CBSE Class 10 Science Chapter 2: Acids, Bases and Salts NCERT Solutions

Bases have the property of turning red litmus paper blue. Acids, on the other hand, turn blue litmus paper red. The pH scale measures the acidity or alkalinity of a solution. A pH value less than 7 indicates an acidic solution, a pH of 7 indicates a neutral solution, and a pH value greater than 7 indicates a basic (alkaline) solution. Since the given solution turns red litmus blue, it indicates that the solution is basic. Therefore, its pH is likely to be greater than 7. Among the given options (1, 4, and 10), only 10 is greater than 7. Hence, the pH of the solution is likely to be 10.

Eggshells are primarily composed of calcium carbonate (CaCO_3). When an acid reacts with a carbonate, it produces a salt, water, and carbon dioxide gas. The reaction is as follows: Acid + Carbonate → Salt + Water + Carbon Dioxide. Carbon dioxide gas has the property of turning lime-water (calcium hydroxide solution) milky due to the formation of a white precipitate of calcium carbonate. Hydrochloric acid (HCl) is an acid that reacts with calcium carbonate. Sodium chloride (NaCl) and potassium chloride (KCl) are salts and do not react with eggshells in this manner. Therefore, the solution contains HCl.

The reaction is: 2HCl_{(aq)} + CaCO_{3(s)} \longrightarrow CaCl_{2(aq)} + H_2O_{(l)} + CO_{2(g)}

The CO_2 gas produced turns lime water milky: Ca(OH)_{2(aq)} + CO_{2(g)} \longrightarrow CaCO_{3(s)} + H_2O_{(l)}

This problem involves a neutralization reaction between sodium hydroxide (NaOH), a base, and hydrochloric acid (HCl), an acid. The balanced chemical equation for the reaction is: NaOH_{(aq)} + HCl_{(aq)} \longrightarrow NaCl_{(aq)} + H_2O_{(l)}

From the balanced equation, we see that 1 mole of NaOH reacts with 1 mole of HCl. This means that the number of moles of NaOH is equal to the number of moles of HCl required for complete neutralization.

We are given that 10 mL of NaOH solution is neutralized by 8 mL of HCl solution. Let the concentration of NaOH be C_A and the concentration of HCl be C_B. The number of moles is concentration multiplied by volume.

For the first case: Moles of NaOH = C_A \times 10 mL. Moles of HCl = C_B \times 8 mL.

Since moles of NaOH = moles of HCl, we have C_A \times 10 = C_B \times 8.

Now, we need to find the volume of HCl solution required to neutralize 20 mL of the same NaOH solution. Let this volume be V_{HCl}.

For the second case: Moles of NaOH = C_A \times 20 mL. Moles of HCl = C_B \times V_{HCl} mL.

Equating the moles: C_A \times 20 = C_B \times V_{HCl}.

We can rearrange the first equation to find the ratio C_A / C_B: \frac{C_A}{C_B} = \frac{8}{10}\.

Substitute this ratio into the second equation:

\frac{8}{10} \times 20 = V_{HCl}

V_{HCl} = \frac{8 \times 20}{10} = 8 \times 2 = 16 mL.

Alternatively, we can observe that the volume of NaOH solution has been doubled (from 10 mL to 20 mL). Since the concentration of both solutions remains the same, and the reaction is 1:1, the volume of HCl required will also double. Therefore, the amount of HCl solution required will be 8 mL * 2 = 16 mL.

Indigestion is often caused by excess acid produced in the stomach. Antacids are medicines that contain bases (like magnesium hydroxide or aluminium hydroxide) which neutralize the excess acid in the stomach, providing relief from indigestion. Antibiotics are used to treat bacterial infections, analgesics are used to relieve pain, and antiseptics are used to kill or inhibit the growth of microorganisms on living tissues.

These reactions involve the reaction of dilute acids with metals, which typically produce a salt and hydrogen gas.

1. Reaction of dilute sulphuric acid with zinc granules: Word Equation: Sulphuric acid + Zinc → Zinc sulphate + Hydrogen Balanced Equation: H_2SO_{4(aq)} + Zn_{(s)} \longrightarrow ZnSO_{4(aq)} + H_{2(g)} In this reaction, zinc displaces hydrogen from dilute sulphuric acid, forming zinc sulphate and releasing hydrogen gas.
2. Reaction of dilute hydrochloric acid with magnesium ribbon: Word Equation: Hydrochloric acid + Magnesium → Magnesium chloride + Hydrogen Balanced Equation: 2HCl_{(aq)} + Mg_{(s)} \longrightarrow MgCl_{2(aq)} + H_{2(g)} Here, magnesium is more reactive than hydrogen and displaces it from hydrochloric acid, producing magnesium chloride and hydrogen gas.
3. Reaction of dilute sulphuric acid with aluminium powder: Word Equation: Sulphuric acid + Aluminium → Aluminium sulphate + Hydrogen Balanced Equation: 3H_2SO_{4(aq)} + 2Al_{(s)} \longrightarrow Al_2(SO_4)_{3(aq)} + 3H_{2(g)} Aluminium reacts with dilute sulphuric acid to form aluminium sulphate and hydrogen gas. The equation is balanced to ensure the conservation of atoms on both sides.
4. Reaction of dilute hydrochloric acid with iron filings: Word Equation: Hydrochloric acid + Iron → Ferric chloride + Hydrogen Balanced Equation: 6HCl_{(aq)} + 2Fe_{(s)} \longrightarrow 2FeCl_{3(aq)} + 3H_{2(g)} Iron reacts with dilute hydrochloric acid to produce iron(III) chloride (ferric chloride) and hydrogen gas. The equation is balanced with appropriate stoichiometric coefficients.

To demonstrate that compounds like alcohols and glucose, which contain hydrogen, do not behave as acids, we can conduct an experiment involving electrical conductivity. Acids ionize in water to produce hydrogen ions (H^+), making the solution electrically conductive. Alcohols and glucose, however, do not ionize significantly in water.

Activity:

1. Take a 100 mL beaker and place a cork in it.
2. Fit two nails into the cork such that they are separated by a small distance.
3. Connect these two nails to the two terminals of a 6-volt battery using a bulb and a switch.
4. Pour some dilute hydrochloric acid (HCl) into the beaker, ensuring the nails are immersed in the acid.
5. Close the switch. Observe if the bulb glows.
6. Empty the beaker, clean it, and repeat the process with a dilute solution of glucose.
7. Again, empty the beaker, clean it, and repeat the process with a solution of alcohol (like ethanol).

Observations:

• When the bulb is connected to the dilute HCl solution and the switch is closed, the bulb will glow brightly. This indicates that the HCl solution conducts electricity. This is because HCl dissociates in water to form ions (H^+ and Cl^-), which carry the electric current.
• When the same experiment is performed with a glucose solution, the bulb will not glow, or it will glow very dimly. This shows that glucose solution does not conduct electricity well. Glucose molecules dissolve in water but do not ionize to a significant extent.
• Similarly, when the experiment is performed with an alcohol solution, the bulb will not glow. Alcohol molecules also dissolve in water but do not produce ions.

Conclusion:

The glowing of the bulb in the HCl solution demonstrates that acids produce ions in solution, making them electrolytes. The fact that the bulb does not glow in glucose and alcohol solutions, despite them containing hydrogen, proves that they do not ionize in water and therefore do not exhibit acidic properties. This activity clearly distinguishes acids from other hydrogen-containing compounds.