Water — Structure, hard and soft water
Hydrogen
4
JEE Qs
8%
Hard
75
min
Master the chemical reactions and their underlying principles for each method of hard water softening, as questions often test these specifics.
🧮 Key Formulas
✅ Key Points for JEE
- 1Water has a V-shaped structure with sp³ hybridized oxygen and two lone pairs, leading to strong intermolecular hydrogen bonding and its anomalous physical properties (e.g., maximum density at 4°C).
- 2Temporary hardness is caused by dissolved bicarbonates of calcium and magnesium (Ca(HCO₃)₂, Mg(HCO₃)₂). It can be removed by boiling or by adding limewater (Clark's method).
- 3Permanent hardness is caused by dissolved chlorides and sulfates of calcium and magnesium (CaCl₂, MgCl₂, CaSO₄, MgSO₄). It cannot be removed by boiling.
- 4Permanent hardness can be removed by chemical methods like treatment with washing soda, Calgon method (using sodium hexametaphosphate), or by physical methods like ion-exchange resins (zeolite method or synthetic resins) and distillation.
- 5Understanding the specific chemical reactions for each softening method is crucial, especially distinguishing between precipitation and complexation/ion-exchange mechanisms.
⚠️ Common Mistakes
- ✕Confusing the causes of temporary vs. permanent hardness and selecting the wrong removal method for a given type of hardness.
- ✕Incorrectly identifying the products of hardness removal reactions, particularly the precipitate formed (e.g., CaCO₃, Mg(OH)₂).
- ✕Misunderstanding the role of Calgon (sodium hexametaphosphate) as a complexing agent that sequesters metal ions, rather than precipitating them.
- ✕Not differentiating between the regeneration process for zeolite/ion-exchange resins and the initial water softening reaction.
📝 Practice Questions
See allQ34.Given below are two statements, one is labelled as Assertion A and the other is labelled as Reason R. Assertion A: Loss of electron from hydrogen atom results in nucleus of ~1 . 5 × 10 – 3 pm size. Reason R: Proton H+ always exists in combined form. In the light of the above statements, choose the most appropriate answer from the options given below: (1) Both A and R are correct and R is the correct (2) A is correct but R is not correct explanation of A (3) A is not correct but R is correct (4) Both A and R are correct but R is NOT the correct explanation of A
Q41.Which of the following can reduce decomposition of H2O2 on exposure to light (1) Urea (2) Alkali (3) Glass containers (4) Dust
Q31.Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R Assertion A: Hydrogen is an environment friendly fuel. Reason R: Atomic number of hydrogen is 1 and it is a very light element. In the light of the above statements, choose the correct answer from the options given below (1) A is true but R is false (2) Both A and R are true but R is NOT the correct explanation of A (3) A is false but R is true (4) Both A and R are true and R is the correct explanation of A
Q39.Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A): Gypsum is used for making fireproof wall boards. Reason (R): Gypsum is unstable at high temperatures. In the light of the above statements, choose the correct answer from the options given below: (1) Both (A) and (R) are correct but (R) is not the (2) (A) is correct but (R) is not correct. correct explanation of (A). (3) (A) is not correct but (R) is correct. (4) Both (A) and (R) are correct and (R) is the correct explanation of (A).
Q47.The water gas on reacting with cobalt as a catalyst forms (1) Methanal (2) Methanoic acid (3) Ethanol (4) Methanol
Q36.Given below are two reactions, involved in the commercial production of dihydrogen H2. The two reactions are carried out at temperature “T1” and “T2”, respectively T1 Cs + H2Og ⟶COg + H2g T2 COg + H2Og CatalystCO2g + H2g The temperatures T1 and T2 are correctly related as (1) T1 = T2 (2) T1 < T2 (3) T1 = 100 K, T2 = 1270 K (4) T1 > T2
NCERT Chapters
- Class 11 Chemistry Chapter 9: Hydrogen