Water — ICSE Class 9 Chemistry Important Questions

Answer: (b) Universal solvent.

Water dissolves a very large number of substances, more than any other common liquid, so it is called the universal solvent.

Answer: (b) Water of crystallisation.

Hydrated copper sulphate, $CuSO_4\cdot 5H_2O$CuSO_4· 5H_2O, is blue because of its water of crystallisation. On heating it loses this water and the anhydrous salt, $CuSO_4$CuSO_4, is white.

Answer: (c) Deliquescent.

A deliquescent substance, such as $NaOH$NaOH or $CaCl_2$CaCl_2, absorbs so much water vapour from air that it eventually dissolves to form a solution.

Answer: (b) $16\ \text{g}$16 g.

Solubility means $32\ \text{g}$32 g dissolves in $100\ \text{g}$100 g water. For $50\ \text{g}$50 g water, mass $= \dfrac{32}{100}\times 50 = 16\ \text{g}$= 32/100× 50 = 16 g.

Answer: (a) Washing soda is efflorescent; it loses water of crystallisation to the dry air and crumbles into a white powder of monohydrate, so R correctly explains A.

(i) Saturated solution: a solution which, at a given temperature, contains the maximum amount of solute that can dissolve in a given amount of solvent, so no more solute can dissolve at that temperature.

(ii) Unsaturated solution: a solution which contains less solute than it can dissolve at that temperature, so more solute can still be dissolved in it.

Soft water readily forms a lather (foam) with soap, whereas hard water does not form a lather easily and instead forms an insoluble white scum, wasting soap.

Hardness is caused by the dissolved bicarbonates, chlorides and sulphates of calcium and magnesium, e.g. $Ca(HCO_3)_2$Ca(HCO_3)_2, $CaSO_4$CaSO_4, $MgCl_2$MgCl_2.

Efflorescence: the property of certain hydrated salts of losing their water of crystallisation to the atmosphere and crumbling to powder. Example: $Na_2CO_3\cdot 10H_2O$Na_2CO_3· 10H_2O (washing soda).

Deliquescence: the property of certain substances of absorbing so much moisture from air that they dissolve to form a solution. Example: $NaOH$NaOH (or $CaCl_2$CaCl_2).

Hygroscopy: the property of certain substances of absorbing moisture from air without dissolving in it and without changing state. Example: concentrated $H_2SO_4$H_2SO_4 (or $CaO$CaO).

(i) Solubility of a solute at a given temperature is the maximum mass in grams of the solute that dissolves in $100\ \text{g}$100 g of solvent to form a saturated solution at that temperature.

(ii) Mass of $KCl = \dfrac{37}{100}\times 250 = 92.5\ \text{g}$KCl = 37/100× 250 = 92.5 g.

(iii) In $100\ \text{g}$100 g water, $37\ \text{g}$37 g $KCl$KCl dissolves, giving $137\ \text{g}$137 g of solution. So $274\ \text{g}$274 g of solution is exactly twice this and contains $2\times 100 = 200\ \text{g}$2× 100 = 200 g of water (and $74\ \text{g}$74 g of $KCl$KCl).

(i) As the temperature rises, the solubility of the salt increases (the curve rises upward).

(ii) Such a graph of solubility against temperature is called a solubility curve.

(iii) A solubility curve is used to find the solubility of a salt at any given temperature, and to predict how much solute will crystallise out when a saturated solution is cooled.

(a) Water of crystallisation is the fixed number of water molecules chemically bound in one formula unit of a salt as it crystallises, giving the crystal its shape and often its colour.

• Hydrated copper sulphate: $CuSO_4\cdot 5H_2O$CuSO_4· 5H_2O, blue.

• Hydrated iron(II) sulphate: $FeSO_4\cdot 7H_2O$FeSO_4· 7H_2O, green.

(b) Heat a few blue copper sulphate crystals in a dry test tube. Colourless droplets of water collect on the cooler upper walls, and the crystals turn white. Adding a few drops of this water back to the white powder turns it blue again. This proves the crystals contained water of crystallisation.

(c) $CuSO_4\cdot 5H_2O \xrightarrow{\Delta} CuSO_4 + 5H_2O$CuSO_4· 5H_2O → (Δ) CuSO_4 + 5H_2O

(blue hydrated salt gives white anhydrous salt).

(a) Water is a good solvent because it is a highly polar molecule; its partially positive hydrogen and partially negative oxygen ends attract and pull apart the ions or polar molecules of many substances, so a large number of ionic and polar solids dissolve in it.

(b) As the saturated solution cools from $60^{\circ}C$60^C to $20^{\circ}C$20^C, the solubility falls, so the excess solute that can no longer stay dissolved separates out as crystals of potassium nitrate at the bottom of the vessel.

(c) For $100\ \text{g}$100 g water, mass crystallising $= 110 - 32 = 78\ \text{g}$= 110 - 32 = 78 g.

For $200\ \text{g}$200 g water, mass crystallising $= \dfrac{78}{100}\times 200 = 156\ \text{g}$= 78/100× 200 = 156 g.

(i) Solid Y ($NaOH$NaOH) absorbs moisture and dissolves to a paste, so it is deliquescent (shows deliquescence).

(ii) Solid Z (washing soda) loses water and becomes a dry powder, so it is efflorescent (shows efflorescence).

(iii) $Na_2CO_3\cdot 10H_2O \rightarrow Na_2CO_3\cdot H_2O + 9H_2O$Na_2CO_3· 10H_2O → Na_2CO_3· H_2O + 9H_2O; the crystals lose water of crystallisation to the air and crumble to a white powder.

(iv) Sodium chloride is neither deliquescent nor efflorescent under normal conditions; it does not absorb enough moisture from ordinary air to dissolve, so it stays unchanged.