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, $CuSO_4\cdot 5H_2O$CuSO₄· 5H₂O, is blue because of its water of crystallisation. On heating it loses this water and the anhydrous salt, $CuSO_4$CuSO₄, is white.

Answer: (c) Deliquescent.

A deliquescent substance, such as $NaOH$NaOH or $CaCl_2$CaCl₂, absorbs so much water vapour from air that it eventually dissolves to form a solution.

Answer: (b) dehydrating agent. A dehydrating agent removes chemically combined hydrogen and oxygen (as water) from a compound. A drying agent only removes moisture (free water) from a substance, e.g. quicklime drying ammonia or anhydrous $CaCl_2$CaCl₂ drying gases.

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(HCO₃)₂, $CaSO_4$CaSO₄, $MgCl_2$MgCl₂.

A supersaturated solution contains more dissolved solute than a saturated solution can hold at that temperature. It is unstable.

Preparation: make a saturated solution at a high temperature, then let it cool slowly without disturbing it. The extra solute stays dissolved; adding a single crystal (or shaking) makes the excess crystallise out at once.

Efflorescence: the property of certain hydrated salts of losing their water of crystallisation to the atmosphere and crumbling to powder. Example: $Na_2CO_3\cdot 10H_2O$Na₂CO₃· 10H₂O (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 $CaCl_2$CaCl₂).

Hygroscopy: the property of certain substances of absorbing moisture from air without dissolving in it and without changing state. Example: concentrated $H_2SO_4$H₂SO₄ (or $CaO$CaO).

(a) Temporary hardness is caused by the bicarbonates of calcium and magnesium, $Ca(HCO_3)_2$Ca(HCO₃)₂ and $Mg(HCO_3)_2$Mg(HCO₃)₂; it is removed by boiling. Permanent hardness is caused by the chlorides and sulphates of calcium and magnesium; it is not removed by boiling.

(b)(i) Boiling: $Ca(HCO_3)_2\xrightarrow{\Delta}CaCO_3\downarrow+H_2O+CO_2\uparrow$Ca(HCO₃)₂→ (Δ)CaCO₃↓+H₂O+CO₂↑

(ii) Washing soda: $CaSO_4+Na_2CO_3\rightarrow CaCO_3\downarrow+Na_2SO_4$CaSO₄+Na₂CO₃→ CaCO₃↓+Na₂SO₄ (also removes temporary hardness).

(i) Potassium nitrate: its curve rises steeply, while the sodium chloride curve is almost flat. (Calcium sulphate, about $0.2\text{ g}$0.2 g per $100\text{ g}$100 g water, is too small to show here; its solubility falls slightly as temperature rises.)

(ii) About $110\text{ g}$110 g per $100\text{ g}$100 g water at $60^\circ\text{C}$60° C, and about $32\text{ g}$32 g at $20^\circ\text{C}$20° C.

(iii) Mass that separates $=110-32=\boxed{78\text{ g}}$=110-32=78 g of potassium nitrate crystals, since only $32\text{ g}$32 g can stay dissolved at $20^\circ\text{C}$20° C.

All three are mixtures: the substances are not chemically combined.

(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: $CuSO_4\cdot 5H_2O$CuSO₄· 5H₂O, blue.

• Hydrated iron(II) sulphate: $FeSO_4\cdot 7H_2O$FeSO₄· 7H₂O, 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) $CuSO_4\cdot 5H_2O \xrightarrow{\Delta} CuSO_4 + 5H_2O$CuSO₄· 5H₂O → (Δ) CuSO₄ + 5H₂O

(blue hydrated salt gives white anhydrous salt).

(a) Water dissolves more substances than any other common liquid, because its polar molecules attract and pull apart the ions or molecules of many solids, liquids and gases.

(b) Crystals of potassium nitrate separate out. As the solution cools its solubility falls, so the extra solute can no longer stay dissolved.

(c) (i) Potassium nitrate: solubility increases sharply. (ii) Sodium chloride: solubility increases only very slightly. (iii) Calcium sulphate: solubility decreases slightly at higher temperatures.

(d) Advantage: hard water contains calcium and magnesium salts that are useful for bones and teeth and give drinking water a pleasant taste. Disadvantage: it wastes soap (forming scum) and deposits scale in kettles and boilers, wasting fuel.

(a)

(i) Alum: added to muddy water, it makes the fine suspended particles clump together so that they settle quickly (coagulation and sedimentation).

(ii) Filtration: the clear water is passed through beds of sand and gravel, which remove the remaining suspended particles.

(iii) Chlorination: a small, controlled amount of chlorine (or bleaching powder) is added to kill disease-causing germs.

(iv) Defluoridation: excess fluoride is removed from water in areas where it is naturally high, because too much fluoride damages teeth and bones (fluorosis).

(b) Any four of: diarrhoea, typhoid, cholera, dysentery, hepatitis A, polio. Water polluted with toxic chemicals can also cause cancer and organ damage.

(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) $Na_2CO_3\cdot 10H_2O \rightarrow Na_2CO_3\cdot H_2O + 9H_2O$Na₂CO₃· 10H₂O → Na₂CO₃· H₂O + 9H₂O; 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.