Why is our water not “pure” and what makes it hard or soft?

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Presentation transcript:

Why is our water not “pure” and what makes it hard or soft? Hard and soft water Why is our water not “pure” and what makes it hard or soft? Starter: What has happened to this kettle?

Glossary Hard water – water in which it is difficult to form a lather with soap. It contains calcium and / or magnesium ions. Soft water – water containing no dissolved calcium and/or magnesium salts so it easily forms a lather with soap.

Clean water Which one of these waters is drinkable and why?

How is this done? (treating/purifying water) Must be done so we do not drink too many ions or microbes. Add chlorine to kill microbes Add fluorine to help dental health Filter with carbon, silver and ion exchange resins. Want pure water? Distil it.

Label diagram

Types of water Hard water Soft water Soap forms a lather readily. Reacts with soap to form a scum so you need more soap to form a lather. Contains dissolved compounds of calcium and magnesium from rocks. These compounds have dissolved as water meets rocks. There are 2 types of hard water; permanent and temporary. Soap forms a lather readily. Not as “good” for you as NO calcium present which is good for bones and teeth. Cheaper than hard water because less soap is needed. No limescale to damage appliances. (unlike temporary hard water)

Temporary Hard Water This can be softened by boiling. Contains HCO3- ions that decompose when heated to produce carbonate ions which react with calcium and magnesium to produce precipitate (lime scale). Carbonate + Calcium  calcium carbonate + water + carbon dioxide Ca2+ (aq) + 2HCO3- (aq)  CaCO3 (s) +H2O (l) + CO2 (g) Carbonate + magnesium  ??

Softening water Remove the calcium and magnesium from the water. Method 1: Add sodium carbonate which reacts to produce magnesium carbonate and calcium carbonate (precipitate can be removed) Ca 2+ (aq) + CO32- (aq)  CaCO3 (s) Method 2: Use commercial water softeners e.g. ion exchange columns with hydrogen or sodium ions which are swapped with the Mg+ or Ca+ ions.

Purifying water

Tasks 1) copy the diagram of water treatment from page 213 2) complete the past paper questions.