinc chloride and water. When added to dilute $\text{NaOH}$, it reacts to form sodium zincate and water.

(CaO, MgO, $\text{CO}_{2}$, $\text{SO}_{3}$, $\text{Na}_{2}\text{O}$, ZnO, $\text{Al}_{2}\text{O}_{3}$, $\text{Fe}_{2}\text{O}_{3}$)

• Answer

  • Acidic Oxides: $\text{CO}_{2}$ (Carbon dioxide), $\text{SO}_{3}$ (Sulphur trioxide)

  • Basic Oxides: $\text{CaO}$ (Calcium oxide), $\text{MgO}$ (Magnesium oxide), $\text{Na}_{2}\text{O}$ (Sodium oxide), $\text{Fe}_{2}\text{O}_{3}$ (Ferric oxide)

  • Amphoteric Oxides: $\text{ZnO}$ (Zinc oxide), $\text{Al}_{2}\text{O}_{3}$ (Aluminium oxide)

• a) Acid: An acid is a substance that dissolves in water to release hydrogen ions ($\text{H}^{+}$) as the only positively charged ions. They have a sour taste and turn blue litmus paper red.

• b) Base: A base is a substance that can neutralize an acid or accept protons, releasing hydroxide ions ($\text{OH}^{-}$) when dissolved in water. They have a bitter taste and turn red litmus paper blue.

• c) Neutralization: A chemical reaction in which an acid and a base react together to form salt and water, effectively nullifying each other's properties.

• d) Cation: A positively charged ion formed when an atom or molecule loses one or more electrons during ionization or electrolysis.

• e) Anion: A negatively charged ion formed when an atom or molecule gains one or more electrons.

• Acids: Have a pH value less than $7$, taste sour, and turn blue litmus red. Example: $\text{HCl}$.

• Bases: Have a pH value greater than $7$, taste bitter, and turn red litmus blue. Example: $\text{NaOH}$.

• Cation: Positively charged particle formed by losing electrons; moves towards the cathode during electrolysis.

• Anion: Negatively charged particle formed by gaining electrons; moves towards the anode during electrolysis.

• Solution:

  • Molecular mass of $\text{HCl} = 1 + 35.5 = 36.5\text{~g/mol}$.

  • Moles of $\text{HCl} = 7.3 \div 36.5 = 0.2\text{~mol}$.

  • Volume in Litres $= 100\text{~ml} = 0.1\text{~L}$.

  • Concentration in $\text{g/L} = 7.3\text{~g} \div 0.1\text{~L} = \mathbf{73}\text{~g/L}$.

  • Concentration in $\text{mol/L} = 0.2\text{~mol} \div 0.1\text{~L} = \mathbf{2}\text{~mol/L}$.

• Solution:

  • Molecular mass of $\text{NaOH} = 23 + 16 + 1 = 40\text{~g/mol}$.

  • Moles $= 2 \div 40 = 0.05\text{~mol}$.

  • Volume $= 50\text{~ml} = 0.05\text{~L}$.

  • Concentration in $\text{g/L} = 2\text{~g} \div 0.05\text{~L} = \mathbf{40}\text{~g/L}$.

  • Concentration in $\text{mol/L} = 0.05\text{~mol} \div 0.05\text{~L} = \mathbf{1}\text{~mol/L}$.

• Solution:

  • Molecular mass of acetic acid $= (2 \times 12) + (4 \times 1) + (2 \times 16) = 60\text{~g/mol}$.

  • Moles $= 3 \div 60 = 0.05\text{~mol}$. Volume $= 0.1\text{~L}$.

  • Concentration in $\text{g/L} = 3 \div 0.1 = \mathbf{30}\text{~g/L}$.

  • Concentration in $\text{mol/L} = 0.05 \div 0.1 = \mathbf{0.5}\text{~mol/L}$.

• Solution:

  • Molecular mass of $\text{H}_{2}\text{SO}_{4} = (2 \times 1) + 32 + (4 \times 16) = 98\text{~g/mol}$.

  • Moles $= 4.9 \div 98 = 0.05\text{~mol}$. Volume $= 0.2\text{~L}$.

  • Concentration in $\text{g/L} = 4.9 \div 0.2 = \mathbf{24.5}\text{~g/L}$.

  • Concentration in $\text{mol/L} = 0.05 \div 0.2 = \mathbf{0.25}\text{~mol/L}$.

• Answer

  • Monobasic Acids: Produce one hydrogen ion per molecule. Example: Hydrochloric acid ($\text{HCl}$).

  • Dibasic Acids: Produce two hydrogen ions per molecule. Example: Sulphuric acid ($\text{H}_{2}\text{SO}_{4}$).

  • Tribasic Acids: Produce three hydrogen ions per molecule. Example: Phosphoric acid ($\text{H}_{3}\text{PO}_{4}$).

• Answer Electrolysis of water is the process of passing an electric current through acidified water to break it down into its constituent elements: hydrogen gas and oxygen gas. Because pure water is a poor conductor, a small amount of acid or salt is added to help carry the current.

• Answer Free hydrogen ions ($\text{H}^{+}$) are extremely unstable and cannot exist independently in water. Therefore, the moment an acid dissolves in water, the $\text{H}^{+}$ ion immediately combines with a neutral water molecule ($\text{H}_{2}\text{O}$) to form a stable hydronium ion ($\text{H}_{3}\text{O}^{+}$).

• Answer

  • Explanation: Neutralization happens when an acid reacts with a base to produce salt and water. For example, mixing sodium hydroxide and hydrochloric acid yields sodium chloride and water ($\text{NaOH} + \text{HCl} \rightarrow \text{NaCl} + \text{H}_{2}\text{O}$).

  • Daily Life Applications:

    1. Acidity relief: Taking a mild antacid (base) like milk of magnesia to neutralize excess stomach acid during indigestion.

    2. Tooth decay prevention: Toothpastes are basic and neutralize the acids produced by bacteria in our mouth after eating sugary foods.

$$\text{NaOH}_{(aq)} + \text{HCl}_{(aq)} \rightarrow \text{NaCl}_{(aq)} + \text{H}_{2}\text{O}_{(l)}$$

• Explanation: When sodium hydroxide (a strong base) mixes with hydrochloric acid (a strong acid), a neutralization reaction occurs, producing common salt ($\text{NaCl}$) and water ($\text{H}_{2}\text{O}$).

$$\text{Zn}_{(s)} + \text{H}_{2}\text{SO}_{4}_{(aq)}^{} \rightarrow \text{ZnSO}_{4}_{(aq)}^{} + \text{H}_{2} \uparrow$$

• Explanation: An active metal like zinc reacts with dilute sulphuric acid, displacing hydrogen to form a metal salt (zinc sulphate) while releasing bubbles of hydrogen gas.

$$\text{CaO}_{(s)} + 2\text{HNO}_{3}_{(aq)}^{} \rightarrow \text{Ca(NO}_{3})_{2}_{(aq)}^{} + \text{H}_{2}\text{O}_{(l)}$$

• Explanation: Calcium oxide is a basic oxide. When it reacts with dilute nitric acid, it neutralizes the acid to form a soluble salt (calcium nitrate) and water.

$$\text{CO}_{2}_{(g)}^{} + 2\text{KOH}_{(aq)} \rightarrow \text{K}_{2}\text{CO}_{3}_{(aq)}^{} + \text{H}_{2}\text{O}_{(l)}$$

• Explanation: Carbon dioxide acts as an acidic oxide. When bubbled through potassium hydroxide (an alkali), it reacts to form potassium carbonate and water.

$$\text{NaHCO}_{3}_{(s)}^{} + \text{HCl}_{(aq)} \rightarrow \text{NaCl}_{(aq)} + \text{H}_{2}\text{O}_{(l)} + \text{CO}_{2} \uparrow$$

• Explanation: Baking soda (sodium hydrogen carbonate) reacts vigorously with hydrochloric acid, producing sodium chloride, water, and rapid effervescence due to the release of carbon dioxide gas.

• Acids: Taste sour, have a pH below $7$, turn blue litmus paper red, and release $\text{H}^{+}$ ions in water. Example: $\text{HCl}$.

• Bases: Taste bitter, have a pH above $7$, turn red litmus paper blue, and release $\text{OH}^{-}$ ions in water. Example: $\text{NaOH}$.

• Cation: A positively charged ion formed by losing electrons; it moves towards the cathode (negative electrode) during electrolysis. Example: $\text{Na}^{+}$.

• Anion: A negatively charged ion formed by gaining electrons; it moves towards the anode (positive electrode) during electrolysis. Example: $\text{Cl}^{-}$.

• Negative electrode (Cathode): Connected to the negative terminal of a power source; attracts positively charged cations and is the site where reduction (gain of electrons) takes place.

• Positive electrode (Anode): Connected to the positive terminal of a power source; attracts negatively charged anions and is the site where oxidation (loss of electrons) takes place.

(Substances: Common salt, sodium acetate, hydrochloric acid, carbon dioxide, potassium bromide, calcium hydroxide, ammonium chloride, vinegar, sodium carbonate, ammonia, sulphur dioxide)

• Answer Grouped by pH:

  • Group 1: pH equal to 7 (Neutral Solutions)

    • Common salt ($\text{NaCl}$) and Potassium bromide ($\text{KBr}$)

    • Reason: They are salts formed from a strong acid and a strong base, making their aqueous solutions neutral.

  • Group 2: pH more than 7 (Basic / Alkaline Solutions)

    • Sodium acetate ($\text{CH}_{3}\text{COONa}$), Calcium hydroxide ($\text{Ca(OH)}_{2}$), Sodium carbonate ($\text{Na}_{2}\text{CO}_{3}$), and Ammonia ($\text{NH}_{3}$)

    • Reason: They either release hydroxide ions directly or form alkaline solutions due to basic properties or hydrolysis (salts of weak acids and strong bases).

  • Group 3: pH less than 7 (Acidic Solutions)

    • Hydrochloric acid ($\text{HCl}$), Carbon dioxide ($\text{CO}_{2}$ - forms carbonic acid in water), Ammonium chloride ($\text{NH}_{4}\text{Cl}$ - acidic salt), Vinegar (acetic acid), and Sulphur dioxide ($\text{SO}_{2}$ - forms sulphurous acid in water)

    • Reason: They release hydronium/hydrogen ions in water, causing the solution to become acidic.