Textbook - Acids, Bases and Salts Questions in English

(A) Curd and other sour substances contain acids. When these substances are kept in brass and copper vessels,the acids react with the metal to form metallic salts and release hydrogen gas. These metallic salts are toxic and can cause food poisoning,thereby spoiling the food.

(N/A) Hydrogen gas $(H_2)$ is usually liberated when an acid reacts with a metal.
Example: When zinc granules react with dilute sulfuric acid $(H_2SO_4)$,zinc sulfate $(ZnSO_4)$ and hydrogen gas are produced.
The chemical equation is:
$Zn(s) + H_2SO_4(aq) \to ZnSO_4(aq) + H_2(g) \uparrow$
Test for Hydrogen gas:
To test for the presence of hydrogen gas,pass the gas produced through a soap solution. Bubbles are formed in the soap solution,which contain hydrogen gas. When a burning candle is brought near these soap bubbles,the gas burns with a 'pop' sound,which confirms the presence of hydrogen gas.

(N/A) The gas that extinguishes a burning candle is carbon dioxide $(CO_2)$. Since the reaction produces $CO_2$ and calcium chloride $(CaCl_2)$ from a metal compound $A$ and hydrochloric acid $(HCl)$,the metal compound $A$ must be calcium carbonate $(CaCO_3)$.
The balanced chemical equation is:
$CaCO_{3(s)} + 2HCl_{(aq)} \rightarrow CaCl_{2(aq)} + CO_{2(g)} + H_2O_{(l)}$

(N/A) The dissociation of $HCl$ or $HNO_3$ to form hydrogen ions occurs only in the presence of water.
Hydrogen ions $(H^+)$ combine with $H_2O$ to form hydronium ions $(H_3O^+)$,which are responsible for acidic properties.
The reactions are as follows:
$HCl_{(aq)} \rightarrow H^+_{(aq)} + Cl^-_{(aq)}$
$H^+_{(aq)} + H_2O_{(l)} \rightarrow H_3O^+_{(aq)}$
Although aqueous solutions of glucose and alcohol contain hydrogen atoms in their molecular structure,these hydrogen atoms do not dissociate into $H^+$ ions in water.
Since they do not release hydrogen ions in solution,they do not exhibit acidic character.

(B) Acids are substances that dissociate in an aqueous solution to produce hydrogen ions $(H^+)$ and corresponding anions.
These free ions act as charge carriers in the solution.
Since electricity is the flow of charge,the movement of these ions allows the aqueous solution of an acid to conduct electricity.

(B) The colour of litmus paper is changed by the presence of hydrogen ions $(H^+)$.
Dry $HCl$ gas does not contain free $H^+$ ions.
An acid only dissociates to release $H^+$ ions when it is in an aqueous solution.
Since the $HCl$ gas is dry and the litmus paper is also dry,no dissociation occurs,and therefore,the colour of the litmus paper does not change.

(C) The process of dissolving a concentrated acid in water is a highly exothermic reaction,meaning it releases a large amount of heat.
If water is added to a concentrated acid,the heat generated may cause the mixture to splash out and cause severe acid burns.
Additionally,the glass container may break due to excessive local heating.
Therefore,it is always recommended to add acid slowly to water with constant stirring.

(B) When an acid is diluted by adding water,the number of hydronium ions $(H_3O^+)$ per unit volume decreases.
This process is known as dilution.
As the volume of the solution increases due to the addition of water,the concentration of the acid decreases,resulting in a lower concentration of $(H_3O^+)$ ions.

(A) When an excess base is dissolved in a solution of sodium hydroxide $(NaOH)$,it releases more hydroxide ions $(OH^-)$ into the solution.
Since sodium hydroxide is a strong base that dissociates completely,adding more base increases the total number of $(OH^-)$ ions per unit volume.
Therefore,the concentration of hydroxide ions $(OH^-)$ increases.

(A) The $pH$ scale measures the acidity or alkalinity of a solution.
$A$ $pH$ value of less than $7$ indicates an acidic solution,while a $pH$ value greater than $7$ indicates a basic (alkaline) solution.
Since the $pH$ of solution $A$ is $6$ $(< 7)$,it is acidic.
Since the $pH$ of solution $B$ is $8$ $(> 7)$,it is basic.
The concentration of hydrogen ions $[H^+]$ is inversely proportional to the $pH$ value.
Therefore,solution $A$ with a lower $pH$ $(6)$ has a higher hydrogen ion concentration than solution $B$ with a higher $pH$ $(8)$.

(A) The nature of a solution is directly dependent on the concentration of $H^+_{(aq)}$ ions.
$1$. When the concentration of $H^+_{(aq)}$ ions increases,the solution becomes more acidic.
$2$. Conversely,when the concentration of $H^+_{(aq)}$ ions decreases,the relative concentration of $OH^-$ ions increases,making the solution more basic.

(N/A) Yes,basic solutions also contain $H^+_{(aq)}$ ions. However,their concentration is significantly lower than the concentration of $OH^-$ ions. Because the concentration of $OH^-$ ions is higher than that of $H^+_{(aq)}$ ions,the solution exhibits basic properties.

(B) The substances mentioned,such as quick lime $(CaO)$,slaked lime $(Ca(OH)_2)$,and chalk $(CaCO_3)$,are basic in nature.
If the soil is too acidic,it becomes unsuitable for the growth of many crops.
To neutralize the excess acidity and improve the soil quality for cultivation,a farmer adds these basic substances to the field.
Therefore,the farmer treats the soil with these substances when the soil is too acidic.

(B) The chemical formula $CaOCl_2$ represents Calcium oxychloride.
Its common name is bleaching powder.
It is widely used for disinfecting water and bleaching cotton and linen in the textile industry.

(C) Calcium hydroxide,represented by the chemical formula $Ca(OH)_2$,reacts with chlorine gas $(Cl_2)$ to produce bleaching powder,which is chemically known as calcium oxychloride $(CaOCl_2)$. The chemical equation for this reaction is: $Ca(OH)_2 + Cl_2 \rightarrow CaOCl_2 + H_2O$.

(D) Washing soda,chemically known as sodium carbonate decahydrate $(Na_2CO_3 \cdot 10H_2O)$,is used for softening hard water. It reacts with the calcium and magnesium salts present in hard water to form insoluble precipitates,thereby removing the hardness.

(N/A) When a solution of sodium hydrogencarbonate $(NaHCO_3)$ is heated,it undergoes thermal decomposition to form sodium carbonate $(Na_2CO_3)$,water $(H_2O)$,and carbon dioxide $(CO_2)$ gas.
The chemical equation for this reaction is:
$2NaHCO_3(s) \xrightarrow{\Delta} Na_2CO_3(s) + H_2O(l) + CO_2(g) \uparrow$

(N/A) The chemical equation for the reaction of Plaster of Paris and water is as follows:
$CaSO_4 \cdot \frac{1}{2}H_2O + 1\frac{1}{2}H_2O \rightarrow CaSO_4 \cdot 2H_2O$
In this reaction,Plaster of Paris (calcium sulphate hemihydrate) reacts with water to form Gypsum (calcium sulphate dihydrate),which is a hard solid mass.

(C) Bases turn red litmus blue,while acids turn blue litmus red.
$A$ basic solution has a $pH$ value greater than $7$.
Since the given solution turns red litmus blue,it is basic in nature.
Therefore,its $pH$ is likely to be $10$ (as $10 > 7$).

(D) Crushed egg-shells consist mainly of calcium carbonate $(CaCO_3)$.
When an acid,such as hydrochloric acid $(HCl)$,reacts with calcium carbonate,it produces carbon dioxide gas $(CO_2)$.
The reaction is: $CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2 \uparrow$.
Carbon dioxide gas turns lime water $(Ca(OH)_2)$ milky due to the formation of insoluble calcium carbonate $(CaCO_3)$.
Therefore,the solution must contain $HCl$.

(A) According to the law of stoichiometry,the neutralization reaction between $NaOH$ and $HCl$ occurs in a fixed ratio.
Given that $10 \,mL$ of $NaOH$ requires $8 \,mL$ of $HCl$ for complete neutralization.
If the volume of $NaOH$ is doubled to $20 \,mL$,the amount of $HCl$ required will also be doubled proportionally.
Calculation: $(20 \,mL \,NaOH / 10 \,mL \,NaOH) \times 8 \,mL \,HCl = 2 \times 8 \,mL = 16 \,mL$.
Therefore,$16 \,mL$ of $HCl$ solution is required.

(B) Indigestion is caused by the production of excess acid in the stomach. Antacids are mild bases that neutralize this excess acid,thereby providing relief from pain and irritation. Therefore,antacids are used for treating indigestion.

(N/A) Word equation: $\text{Sulphuric acid} + \text{Zinc} \rightarrow \text{Zinc sulphate} + \text{Hydrogen}$
Balanced equation: $H_2SO_{4(aq)} + Zn_{(s)} \rightarrow ZnSO_{4(aq)} + H_{2(g)}$
$(b)$ Word equation: $\text{Hydrochloric acid} + \text{Magnesium} \rightarrow \text{Magnesium chloride} + \text{Hydrogen}$
Balanced equation: $2HCl_{(aq)} + Mg_{(s)} \rightarrow MgCl_{2(aq)} + H_{2(g)}$

(N/A) $1$. Take two iron nails and fix them on a rubber cork. Place the cork in a $100\,mL$ beaker.
$2$. Connect the nails to the two terminals of a $6\,V$ battery through a bulb and a switch.
$3$. Pour some dilute $HCl$ into the beaker and switch on the current. The bulb will glow,indicating that $HCl$ conducts electricity due to the presence of $H^+$ ions.
$4$. Repeat the same experiment using glucose solution and alcohol solution separately.
$5$. Observation: The bulb does not glow in the case of glucose and alcohol solutions.
$6$. Conclusion: Glucose and alcohol do not dissociate into ions in aqueous solution. Since they do not produce $H^+$ ions,they do not show acidic properties,even though they contain hydrogen.

(C) Distilled water is a pure form of water and is devoid of any dissolved ionic species. Since electricity conduction requires the movement of ions,distilled water does not conduct electricity.
Rain water,while falling through the atmosphere,dissolves gases like $CO_2$ and $SO_2$ to form acids (like carbonic acid and sulfuric acid). These acids dissociate into ions,which allow the rain water to conduct electricity.

(A) Acids do not show acidic behaviour in the absence of water because the dissociation of hydrogen ions $(H^+)$ from an acid occurs only in the presence of water.
It is the presence of these hydrogen ions $(H^+)$ that is responsible for the acidic behaviour of the substance.
In the absence of water,the acid molecules do not release $H^+$ ions,and therefore,they do not exhibit acidic properties.

(A) Neutral: Solution $D$ $(pH = 7)$.
$(b)$ Strongly alkaline: Solution $C$ $(pH = 11)$.
$(c)$ Strongly acidic: Solution $B$ $(pH = 1)$.
$(d)$ Weakly acidic: Solution $A$ $(pH = 4)$.
$(e)$ Weakly alkaline: Solution $E$ $(pH = 9)$.
The concentration of hydrogen ions $[H^+]$ is inversely proportional to the $pH$ value. Therefore,the increasing order of hydrogen-ion concentration is:
$pH \, 11 < pH \, 9 < pH \, 7 < pH \, 4 < pH \, 1$ (i.e.,$C < E < D < A < B$).

(A) The fizzing will occur more vigorously in test tube $A$,to which hydrochloric acid $(HCl)$ is added.
This is because $HCl$ is a strong acid,whereas $CH_3COOH$ is a weak acid.
Strong acids dissociate completely in water to produce a higher concentration of hydrogen ions $(H^+)$ compared to weak acids.
Since the reaction between magnesium and acid depends on the concentration of $H^+$ ions,the reaction in test tube $A$ proceeds at a faster rate,resulting in more vigorous evolution of hydrogen gas.

(N/A) The $pH$ of fresh milk is $6$. As milk turns into curd,the process involves the fermentation of lactose sugar into lactic acid by bacteria such as $Lactobacillus$. Since lactic acid is acidic in nature,the concentration of hydrogen ions $(H^+)$ increases in the solution. Consequently,the $pH$ value will decrease to below $6$ as the milk becomes more acidic.

(N/A) The milkman shifts the $pH$ of the fresh milk from $6$ to slightly alkaline to prevent the milk from turning sour quickly. In an alkaline condition,the growth of lactic acid-producing bacteria is inhibited,which prevents the milk from spoiling.
$(b)$ Since this milk is slightly basic,the lactic acid produced by bacteria during the fermentation process is neutralized by the added baking soda. Therefore,it takes a longer time for the milk to reach the required acidity level to set as curd.

(N/A) Plaster of Paris $(POP)$ should be stored in a moisture-proof container because it is a powdery substance that reacts with atmospheric moisture to form a hard,solid mass known as gypsum.
The chemical reaction is as follows:
$CaSO_4 \cdot \frac{1}{2}H_2O + 1\frac{1}{2}H_2O \rightarrow CaSO_4 \cdot 2H_2O$ (Gypsum,a hard solid).

(A) reaction in which an acid and a base react with each other to produce salt and water is termed as a neutralization reaction. In this reaction,energy is evolved in the form of heat.
Example $(i)$: The reaction between sodium hydroxide $(NaOH)$ and hydrochloric acid $(HCl)$:
$NaOH + HCl \rightarrow NaCl + H_2O$
Example $(ii)$: During indigestion,caused by the production of excess hydrochloric acid in the stomach,we take an antacid like milk of magnesia $(Mg(OH)_2)$. The antacid neutralizes the excess acid and provides relief.
$Mg(OH)_2 + 2HCl \rightarrow MgCl_2 + 2H_2O$

(N/A) Two important uses of washing soda and baking soda are as follows:
$(1)$ Washing soda:
$(a)$ It is used in glass,soap,and paper industries.
$(b)$ It is used to remove permanent hardness of water.
$(2)$ Baking soda:
$(a)$ It is used as baking powder. Baking powder is a mixture of baking soda and a mild acid known as tartaric acid. When it is heated or mixed in water,it releases $CO_{2}$ that makes bread or cake fluffy.
$(b)$ It is used in soda-acid fire extinguishers.

(N/A) Word equation: $\text{Sulphuric acid} + \text{Aluminium} \rightarrow \text{Aluminium sulphate} + \text{Hydrogen}$
Balanced equation: $3H_{2}SO_{4(aq)} + 2Al_{(s)} \rightarrow Al_{2}(SO_{4})_{3(aq)} + 3H_{2(g)}$
$(b)$ Word equation: $\text{Hydrochloric acid} + \text{Iron} \rightarrow \text{Iron(II) chloride} + \text{Hydrogen}$
Balanced equation: $Fe_{(s)} + 2HCl_{(aq)} \rightarrow FeCl_{2(aq)} + H_{2(g)}$

(N/A) $1$. Place a drop of each solution on the red litmus paper. The solution that turns the red litmus paper blue is the basic solution.
$2$. Now,we have two test tubes left: one containing distilled water (neutral) and the other containing an acidic solution.
$3$. Take the identified basic solution and mix a drop of it with a drop of each of the two remaining unknown solutions separately.
$4$. Test these mixtures with the red litmus paper. The mixture that turns the red litmus paper blue indicates that the added solution was neutral (distilled water),as the basic property remains dominant.
$5$. The mixture that does not change the colour of the red litmus paper indicates that the added solution was acidic,as the acid neutralized the base.