Acids, Bases and SaltsClass 10 Chemistry NCERT Solutions

A solution that turns red litmus blue is basic in nature. Basic solutions have a pH value greater than 7. Among the given options, only 10 is greater than 7.

Therefore, the correct option is (d) 10.

Crushed egg-shells are primarily made of calcium carbonate ($CaCO_3$). The gas that turns lime-water milky is carbon dioxide ($CO_2$). The reaction between a metal carbonate and an acid produces carbon dioxide gas. Among the given options, HCl (hydrochloric acid) is an acid.

The reaction is: $CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)$.

Therefore, the correct option is (b) HCl.

For a complete neutralization reaction, the amount of acid required is directly proportional to the amount of base.

Given: 10 mL of NaOH solution is neutralized by 8 mL of HCl solution.

Now, we have 20 mL of the same NaOH solution, which is double the initial amount (20 mL = 2 x 10 mL).

Therefore, the amount of HCl solution required will also be double. Amount of HCl required = 2 x 8 mL = 16 mL.

Therefore, the correct option is (d) 16 mL.

Indigestion is caused by the production of excess acid in the stomach. To treat this condition, a medicine that can neutralize the excess acid is required. Antacids are mild bases that are used for this purpose.

Therefore, the correct option is (c) Antacid.

(a) Dilute sulphuric acid reacts with zinc granules. Word Equation: Zinc + Sulphuric acid → Zinc sulphate + Hydrogen gas Balanced Equation: $Zn(s) + H_2SO_4(aq) \rightarrow ZnSO_4(aq) + H_2(g)$

(b) Dilute hydrochloric acid reacts with magnesium ribbon. Word Equation: Magnesium + Hydrochloric acid → Magnesium chloride + Hydrogen gas Balanced Equation: $Mg(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2(g)$

(c) Dilute sulphuric acid reacts with aluminium powder. Word Equation: Aluminium + Sulphuric acid → Aluminium sulphate + Hydrogen gas Balanced Equation: $2Al(s) + 3H_2SO_4(aq) \rightarrow Al_2(SO_4)_3(aq) + 3H_2(g)$

(d) Dilute hydrochloric acid reacts with iron filings. Word Equation: Iron + Hydrochloric acid → Iron(II) chloride + Hydrogen gas Balanced Equation: $Fe(s) + 2HCl(aq) \rightarrow FeCl_2(aq) + H_2(g)$

To prove that compounds like alcohol and glucose are not acids, we can perform an activity to check if they conduct electricity in an aqueous solution, which is a property of acids.

Distilled water is pure water, which is a covalent compound and does not contain any dissolved ions. Due to the absence of free ions, distilled water does not conduct electricity.

Rainwater, on the other hand, while passing through the atmosphere, dissolves atmospheric gases like carbon dioxide ($CO_2$) and sulphur dioxide ($SO_2$). These gases react with water to form acids like carbonic acid ($H_2CO_3$) and sulphurous acid ($H_2SO_3$). These acids dissociate in water to produce ions (e.g., $H^+$ and $HCO_3^-$). The presence of these ions makes rainwater a conductor of electricity.

The acidic behaviour of an acid is due to the presence of hydrogen ions ($H^+$). Acids release $H^+$ ions only when they are dissolved in water. In the absence of water, an acid exists as molecules and does not dissociate to produce $H^+$ ions. Since there are no free $H^+$ ions, the acid does not exhibit its acidic properties, such as changing the colour of litmus paper or conducting electricity.

Given pH values: A = 4, B = 1, C = 11, D = 7, E = 9

(a) Neutral: A solution with pH = 7 is neutral. So, solution D is neutral. (b) Strongly alkaline: A solution with a high pH (much greater than 7) is strongly alkaline. So, solution C (pH = 11) is strongly alkaline. (c) Strongly acidic: A solution with a very low pH (much less than 7) is strongly acidic. So, solution B (pH = 1) is strongly acidic. (d) Weakly acidic: A solution with a pH slightly less than 7 is weakly acidic. So, solution A (pH = 4) is weakly acidic. (e) Weakly alkaline: A solution with a pH slightly greater than 7 is weakly alkaline. So, solution E (pH = 9) is weakly alkaline.

Arrangement in increasing order of hydrogen-ion concentration: Hydrogen-ion concentration is inversely proportional to the pH value. Therefore, increasing order of $H^+$ concentration corresponds to decreasing order of pH.

The order of pH values from highest to lowest is: 11 > 9 > 7 > 4 > 1.

Thus, the arrangement of solutions in increasing order of hydrogen-ion concentration is: C < E < D < A < B

The fizzing will occur more vigorously in test tube A.

Reason: The fizzing is due to the evolution of hydrogen gas when magnesium reacts with the acid. The rate of this reaction depends on the concentration of hydrogen ions ($H^+$) in the solution.

Since the concentration of $H^+$ ions is higher in test tube A, the reaction with magnesium is much faster, leading to more vigorous fizzing.

When fresh milk turns into curd, its pH will decrease (become lower than 6).

Explanation: The process of curdling involves the action of bacteria, such as Lactobacillus, on the lactose (sugar) present in milk. These bacteria convert lactose into lactic acid through fermentation. The formation of lactic acid increases the concentration of hydrogen ions ($H^+$) in the milk, making it more acidic. This increase in acidity causes the pH to drop below 6.

(a) Baking soda (sodium hydrogencarbonate, $NaHCO_3$) is a mild base. A milkman adds it to fresh milk to make the milk slightly alkaline (increasing the pH above 6). This is done to neutralize the lactic acid that is naturally produced by bacteria over time. By making the milk alkaline, he prevents it from turning sour quickly, thus increasing its shelf life.

(b) The process of setting curd requires an acidic environment (a low pH) for the bacteria to work efficiently and convert lactose into lactic acid. Since the milkman has made the milk alkaline by adding baking soda, the lactic acid produced by the bacteria must first neutralize this added alkali. Only after the alkali is neutralized can the pH start to drop to the acidic level required for curdling. This delays the process, causing the milk to take a longer time to set as curd.

Plaster of Paris is calcium sulphate hemihydrate ($CaSO_4 \cdot \frac{1}{2}H_2O$). It has the property of reacting with water (or moisture) to set into a hard, solid mass called gypsum ($CaSO_4 \cdot 2H_2O$).

Reaction: $CaSO_4 \cdot \frac{1}{2}H_2O + 1\frac{1}{2}H_2O \rightarrow CaSO_4 \cdot 2H_2O$

If Plaster of Paris is exposed to moisture from the air, it will slowly absorb the water and turn into gypsum, losing its powdery form and setting property. This would make it useless for its intended applications. Therefore, it must be stored in a moisture-proof container to prevent contact with moisture.

A neutralisation reaction is a chemical reaction in which an acid and a base react with each other to form a salt and water. In this reaction, the acidic properties of the acid and the basic properties of the base are destroyed or 'neutralised'.

The general form of the reaction is: Acid + Base → Salt + Water

To identify the contents of each test tube using only red litmus paper, follow these steps:

Curd and other sour substances contain acids. These acids react with metals like copper and brass (an alloy of copper and zinc) to form toxic metal salts. These salts are poisonous in nature and can cause food poisoning, which can be damaging to health. Therefore, to prevent the formation of these harmful compounds, curd and sour substances should not be kept in brass and copper vessels.

Hydrogen gas ($H_2$) is usually liberated when an acid reacts with a metal.

Example: When zinc granules react with dilute sulphuric acid, hydrogen gas is evolved and zinc sulphate is formed. Reaction: $Zn(s) + H_2SO_4(aq) \rightarrow ZnSO_4(aq) + H_2(g)$

Test for Hydrogen Gas: To test for the presence of hydrogen gas, bring a burning candle or a burning matchstick near the mouth of the test tube from which the gas is evolving. If hydrogen gas is present, it will burn with a characteristic 'pop' sound.

The information provided indicates the following:

From these points, we can conclude that the metal compound A is calcium carbonate ($CaCO_3$).

The balanced chemical equation for the reaction is: $CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)$

The acidic character of a substance is due to the presence of hydrogen ions ($H^+$) or hydronium ions ($H_3O^+$). Compounds like HCl and $HNO_3$ are acids because they dissociate in water to produce $H^+$ ions.

For example: $HCl(aq) \rightarrow H^+(aq) + Cl^-(aq)$

On the other hand, compounds like alcohol and glucose contain hydrogen atoms, but they do not dissociate in water to release $H^+$ ions. Since they do not produce hydrogen ions in solution, they do not show acidic character.

An aqueous solution of an acid conducts electricity because acids dissociate in water to form ions. For example, hydrochloric acid (HCl) dissociates into hydrogen ions ($H^+$) and chloride ions ($Cl^-$). The presence of these mobile, charged particles (ions) in the solution allows for the flow of electric current. The movement of these ions through the solution is what constitutes the electric current.

Dry HCl gas does not change the colour of dry litmus paper because it does not dissociate to produce hydrogen ions ($H^+$). The acidic properties of HCl are exhibited only in the presence of water, where it ionizes to form $H^+$ ions. Since both the HCl gas and the litmus paper are dry, there is no water present for ionization to occur. Therefore, no acidic character is shown.

The process of dissolving a concentrated acid, especially sulphuric acid or nitric acid, in water is a highly exothermic reaction (it releases a large amount of heat).

If water is added to a concentrated acid, the heat generated is so large that it can cause the mixture to splash out of the container and cause severe acid burns. The sudden temperature increase can also cause the glass container to crack or break.

To avoid this, it is recommended to add the acid slowly to water with constant stirring. This allows the heat to be dissipated gradually and safely into the larger volume of water.

When a solution of an acid is diluted by adding water, the volume of the solution increases. While the total number of hydronium ions ($H_3O^+$) in the solution remains the same, they are now distributed in a larger volume. This results in a decrease in the concentration of hydronium ions per unit volume.

Sodium hydroxide (NaOH) is a base that dissociates in water to produce sodium ions ($Na^+$) and hydroxide ions ($OH^-$). When an excess base is dissolved in a solution of sodium hydroxide, more hydroxide ions are added to the solution. This leads to an increase in the concentration of hydroxide ions ($OH^-$) per unit volume.

The pH scale is inversely related to the hydrogen ion concentration. A lower pH value indicates a higher concentration of hydrogen ions.

The concentration of $H^+(aq)$ ions determines the acidic nature of a solution.

Yes, basic solutions also have $H^+(aq)$ ions. In any aqueous solution, there is an equilibrium between water molecules and their ions ($H^+$ and $OH^-$). However, a solution is considered basic because the concentration of hydroxide ions ($OH^-(aq)$) is significantly greater than the concentration of hydrogen ions ($H^+(aq)$). The excess of $OH^-$ ions is what gives the solution its basic properties.

A farmer would treat the soil with substances like quick lime (calcium oxide), slaked lime (calcium hydroxide), or chalk (calcium carbonate) when the soil is too acidic. Plants require a specific pH range for healthy growth, and highly acidic soil can be detrimental. These compounds are basic in nature and are used to neutralize the excess acid in the soil, thereby raising the soil's pH to a more neutral or optimal level for cultivation.

The common name of the compound $CaOCl_2$ is Bleaching powder.

The substance that yields bleaching powder on treatment with chlorine is dry slaked lime, which is calcium hydroxide ($Ca(OH)_2$).

The sodium compound used for softening hard water is washing soda. Its chemical name is sodium carbonate decahydrate ($Na_2CO_3 \cdot 10H_2O$).

When a solution of sodium hydrogencarbonate (baking soda) is heated, it decomposes to form sodium carbonate, water, and carbon dioxide gas.

The equation for the reaction is: $2NaHCO_3(s) \xrightarrow{Heat} Na_2CO_3(s) + H_2O(l) + CO_2(g)$

When Plaster of Paris (calcium sulphate hemihydrate) is mixed with water, it undergoes a chemical reaction to form a hard solid mass known as gypsum (calcium sulphate dihydrate).

The equation for the reaction is: $\underset{\text{(Plaster of Paris)}}{CaSO_4 \cdot \frac{1}{2}H_2O} + 1\frac{1}{2}H_2O \rightarrow \underset{\text{(Gypsum)}}{CaSO_4 \cdot 2H_2O}$