[3 Mark Question Answer]

Question 13 Marks

Give balanced equations to show how $–$
$(a)$ Temporary hardness enters into water
$(b)$ Temporary hardness in water can be removed by boiling
$(c)$ Permanent hardness in water can be removed by addition of washing soda.

Answer

$(a)$ Temporary hardness : Enters in water when rain water containing $\ce{CO_2}$ falls on earth and combines with limestone $[\ce{CaCO_3]}\ \&$ magnesite $[ \ce{MgCO _3]}$ layers forming respective soluble bicarbonates.
$\ce{CaCO_3 + H_2 + CO_2 \rightarrow Ca \left[HCO_3\right]_2} ($Cal. bicarbonate$)$
$\ce{MgCO _3 + H _2 O + CO _2 \rightarrow Mg \left[HCO_3\right]_2} ($Magnesium bicarbonate$)$
$(b)$ To remove temporary hardness by boiling : Bicarbonate changes to insoluble carbonate and water becomes soft.

$(c)$ Removal of permanent hardness in water by adding washing soda: $\ce{CaCl _2}$ or $\ce{CaSO _4}$ present in permanent hard water exchanges the ions and forms $\ce{CaCO_4}$ which being in soluble can be filtered.

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Question 23 Marks

Give a reason why metals are arranged in a series called $–$ ‘activity series of metalsName a metal which reacts with
$(a)$ Cold water
$(b)$ Boiling water
$(c)$ Steam $–$ to liberate hydrogen in each case.

Answer

Metals arranged in series is called 'activity series of metals' because metals differ in tendency to lose valence electrons and can be arranged in a series. i.e. metals are arranged in decreasing order of their reactivity. The most reactive metal at top of the series and then with decreasing reactivity.
Metal that react with
$(a)$ Cold water $-$ Sodium
$\ce{2Na + 2H_2O \rightarrow 2 NaOH + H _2}$
$(b)$ Boiling water $-$ Magnesium $\ce{[Mg]}$
$(c)$ Steam $-$ Iron $\ce{[Fe]}$

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Question 33 Marks

Differentiate the function of concentrated sulphuric acid as a drying agent $\&$ as a dehydrating agent. Is fused calcium chloride a dehydrating or a desiccating agent. Give reasons.

Answer

Sulphuric acid $($cone.$)$ acts as drying agent when it removes water from other substances i.e. for drying gases.
Cone, sulphuric acid acts as dehydrating agent when it removes chemically combined water from compounds due to it strong affinity for water.

Cone. $\ce{H_2 SO_4}$ has removed $\ce{H_2O}$ from ethanol and changed it to ethene. Fused calcium chloride $\left[\ce{CaCl _2}\right]$ is desiccating agent as it is drying the substance.

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Question 43 Marks

State how a ‘supersaturated’ solution differs from a ‘saturated solution’. State briefly how you would prepare a supersaturated solution using potassium chloride and water.

Answer

Super saturated solution contains more amount of solute than saturated solution in hot solution. On cooling, it gives out excess salt which separate out. Preparation of supersaturated solution of potassium chloride in water : Take $100 g$ of water and add little by little of $\ce{KCl}$ salt and stir it, if it gets dissolved, add a little more and stir it. We find that $35 g$ of $\ce{KCl}$ can make the solution saturated at $20^{\circ} C$. Now heat the saturated solution and add more of $\ce{KCl}$. At $40^{\circ} C$ it will dissolve $40 g$ and $46 g$ at $60^{\circ} C$ and $54 g$ at $80^{\circ} C$. This super saturated solution at $80^{\circ} C$. On cooling this super saturated solution excess of solute $[\ce{KCl]}$ crystals separates out.

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Question 53 Marks

Draw a table to differentiate $–$ True solutions, colloidal solutions $\&$ suspensions with respect to $–$
$(i)$ Type of mixture
$(ii)$ Solute particles in the mixture.

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Question 63 Marks

Give a reason why $– (a)$ Alcohol and water form a miscible mixture while oil $\&$ water do not. $(b)$ Boiled water tastes flat, $(c)$ On opening a bottle of soda $[$carbon dioxide dissolved in water$]$ the gas escapes out with a ‘fizz’.

Answer

$(a)$ Alcohol and water form miscible mixture as alcohol is soluble in water. oil does not dissolve in water form immiscible mixture.
$(b)$ Boiled water loses the gases and mineral and the water tastes flat.
$(c)$ Solubility of gases in liquid decreases on decreasing the pressure and $CO$ come out of water with a ‘fizz’ when pressure decreases on opening the bottle.

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Question 73 Marks

Define the term 'solubility ' of a solute in water. If $10 g$. of a solute is added to $100 g$. of water at $50^{\circ} C$ and stirred, it is seen that the total solute dissolves at that temperature giving an unsaturated solution. is $10 g$, the solubility of the solute in water. Give reasons.

Answer

Solubility is the maximum amount of solute in grams $-$ that will saturate $100 g$ of water at $0^{\circ} C$.
OR
Maximum amount of solute that can be dissolved in $100 g$ of solvent at specific temperature.
No, $10 g$ is not solubility as the solution is unsaturated and not saturated solution. This means more of solute can be added to make it saturated solution.

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Question 83 Marks

Explain how each of the following factors affects the formation of a solution $–$
$(a)$ Proportion of the solute & the solvent
$(b)$ Particle size of the solute
$(c)$ Temperature of the solvent

Answer

Factors effecting the formation of solution :
$(a)$ Proportion of solute $\&$ the solvent : The solute must be in a small amount compared to the solvent which must be in much greater amount to dissolve the solute.
$(b)$ Particle size of the solute : Smaller the size of solute, quickly and completely they will dissolve and form solution.
$(c)$ Temperature of the solvent : Sometime increase in temperature makes the solution quicker.

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Question 93 Marks

State what is potable water. Give its qualitie characteristics which make it fit for human consumption.

Answer

Potable water : “That water which is fit for human consumption and for drinking purposes.”
Characteristics of potable water :
Water should be free from harmful bacteria, germs, suspended impurities and harmful salts.
Should be clear, colourless and odourless.
Should contain small amounts of dissolved gases and minerals for good taste.
Should have dissolved air and salts of $(\ce{Na, Ca, Mg)}$ which impart taste to the water and are useful for metabolic functions of the human body.

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