Water or hydride of oxygen Questions in English

(B) Zeolite,represented as $Na_2Ze$,acts as an ion exchanger. When hard water containing $Ca^{2+}$ or $Mg^{2+}$ ions is passed through it,the sodium ions $(Na^+)$ in the zeolite are replaced by these hardness-causing ions.
The reaction is: $Na_2Ze(s) + Ca^{2+}(aq) \to CaZe(s) + 2Na^+(aq)$.

(A) The thermal decomposition of ammonium nitrate $(NH_4NO_3)$ is given by the following reaction:
$NH_4NO_3(s) \xrightarrow{\Delta} N_2O(g) + 2H_2O(l)$
Here,$(A)$ is $NH_4NO_3$,$(B)$ is $N_2O$ (a colourless neutral gas),and $(C)$ is $H_2O$ (a liquid at room temperature which is neutral to litmus).
Thus,the correct option is $(A)$.

(B) Clark's method is used to remove temporary hardness of water by adding lime $(Ca(OH)_2)$.
The chemical reaction is:
$Ca(HCO_3)_2 + Ca(OH)_2 \rightarrow 2 CaCO_3 \downarrow + 2 H_2O$

(D) The zeolite process is limited to the exchange of cations ($Ca^{2+}$ and $Mg^{2+}$) only.
$2 NaZ_{(s)} + M^{2+}_{(aq)} \rightarrow MZ_{2(s)} + 2 Na^{+}_{(aq)}$
(where $M = Ca, Mg$)
In contrast,the synthetic resin method is more efficient because it utilizes both cation exchange resins (to remove $Ca^{2+}$ and $Mg^{2+}$) and anion exchange resins (to remove $Cl^{-}$,$HCO_{3}^{-}$,$SO_{4}^{2-}$,etc.),resulting in demineralized water.
$I$. Cation exchange: $2 RNa_{(s)} + M^{2+}_{(aq)} \rightarrow R_{2}M_{(s)} + 2 Na^{+}_{(aq)}$
$II$. Anion exchange: $RNH_{3}^{+}OH^{-}_{(s)} + X^{-}_{(aq)} \rightarrow RNH_{3}^{+}X^{-}_{(s)} + OH^{-}_{(aq)}$
(where $X = Cl^{-}, HCO_{3}^{-}, SO_{4}^{2-}$)

(A) Ionic conductance is directly proportional to the concentration of dissolved ions in water.
Distilled water is purified through distillation,which removes dissolved salts and minerals,resulting in the lowest concentration of ions.
Therefore,distilled water exhibits the lowest ionic conductance compared to well water,saline water,or sea water,which contain significant amounts of dissolved electrolytes.

(A) The structure of $CuSO_{4} \cdot 5H_{2}O$ is represented as $[Cu(H_{2}O)_{4}]SO_{4} \cdot H_{2}O$.
In this structure,four water molecules are coordinated directly to the $Cu^{2+}$ ion.
The fifth water molecule is held by hydrogen bonding between the $SO_{4}^{2-}$ ion and the coordinated water molecules.
Therefore,only $1$ water molecule is hydrogen-bonded.

(N/A) Ice is the crystalline form of water. It takes a hexagonal form if crystallized at atmospheric pressure, but condenses to a cubic form if the temperature is very low.
The three-dimensional structure of ice is highly ordered and involves hydrogen bonding.
Each oxygen atom is surrounded tetrahedrally by four other oxygen atoms at a distance of $276 \, pm$.
The structure also contains wide holes that can hold molecules of appropriate sizes interstitially.

(N/A) Temporary hardness of water is due to the presence of soluble salts of magnesium and calcium in the form of hydrogen carbonates $(M(HCO_3)_2, \text{ where } M = Mg, Ca)$ in water.
Permanent hardness of water is due to the presence of soluble salts of calcium and magnesium in the form of chlorides and sulfates $(CaCl_2, MgCl_2, CaSO_4, MgSO_4)$ in water.

(N/A) The process of treating permanent hardness of water using synthetic resins is based on the exchange of cations (e.g.,$Na^{+}$,$Ca^{2+}$,$Mg^{2+}$) and anions (e.g.,$Cl^{-}$,$SO_{4}^{2-}$,$HCO_{3}^{-}$) present in water by $H^{+}$ and $OH^{-}$ ions respectively.
Synthetic resins are of two types:
$1)$ Cation exchange resins
$2)$ Anion exchange resins
Cation exchange resins are large organic molecules that contain the $-SO_{3}H$ group. The resin is firstly changed to $RNa$ (from $RSO_{3}H$) by treating it with $NaCl$. This resin then exchanges $Na^{+}$ ions with $Ca^{2+}$ and $Mg^{2+}$ thereby making the water soft.
$2RNa + M_{(aq)}^{2+} \to R_{2}M_{(s)} + 2Na_{(aq)}^{+}$
There are cation exchange resins in $H^{+}$ form. The resins exchange $H^{+}$ ions for $Na^{+}$,$Ca^{2+}$ and $Mg^{2+}$ ions.
$2RH + M_{(aq)}^{2+} \leftrightarrow MR_{2(s)} + 2H_{(aq)}^{+}$
Anion exchange resins exchange $OH^{-}$ ions for anions like $Cl^{-}$,$HCO_{3}^{-}$,and $SO_{4}^{2-}$ present in water.
$RNH_{2(s)} + H_{2}O_{(l)} \leftrightarrow RNH_{3}^{+}.OH_{(s)}^{-}$ $\xrightarrow{+X_{(aq)}^{-}} RNH_{3}^{+}.X_{(s)}^{-} + OH_{(aq)}^{-}$
During the complete process,water first passes through the cation exchange process. The water obtained after this process is free from mineral cations and is acidic in nature. This acidic water is then passed through the anion exchange process where $OH^{-}$ ions neutralize the $H^{+}$ ions and de-ionize the water obtained.

(N/A) The amphoteric nature of water can be described on the basis of the following reactions:
$1)$ Reaction with $H_2S$ (Water acts as a base):
$H_2O_{(l)} + H_2S_{(aq)} \rightleftharpoons H_3O^{+}_{(aq)} + HS^{-}_{(aq)}$
$2)$ Reaction with $NH_3$ (Water acts as an acid):
$H_2O_{(l)} + NH_{3_{(aq)}} \rightleftharpoons OH^{-}_{(aq)} + NH^{+}_{4_{(aq)}}$
$3)$ Self-ionization of water:
In this reaction,one water molecule acts as an acid and another as a base:
$H_2O_{(l)} + H_2O_{(l)} \rightleftharpoons H_3O^{+}_{(aq)} + OH^{-}_{(aq)}$

(N/A) Demineralised water is free from all soluble mineral salts. It does not contain any anions or cations.
Demineralised water is obtained by passing water successively through a cation exchange (in the $H^+$ form) and an anion exchange (in the $OH^-$ form) resin.
During the cation exchange process,$H^+$ exchanges for $Na^+, Mg^{2+}, Ca^{2+},$ and other cations present in water.
$2RH_{(s)} + M^{2+}_{(aq)} \leftrightarrow MR_{2(s)} + 2H^+_{(aq)} \dots (1)$
In the anion exchange process,$OH^-$ exchanges for anions such as $CO_3^{2-}, SO_4^{2-}, Cl^-, HCO_3^-$ etc. present in water.
$RNH_{2(s)} + H_2O_{(l)} \leftrightarrow RNH_3^+OH^-_{(s)}$
$RNH_3^+OH^-_{(s)} + X^-_{(aq)} \leftrightarrow RNH_3^+X^-_{(s)} + OH^-_{(aq)} \dots (2)$
$OH^-$ ions liberated in reaction $(2)$ neutralize $H^+$ ions liberated in reaction $(1)$,thereby forming water.
$H^+_{(aq)} + OH^-_{(aq)} \longrightarrow H_2O_{(l)}$

(N/A) No,demineralised or distilled water is not useful for drinking purposes because it lacks essential minerals like $Ca^{2+}$ and $Mg^{2+}$ ions,which are necessary for the human body.
It can be made useful by adding essential minerals in controlled amounts or by mixing it with a small amount of mineral-rich water to restore the required mineral balance.

(N/A) Water is essential for all forms of life. It constitutes around $65 \%$ of the human body and $95 \%$ of plants. Water plays an important role in the biosphere owing to its high specific heat,thermal conductivity,surface tension,dipole moment,and dielectric constant. The high heat of vaporization and heat capacity of water help in moderating the climate and body temperature of all living beings. It acts as a carrier of various nutrients required by plants and animals for various metabolic reactions.

Water is a universal solvent primarily due to its high dielectric constant $(78.39 \, C^{2} / Nm^{2})$ and high dipole moment.
$(i)$ Water dissolves ionic compounds through ion-dipole interactions and covalent compounds that are capable of forming hydrogen bonds with water.
$(ii)$ Water hydrolyzes various compounds including metallic and non-metallic oxides,hydrides,carbides,phosphides,and nitrides. During hydrolysis,the $H^{+}$ and $OH^{-}$ ions of water react with the substrate.
Examples of hydrolysis reactions:
$CaO + H_{2}O \longrightarrow Ca(OH)_{2}$
$NaH + H_{2}O \longrightarrow NaOH + H_{2}$
$CaC_{2} + H_{2}O \longrightarrow C_{2}H_{2} + Ca(OH)_{2}$

(N/A) Saline hydrides are ionic in nature. They react with water to form a metal hydroxide along with the liberation of hydrogen gas.
The reaction of saline hydrides with water can be represented as:
$AH_{(s)} + H_2O_{(l)} \to AOH_{(aq)} + H_{2(g)}$
(where,$A = Na, Ca, \dots$)
When added to an organic solvent,they react with water present in it. Hydrogen gas escapes into the atmosphere,leaving behind the metallic hydroxide. The dry organic solvent can then be distilled over.

(N/A) Soaps form a precipitate (scum) in hard water due to the presence of $Ca^{2+}$ and $Mg^{2+}$ ions,but they do not form a precipitate in soft water. Therefore,soaps can be used to check the hardness of water.
Synthetic detergents,on the other hand,do not form a precipitate in either hard water or soft water because their calcium and magnesium salts are soluble in water. Therefore,synthetic detergents cannot be used to check the hardness of water.

$A$ major part of all living organisms is made up of water. The human body contains about $65 \%$ water,and some plants contain as much as $95 \%$ water.
It is a crucial compound for the survival of all life forms.
It acts as a solvent of great importance.
The high heat of vaporization and high heat capacity of water are responsible for the moderation of the climate and the regulation of body temperature in living beings.
It serves as an excellent solvent for the transportation of ions and molecules required for plant and animal metabolism.
The distribution of water over the earth's surface is not uniform. The estimated world water supply is given in the following table:

Source	$\%$ of Total
Oceans	$97.33$
Saline lakes and inland seas	$0.008$
Polar ice and glaciers	$2.04$
Ground water	$0.61$
Lakes	$0.009$
Soil moisture	$0.005$
Atmospheric water vapour	$0.001$
Rivers	$0.0001$

(N/A) major part of all living organisms is made up of water. The human body contains about $65 \%$ water,and some plants contain as much as $95 \%$ water.
It is a crucial compound for the survival of all life forms.
It is a solvent of great importance.
The distribution of water over the earth's surface is not uniform. The estimated world water supply is given in the following table.
The high heat of vaporization and heat capacity are responsible for the moderation of the climate and the body temperature of living beings.
It is an excellent solvent for the transportation of ions and molecules required for plant and animal metabolism.
Estimated World Water Supply

Source	$\%$ of Total
Oceans	$97.33$
Saline lakes and inland seas	$0.008$
Polar ice and glaciers	$2.04$
Ground water	$0.61$
Lakes	$0.009$
Soil moisture	$0.005$
Atmospheric water vapour	$0.001$
Rivers	$0.0001$

$A$ major part of all living organisms is made up of water. The human body contains about $65 \%$ water,and some plants contain as much as $95 \%$ water.
It is a crucial compound for the survival of all life forms.
It acts as a solvent of great importance for biological processes.
The high heat of vaporization and high heat capacity of water are responsible for the moderation of the climate and the maintenance of body temperature in living beings.
It serves as an excellent solvent for the transportation of ions and molecules required for plant and animal metabolism.
The distribution of water over the earth's surface is not uniform. The estimated world water supply is given in the following table:

Source	$\%$ of Total
Oceans	$97.33$
Saline lakes and inland seas	$0.008$
Polar ice and glaciers	$2.04$
Ground water	$0.61$
Lakes	$0.009$
Soil moisture	$0.005$
Atmospheric water vapour	$0.001$
Rivers	$0.0001$

(N/A) Water is a polar molecule with a high dielectric constant ($78.39$ at $298 \ K$),which allows it to dissolve ionic compounds by weakening the electrostatic forces between ions. It also forms hydrogen bonds with polar covalent molecules,enabling the dissolution of substances like alcohols and carbohydrates.
$(i)$ Water dissolves ionic compounds (e.g.,$NaCl$) and polar covalent compounds (e.g.,glucose,ethanol) through ion-dipole and hydrogen bonding interactions.
$(ii)$ Water hydrolyses many covalent and ionic compounds,such as metal oxides,non-metal oxides,and halides. For example,$P_4O_{10} + 6H_2O \rightarrow 4H_3PO_4$ and $SiCl_4 + 2H_2O \rightarrow SiO_2 + 4HCl$.

(N/A) Water is a colourless and tasteless liquid. The unusual properties of water in the condensed phase (liquid and solid states) are due to the presence of extensive hydrogen bonding between water molecules.
This leads to high freezing point,high boiling point,high heat of vaporisation,and high heat of fusion in comparison to $H_{2}S$ and $H_{2}Se$.
In comparison to other liquids,water has a higher specific heat,thermal conductivity,surface tension,dipole moment,and dielectric constant.
These properties allow water to play a key role in the biosphere.
It is an excellent solvent for the transportation of ions and molecules required for plant and animal metabolism.
Due to hydrogen bonding with polar molecules,even covalent compounds like alcohol and carbohydrates dissolve in water. Physical properties of $H_{2}O$ and $D_{2}O$ are summarized below:

Property	$H_{2}O$ vs $D_{2}O$
Molecular mass $(g \ mol^{-1})$	$H_{2}O: 18.0151, D_{2}O: 20.0276$
Melting point $(K)$	$H_{2}O: 273.0, D_{2}O: 276.8$
Boiling point $(K)$	$H_{2}O: 373.0, D_{2}O: 374.4$
Enthalpy of formation $(kJ \ mol^{-1})$	$H_{2}O: -285.9, D_{2}O: -294.6$
Enthalpy of vaporisation $(373 \ K)$ $(kJ \ mol^{-1})$	$H_{2}O: 40.66, D_{2}O: 41.61$
Enthalpy of fusion $(kJ \ mol^{-1})$	$H_{2}O: 6.01, D_{2}O: 6.28$
Temp of max. density $(K)$	$H_{2}O: 276.98, D_{2}O: 284.2$
Density $(298 \ K)$ $(g \ cm^{-3})$	$H_{2}O: 1.0000, D_{2}O: 1.1059$
Viscosity (centipoise)	$H_{2}O: 0.8903, D_{2}O: 1.107$
Dielectric constant	$H_{2}O: 78.39, D_{2}O: 78.06$
Electrical conductivity $(293 \ K)$ $(ohm^{-1} \ cm^{-1})$	$H_{2}O: 5.7 \times 10^{-8}, D_{2}O: 3.0 \times 10^{-8}$

(N/A) In the gas phase, water is a bent molecule with a bond angle of $104.5^{\circ}$ and an $O-H$ bond length of $95.7 \text{ pm}$.
It is a highly polar molecule due to the electronegativity difference between oxygen and hydrogen atoms.
In the liquid phase, water molecules are associated together by hydrogen bonds.
The crystalline form of water is ice. At atmospheric pressure, ice crystallizes in the hexagonal form, but at very low temperatures, it condenses to a cubic form.
The density of ice is less than that of water, which is why an ice cube floats on water.
In the winter season, ice formed on the surface of a lake provides thermal insulation, which ensures the survival of aquatic life.

(N/A) Ice has a highly ordered three-dimensional hydrogen-bonded structure.
Examination of ice crystals with $X$-rays shows that each oxygen atom is surrounded tetrahedrally by four other oxygen atoms at a distance of $276 \text{ pm}$.
Hydrogen bonding gives ice a rather open-type structure with wide holes. These holes can hold some other molecules of appropriate size interstitially.

(N/A) Ice has a highly ordered three-dimensional hydrogen-bonded structure as shown in the figure.
Examination of ice crystals with $X$-rays shows that each oxygen atom is surrounded tetrahedrally by four other oxygen atoms at a distance of $276 \text{ pm}$.
Hydrogen bonding gives ice a rather open-type structure with wide holes. These holes can hold some other molecules of appropriate size interstitially.

Water reacts with a large number of substances. Some of the important reactions are given below.
$(i)$ Amphoteric Nature: It has the ability to act as an acid as well as a base,i.e.,it behaves as an amphoteric substance. In the Br\u00f6nsted sense,it acts as an acid with $NH_{3}$ and a base with $H_{2}S$.
$H_{2}O_{(l)} + NH_{3(aq)} \rightleftharpoons OH^{-}_{(aq)} + NH_{4(aq)}^{+}$
$H_{2}O_{(l)} + H_{2}S_{(aq)} \rightleftharpoons H_{3}O^{+}_{(aq)} + HS^{-}_{(aq)}$
The auto-protolysis (self-ionization) of water takes place as follows:
$H_{2}O_{(l)} + H_{2}O_{(l)} \rightleftharpoons H_{3}O^{+}_{(aq)} + OH^{-}_{(aq)}$
$(ii)$ Redox Reactions Involving Water: Water can be easily reduced to dihydrogen by highly electropositive metals.
$2H_{2}O_{(l)} + 2Na_{(s)} \longrightarrow 2NaOH_{(aq)} + H_{2(g)}$
Thus,it is a great source of dihydrogen. Water is oxidised to $O_{2}$ during photosynthesis.
$6CO_{2(g)} + 12H_{2}O_{(l)} \longrightarrow C_{6}H_{12}O_{6(aq)} + 6H_{2}O_{(l)} + 6O_{2(g)}$
With fluorine also,it is oxidised to $O_{2}$:
$2F_{2(g)} + 2H_{2}O_{(l)} \longrightarrow 4H^{+}_{(aq)} + 4F^{-}_{(aq)} + O_{2(g)}$

Rain water is almost pure. Being a good solvent,when it flows on the surface of the earth,it dissolves many salts.
Presence of calcium and magnesium salts in the form of hydrogencarbonate,chloride,and sulphate in water makes water 'hard'.
Hard water does not give lather with soap. Water free from soluble salts of calcium and magnesium is called 'soft' water. It gives lather with soap easily.
Hard water forms scum/precipitate with soap. Soap containing sodium stearate $(C_{17}H_{35}COONa)$ reacts with hard water to precipitate out $Ca^{2+}$ or $Mg^{2+}$ stearate.
$2 C_{17}H_{35}COONa_{(aq)} + M^{2+}_{(aq)} \longrightarrow (C_{17}H_{35}COO)_{2} M_{(s)} \downarrow + 2 Na^{+}_{(aq)}$
Here,$M$ represents $Ca^{2+}$ or $Mg^{2+}$.
It is,therefore,unsuitable for laundry. It is also harmful for boilers because of the deposition of salts in the form of scale,which reduces the efficiency of the boiler. The hardness of water is of two types: $(i)$ temporary hardness,and $(ii)$ permanent hardness.

(N/A) $(i)$ Boiling: During boiling,the soluble $Mg(HCO_{3})_{2}$ is converted into insoluble $Mg(OH)_{2}$ and $Ca(HCO_{3})_{2}$ is changed to insoluble $CaCO_{3}$. Because the solubility product of $Mg(OH)_{2}$ is lower than that of $MgCO_{3}$,$Mg(OH)_{2}$ precipitates out. These precipitates can be removed by filtration to obtain soft water.
$Mg(HCO_{3})_{2} \xrightarrow{\Delta} Mg(OH)_{2} \downarrow + 2CO_{2} \uparrow$
$Ca(HCO_{3})_{2} \xrightarrow{\Delta} CaCO_{3} \downarrow + H_{2}O + CO_{2} \uparrow$
$(ii)$ Clark's method: In this method,a calculated amount of lime $(Ca(OH)_{2})$ is added to hard water. It precipitates calcium carbonate and magnesium hydroxide,which can be filtered off.
$Ca(HCO_{3})_{2} + Ca(OH)_{2} \longrightarrow 2CaCO_{3} \downarrow + 2H_{2}O$
$Mg(HCO_{3})_{2} + 2Ca(OH)_{2} \longrightarrow 2CaCO_{3} \downarrow + Mg(OH)_{2} \downarrow + 2H_{2}O$

(N/A) Permanent hardness is due to the presence of soluble salts of magnesium and calcium in the form of chlorides and sulphates in water.
Permanent hardness is not removed by boiling. It can be removed by the following methods:
$(i)$ Treatment with washing soda (sodium carbonate): Washing soda reacts with soluble calcium and magnesium chlorides and sulphates in hard water to form insoluble carbonates.
$MCl_{2} + Na_{2}CO_{3} \longrightarrow MCO_{3} \downarrow + 2NaCl$ $(M = Mg, Ca)$
$MSO_{4} + Na_{2}CO_{3} \longrightarrow MCO_{3} \downarrow + Na_{2}SO_{4}$
$(ii)$ Calgon's method: Sodium hexametaphosphate $(Na_{6}P_{6}O_{18})$,commercially called 'calgon',when added to hard water,the following reactions take place:
$Na_{6}P_{6}O_{18} \longrightarrow 2Na^{+} + Na_{4}P_{6}O_{18}^{2-}$
$M^{2+} + [Na_{4}P_{6}O_{18}]^{2-} \longrightarrow [Na_{2}MP_{6}O_{18}]^{2-} + 2Na^{+}$ $(M = Mg, Ca)$
The complex anion keeps the $Mg^{2+}$ and $Ca^{2+}$ ions in solution.
$(iii)$ Ion-exchange method: This method is also called the zeolite/permutit process. Hydrated sodium aluminium silicate is zeolite/permutit.
For the sake of simplicity,sodium aluminium silicate $(NaAlSiO_{4})$ can be written as $NaZ$. When this is added to hard water,exchange reactions take place:
$2NaZ_{(s)} + M^{2+}_{(aq)} \longrightarrow MZ_{2(s)} + 2Na^{+}_{(aq)}$ $(M = Mg, Ca)$

(N/A) Demineralised water is water that is free from all soluble mineral salts,such as those containing $Ca^{2+}$,$Mg^{2+}$,$Na^{+}$,$Cl^{-}$,$SO_{4}^{2-}$,and $HCO_{3}^{-}$ ions.
It is obtained by passing water successively through a cation exchange resin (in the $H^{+}$ form) and an anion exchange resin (in the $OH^{-}$ form).
$1.$ In the cation exchange process,$H^{+}$ ions are exchanged for $Na^{+}$,$Ca^{2+}$,$Mg^{2+}$,and other cations present in the water:
$2RH_{(s)} + M^{2+}_{(aq)} \longrightarrow MR_{2(s)} + 2H^{+}_{(aq)}$
$2.$ In the anion exchange process,$OH^{-}$ ions are exchanged for anions like $Cl^{-}$,$HCO_{3}^{-}$,and $SO_{4}^{2-}$:
$R'NH_{2(s)} + H_{2}O_{(l)} \longrightarrow R'NH_{3}^{+}OH^{-}_{(s)}$
$R'NH_{3}^{+}OH^{-}_{(s)} + X^{-}_{(aq)} \longrightarrow R'NH_{3}^{+}X^{-}_{(s)} + OH^{-}_{(aq)}$
Finally,the $H^{+}$ and $OH^{-}$ ions released in the process combine to produce water:
$H^{+}_{(aq)} + OH^{-}_{(aq)} \longrightarrow H_{2}O_{(l)}$

(N/A) Demineralised water is not suitable for drinking because it lacks essential minerals like $Ca^{2+}$,$Mg^{2+}$,and other trace elements required by the human body. Distilled water is also not ideal for drinking as it lacks these essential minerals. To make such water useful for drinking,it must be remineralized by adding controlled amounts of essential minerals and salts,and it should be aerated to improve its taste.

(N/A) Permanent hardness of water is due to the presence of soluble salts of magnesium and calcium in the form of chlorides and sulfates in water.
$(i)$ Treatment with washing soda (sodium carbonate): Washing soda reacts with soluble calcium and magnesium sulfates and chlorides in hard water to form insoluble carbonates.
$MCl_{2} + Na_{2}CO_{3} \longrightarrow MCO_{3} \downarrow + 2NaCl$ $(M = Mg, Ca)$
$MSO_{4} + Na_{2}CO_{3} \longrightarrow MCO_{3} \downarrow + Na_{2}SO_{4}$
$(ii)$ Calgon's method: Sodium hexametaphosphate $(Na_{6}P_{6}O_{18})$,commercially called 'Calgon',is added to hard water. The reactions are as follows:
$Na_{6}P_{6}O_{18} \longrightarrow 2Na^{+} + Na_{4}P_{6}O_{18}^{2-}$
$M^{2+} + Na_{4}P_{6}O_{18}^{2-} \longrightarrow [Na_{2}MP_{6}O_{18}]^{2-} + 2Na^{+}$ $(M = Mg, Ca)$
This complex anion keeps $Mg^{2+}$ and $Ca^{2+}$ ions in solution.
$(iii)$ Ion-exchange method: This is also known as the zeolite/permutit process. Hydrated sodium aluminium silicate is known as zeolite/permutit. Represented as $NaZ$,when added to hard water,an exchange reaction occurs:
$2NaZ_{(s)} + M^{2+}_{(aq)} \longrightarrow MZ_{2(s)} + 2Na^{+}_{(aq)}$ $(M = Mg, Ca)$
When all sodium in the zeolite is used up,it becomes ineffective,but it can be regenerated by treatment with an aqueous sodium chloride solution:
$MZ_{2(s)} + 2NaCl_{(aq)} \longrightarrow 2NaZ_{(s)} + MCl_{2(aq)}$
$(iv)$ Synthetic resin method: Currently,synthetic cation exchangers are used to soften hard water. This method is more efficient than the zeolite process.
Synthetic resins contain a large organic molecule with a $-SO_{3}H$ group. The resin $(RSO_{3}H)$ is changed to $RNa$ by treatment with $NaCl$. When hard water is passed through this resin $(RNa)$,$Ca^{2+}$ and $Mg^{2+}$ ions are exchanged for $Na^{+}$ ions:
$2RNa_{(s)} + M^{2+}_{(aq)} \longrightarrow R_{2}M_{(s)} + 2Na^{+}_{(aq)}$
The resin can be regenerated by treatment with aqueous sodium chloride solution.

(N/A) Heavy water $(D_2O)$ is injurious to human beings,plants,and animals because it slows down the rates of biochemical reactions occurring in them.
Thus,heavy water does not support life and is not suitable for drinking.
Heavy water $(D_2O)$ is an oxide of deuterium,an isotope of hydrogen. It is extensively used as a moderator in nuclear reactors and in exchange reactions for the study of reaction mechanisms.
It can be prepared by the exhaustive electrolysis of water or as a by-product in some fertilizer industries.
Chemical reactions of $D_2O$:
$CaC_2 + 2 D_2O \longrightarrow C_2D_2 + Ca(OD)_2$
$SO_3 + D_2O \longrightarrow D_2SO_4$
$Al_4C_3 + 12 D_2O \longrightarrow 3 CD_4 + 4 Al(OD)_3$

(N/A) Heavy water,denoted as $D_{2}O$,is an oxide of deuterium,which is an isotope of hydrogen.
Heavy water is injurious to human beings,plants,and animals because it slows down the rates of biochemical reactions occurring in them. Consequently,it does not support life and is not suitable for drinking.
Uses of heavy water:
$1$. It is extensively used as a moderator in nuclear reactors to slow down fast-moving neutrons.
$2$. It is used in exchange reactions to study reaction mechanisms.
$3$. It serves as a tracer compound in various chemical and biological studies.
Chemical reactions involving heavy water:
$CaC_{2} + 2D_{2}O \longrightarrow C_{2}D_{2} + Ca(OD)_{2}$
$SO_{3} + D_{2}O \longrightarrow D_{2}SO_{4}$
$Al_{4}C_{3} + 12D_{2}O \longrightarrow 3CD_{4} + 4Al(OD)_{3}$

(N/A) Sodium hexametaphosphate is commercially known as 'Calgon'.
Its molecular formula is $Na_{6}P_{6}O_{18}$.

(B) In Clark's method,lime water,which is calcium hydroxide $(Ca(OH)_2)$,is added to water containing temporary hardness.
It reacts with calcium bicarbonate to precipitate calcium carbonate:
$Ca(HCO_3)_2 + Ca(OH)_2 \rightarrow 2CaCO_3 \downarrow + 2H_2O$.

(N/A) The density of water is higher than the density of ice because of the structure formed by hydrogen bonding. In ice,water molecules are arranged in a highly ordered,three-dimensional open cage-like structure due to hydrogen bonding,which creates large empty spaces. When ice melts,these hydrogen bonds break,and the molecules occupy these empty spaces,leading to a more compact structure in liquid water. Consequently,for the same mass,liquid water occupies less volume than ice,resulting in a higher density.

(A) The hardness of water is primarily caused by the presence of dissolved calcium and magnesium ions. Specifically,$Ca^{2+}$ and $Mg^{2+}$ ions are responsible for both temporary and permanent hardness of water.

(D) Hard water contains dissolved salts of calcium $(Ca^{2+})$ and magnesium $(Mg^{2+})$ ions.
These ions react with soap (sodium stearate,$C_{17}H_{35}COONa$) to form an insoluble precipitate known as scum ($ (C_{17}H_{35}COO)_2Ca $ or $ (C_{17}H_{35}COO)_2Mg $).
Due to the formation of this insoluble scum,soap is wasted and cannot form lather,which is necessary for cleaning clothes.
Therefore,hard water is not suitable for washing clothes.

(N/A) Density is defined as mass per unit volume $(d = \frac{m}{V})$.
When water freezes to form ice,the water molecules are held together by hydrogen bonds in a three-dimensional,open,hexagonal honeycomb-like structure.
This structure creates large empty spaces between the water molecules,which increases the volume of the ice compared to an equal mass of liquid water.
Since the volume $(V)$ of ice is greater than the volume of an equal mass of liquid water,the density of ice is lower than that of liquid water.
This is why ice floats on water.

(N/A) $(i)$ During winter,the temperature of lake water decreases. Since cold water is denser,it sinks to the bottom,while warmer water rises to the surface. This continues until the entire lake reaches $277 \ K$ $(4^{\circ}C)$. Below $277 \ K$,the surface water becomes less dense as it cools further,causing it to remain at the top and eventually freeze.
$(ii)$ The density of ice is lower than that of liquid water due to its open cage-like structure formed by hydrogen bonding,which causes it to float on water.

(N/A) Demineralization of water is achieved by passing water through cation and anion exchange resins.
$1$. Cation exchange resin: These resins contain large organic molecules with $-SO_{3}H$ groups. In the $H^{+}$ form,they exchange $H^{+}$ ions for cations like $Na^{+}, Ca^{2+},$ and $Mg^{2+}$ present in water.
$2 RH_{(s)} + M^{2+}_{(aq)} \rightleftharpoons MR_{2(s)} + 2H^{+}_{(aq)}$
This process makes the water acidic due to the release of $H^{+}$ ions.
$2$. Anion exchange resin: These resins contain basic groups like $-NH_{2}$ or $-OH$ groups. They exchange $OH^{-}$ ions for anions like $Cl^{-}, HCO_{3}^{-},$ and $SO_{4}^{2-}$ present in water.
$RNH_{2(s)} + H_{2}O_{(l)} \rightleftharpoons RNH_{3}^{+}OH^{-}_{(s)}$
$RNH_{3}^{+}OH^{-}_{(s)} + X^{-}_{(aq)} \rightleftharpoons RNH_{3}^{+}X^{-}_{(s)} + OH^{-}_{(aq)}$
$3$. Neutralization: The $OH^{-}$ ions released from the anion exchange resin neutralize the $H^{+}$ ions released from the cation exchange resin to form pure water.
$H^{+}_{(aq)} + OH^{-}_{(aq)} \longrightarrow H_{2}O_{(l)}$
The exhausted resins are regenerated using dilute acid and alkali solutions.

(N/A) Heavy water $(D_{2}O)$ is prepared by the prolonged electrolysis of ordinary water $(H_{2}O)$. Since $H_{2}O$ is electrolyzed faster than $D_{2}O$,the remaining water becomes enriched in $D_{2}O$.
The physical properties of $H_{2}O$ and $D_{2}O$ are compared in the table below:

Property	$H_{2}O$	$D_{2}O$
Molecular mass $(g \ mol^{-1})$	$18.0151$	$20.0276$
Melting point $(K)$	$273.0$	$276.8$
Boiling point $(K)$	$373.0$	$374.4$
Enthalpy of formation $(kJ \ mol^{-1})$	$-285.9$	$-294.6$
Enthalpy of vaporisation at $373 \ K$ $(kJ \ mol^{-1})$	$40.66$	$41.61$
Enthalpy of fusion $(kJ \ mol^{-1})$	$6.01$	$6.28$
Temperature of max. density $(K)$	$276.98$	$284.2$
Density at $298 \ K$ $(g \ cm^{-3})$	$1.0000$	$1.1059$
Viscosity (centipoise)	$0.8903$	$1.107$
Dielectric constant $(C^{2} / Nm^{2})$	$78.39$	$78.06$
Electrical conductivity at $298 \ K$ $(ohm^{-1} \ cm^{-1})$	$5.7 \times 10^{-8}$	$3.0 \times 10^{-8}$

(N/A) Water is widely used as a reaction medium due to the following reasons:
$1$. It has a high specific heat capacity,which helps in absorbing heat generated during exothermic reactions.
$2$. It is a polar solvent,making it excellent for dissolving ionic and polar covalent compounds.
$3$. It is chemically stable and relatively inert under many reaction conditions.
$4$. It is inexpensive,non-toxic,and easy to handle.

(A) When hydrogen gas $(H_2)$ and oxygen gas $(O_2)$ combine,they react to form water $(H_2O)$.
The chemical equation for this reaction is: $2H_2(g) + O_2(g) \rightarrow 2H_2O(l)$.
Water is a liquid at room temperature.

(N/A) Water is an example of a substance that exists in three different physical states: solid (ice),liquid (water),and gas (water vapor or steam).
The physical properties of ice,water,and steam are significantly different.
However,the chemical composition of all three states is identical,represented by the formula $H_2O$.
The characteristics of these three states depend on the energy of the molecules and the arrangement of the water molecules.

(N/A) hydrogen ion $(H^{+})$ by itself is a bare proton with a very small size $(\sim 10^{-15} \ m$ radius) and an intense electric field.
Therefore,the proton binds itself to a water molecule at one of the two available lone pairs via a coordinate covalent bond,forming the hydronium ion $(H_{3}O^{+})$.
This hydronium ion $(H_{3}O^{+})$ has a trigonal pyramidal geometry. The existence of the $H_{3}O^{+}$ species has been confirmed in various compounds,such as $H_{3}O^{+}Cl^{-}$ in the solid state.
Similarly,the hydronium ion can be further hydrated to form several other ionic species,such as $H_{5}O_{2}^{+}$,$H_{7}O_{3}^{+}$,and $H_{9}O_{4}^{+}$.
Thus,a proton does not exist independently in an aqueous solution; instead,it exists as a hydronium ion or other hydrated oxonium ions.

(N/A) The melting point,enthalpy of vaporization,and viscosity are physical properties that depend on the magnitude of intermolecular forces of attraction.
As observed from the data,the values for $D_2O$ are consistently higher than those for $H_2O$.
This indicates that the intermolecular forces of attraction (specifically hydrogen bonding) are stronger in $D_2O$ compared to $H_2O$.

(A) The first element of the periodic table is hydrogen $(H)$. When it reacts with dioxygen $(O_2)$,it forms water $(H_2O)$.
Ice (solid $H_2O$) has a lower density than liquid water,which is why it floats on its liquid state.
Water is amphoteric,meaning it can act as both an acid and a base.
Water undergoes autoionisation (also known as self-protolysis) according to the following equilibrium reaction:
$H_2O_{(l)} + H_2O_{(l)} \rightleftharpoons H_3O^{+}_{(aq)} + OH^{-}_{(aq)}$
Thus,the products formed are hydronium ions $(H_3O^+)$ and hydroxide ions $(OH^-)$.

(N/A) $i$. Heavy water $(D_2O)$ is extensively used as a moderator in nuclear reactors to slow down fast-moving neutrons.
$ii$. It is widely used as a tracer compound for studying the reaction mechanisms of various chemical and biochemical processes.
$iii$. Heavy water is used in the preparation of other deuterium compounds such as $CD_4$,$D_2SO_4$,etc.

(N/A) Hard water contains dissolved bicarbonates,chlorides,and sulphates of calcium $(Ca^{2+})$ and magnesium $(Mg^{2+})$ ions. These ions react with soap (sodium stearate,$C_{17}H_{35}COONa$) to form insoluble precipitates of calcium and magnesium stearates.
$2 C_{17}H_{35}COONa_{(aq)} + M^{2+}_{(aq)} \rightarrow (C_{17}H_{35}COO)_2M_{(s)} \downarrow + 2Na^{+}_{(aq)}$
(Where $M = Ca$ or $Mg$)
Because the soap is consumed in forming these insoluble precipitates,it is not available to form lather with water.