Water & Solutions Cheatsheet

Cheatsheet Content

### Water as a Universal Solvent - Water's polarity ($H_2O$) allows it to dissolve many substances. - **Solutions:** Homogeneous mixtures, often of a solid (solute) dissolved in a liquid (solvent), like water. - **Saturated Solution:** A solution containing the maximum amount of solute that can be dissolved at a given temperature. Any additional solute will not dissolve. #### Effect of Temperature on Solubility - **Generally:** Solubility of solids in water increases with temperature. - **Exceptions:** - **Calcium Sulfate ($CaSO_4$):** Solubility *decreases* slightly as temperature increases. - **Potassium Nitrate ($KNO_3$):** Solubility *increases significantly* with temperature. - **Sodium Chloride ($NaCl$):** Solubility *changes little* with temperature. ### Hydrated and Anhydrous Substances #### Hydrated Substances - **Water of Crystallisation:** Water molecules chemically bonded within a crystal structure. - **Examples:** - Copper(II) sulfate pentahydrate ($CuSO_4 \cdot 5H_2O$) - blue crystals - Washing soda ($Na_2CO_3 \cdot 10H_2O$) #### Anhydrous Substances - Substances that do not contain water of crystallisation. - **Examples:** - Anhydrous copper(II) sulfate ($CuSO_4$) - white powder - Anhydrous calcium chloride ($CaCl_2$) #### Properties Related to Water Content - **Efflorescence:** The loss of water of crystallisation from a hydrated salt when exposed to air, forming an anhydrous or lower hydrate powder. - **Example:** $Na_2CO_3 \cdot 10H_2O_{(s)} \rightarrow Na_2CO_3 \cdot H_2O_{(s)} + 9H_2O_{(g)}$ - **Deliquescence:** The process where a substance absorbs so much moisture from the air that it dissolves in the absorbed water to form a solution. - **Example:** Calcium chloride ($CaCl_2$), Sodium hydroxide ($NaOH$) - **Hygroscopy:** The ability of a substance to attract and hold water molecules from the surrounding environment through absorption or adsorption. The substance remains a solid. - **Example:** Concentrated sulfuric acid ($H_2SO_4$), Silica gel ($SiO_2$) ### Drying and Dehydrating Agents - **Drying Agent:** A substance that absorbs moisture from other substances without reacting chemically. Used to remove water. - **Examples:** Anhydrous calcium chloride ($CaCl_2$), Silica gel, Phosphorus pentoxide ($P_4O_{10}$) - **Dehydrating Agent:** A substance that removes hydrogen and oxygen atoms in the form of water from a compound, often by a chemical reaction. - **Examples:** Concentrated sulfuric acid ($H_2SO_4$) (e.g., in charring sugar), Anhydrous alumina ($Al_2O_3$) ### Soft Water and Hard Water #### Meaning (in terms of action of soap) - **Soft Water:** Water that readily forms lather with soap. It contains very few dissolved mineral salts. - **Hard Water:** Water that does not readily form lather with soap. It contains dissolved mineral salts (primarily calcium and magnesium ions) which react with soap to form scum. #### Types and Causes of Hardness - **Temporary Hardness:** Caused by dissolved hydrogen carbonates of calcium and magnesium ($Ca(HCO_3)_2$, $Mg(HCO_3)_2$). - **Removal:** - **Boiling:** Decomposes hydrogen carbonates, precipitating calcium carbonate. $$Ca(HCO_3)_{2(aq)} \xrightarrow{\Delta} CaCO_{3(s)} + H_2O_{(l)} + CO_{2(g)}$$ - **Addition of Washing Soda ($Na_2CO_3$):** Precipitates calcium and magnesium carbonates. $$Ca(HCO_3)_{2(aq)} + Na_2CO_{3(aq)} \rightarrow CaCO_{3(s)} + 2NaHCO_{3(aq)}$$ - **Permanent Hardness:** Caused by dissolved sulfates and chlorides of calcium and magnesium ($CaSO_4$, $MgSO_4$, $CaCl_2$, $MgCl_2$). - **Removal:** - **Addition of Washing Soda ($Na_2CO_3$):** Precipitates calcium and magnesium carbonates. $$CaSO_{4(aq)} + Na_2CO_{3(aq)} \rightarrow CaCO_{3(s)} + Na_2SO_{4(aq)}$$ - **Ion Exchange (e.g., using Zeolites):** Exchanges hardening ions for non-hardening ions (e.g., $Na^+$). #### Advantages and Disadvantages | Feature | Soft Water | Hard Water | | :------------- | :----------------------------------------- | :---------------------------------------------- | | **Advantages** | - Forms lather easily with soap (less soap used) - No scale build-up in pipes/boilers | - Pleasant taste due to minerals - May provide essential minerals (Ca, Mg) in diet | | **Disadvantages**| - May taste bland - Lacks beneficial minerals | - Forms scum with soap (more soap needed) - Causes scale (limescale) in pipes, kettles, boilers, reducing efficiency and lifespan - Can cause skin/hair issues |