Chemistry of Soaps and Hard Water

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Questions and Answers

What reaction occurs when sodium stearate comes in contact with hard water?

Sodium stearate mutates into glycerol in hard water.

Sodium stearate reacts with calcium and magnesium salts to form soluble salts.

Sodium stearate reacts with calcium and magnesium salts producing insoluble calcium and magnesium stearate. (correct)

Sodium stearate reacts with sodium chloride to produce calcium stearate.

Which of the following statements about soap in hard water is true?

Soap can easily produce leathering in hard water.

Calcium and magnesium salts do not affect soap's ability to lather.

Soft water is more effective for soap lathering than hard water.

Soap forms insoluble precipitates in hard water. (correct)

What is the hardness classification of water with a hardness of 250 mg/L?

Hard water (correct)

Very hard water

Moderate hard water

Soft water

How is the hardness of water expressed?

In terms of calcium carbonate equivalents. Signup and view all the answers

What is the molecular weight used to express hardness as calcium carbonate equivalent?

100 Signup and view all the answers

When calculating hardness, what does the formula rely on?

Amount of hardness-causing salt present. Signup and view all the answers

What happens to calcium and magnesium salts when they dissolve in hard water?

They react with soap to produce insoluble precipitates. Signup and view all the answers

What is the ppm equivalent of hardness if 1 part of CaCO₃ is present in 10⁶ parts of water?

1 ppm Signup and view all the answers

What is the equivalent hardness of water expressed in mg/L for a sample with 1 ppm?

1 mg/L Signup and view all the answers

Which statement correctly describes permanent hardness?

It consists of chlorides and sulphates of various metals. Signup and view all the answers

What is the value of 121 ppm when converted to Degree Clark (°Cl)?

8.47 °Cl Signup and view all the answers

How is temporary hardness primarily caused?

Presence of bicarbonates. Signup and view all the answers

Given that total hardness is the sum of temporary and permanent hardness, if total hardness is 121 mg/L and permanent hardness is 101 mg/L, what is the temporary hardness?

20 mg/L Signup and view all the answers

What characterizes Degree French (°Fr)?

1 part of CaCO₃ per 10^5 parts of water. Signup and view all the answers

What is the relationship between mg/L and meq/L of hardness for CaCO₃?

1 mg/L = 0.02 meq/L Signup and view all the answers

What is the primary method used for the removal of temporary hardness?

Addition of slaked lime. Signup and view all the answers

What is the calculated temporary hardness of the water sample containing Mg(HCO₃)₂ and Ca(HCO₃)₂?

15 ppm Signup and view all the answers

How much FeSO₄ would result in 210.5 ppm of hardness?

0.31996 g/L Signup and view all the answers

What is the total permanent hardness of the water sample including MgCl₂ and CaSO₄?

20 ppm Signup and view all the answers

Which of the following is NOT a requirement for potable water?

High alkalinity Signup and view all the answers

What is the first step involved in the purification of water as outlined?

Sedimentation Signup and view all the answers

What is the role of coagulants in the water purification process?

To remove colloidal impurities Signup and view all the answers

If a sample contains 9.5 mg/L of MgCl₂, what is its contribution to permanent hardness?

10 mg/L Signup and view all the answers

Which parameter specifies that municipal water should not contain objectionable minerals?

Total dissolved solids less than 500 ppm Signup and view all the answers

What ions are primarily responsible for hardness in water?

Ca2+ and Mg2+ Signup and view all the answers

In what pH condition does EDTA form stable complexes with Ca²⁺ and Mg²⁺?

pH 9 or higher Signup and view all the answers

What is the color change observed in the EDTA titration at the endpoint?

Wine red to blue Signup and view all the answers

What is used as an indicator in the EDTA titration method for determining water hardness?

Eriochrome Black-T Signup and view all the answers

What is the main purpose of estimating the hardness of water?

To assess its suitability for drinking and bathing Signup and view all the answers

What happens to the EBT indicator when EDTA is added during the titration?

It changes color and becomes free in solution Signup and view all the answers

What are the compounds that contribute to the total hardness of water?

Bicarbonates, chlorides, and sulfates Signup and view all the answers

What is the role of the disodium salt of EDTA in the complexometric titration?

It is the soluble form used to form complexes Signup and view all the answers

What must be done to turbid water before it is admitted to a zeolite bed?

Remove suspended matter through coagulation and filtration. Signup and view all the answers

Which ion cannot be produced by the zeolite process?

Manganese ion (Mn+2) Signup and view all the answers

What happens to zeolite if acidic ions like HCl are present in the water?

It destroys the zeolite bed. Signup and view all the answers

What is an advantage of the zeolite process in terms of handling water hardness?

It removes hardness of 10 ppm completely. Signup and view all the answers

In comparison to the lime-soda method, how does the permutit method differ regarding space requirements?

It occupies less space than the lime-soda method. Signup and view all the answers

What is a disadvantage of the zeolite process regarding sodium content?

It increases sodium salt content compared to lime-soda process. Signup and view all the answers

What is a limitation of the zeolite process when dealing with high turbidity water?

High turbidity creates deposits blocking the zeolite bed. Signup and view all the answers

Which of the following is true about the operational expenses of the permutit method compared to the lime-soda method?

Operational expenses are higher for the permutit method. Signup and view all the answers

Study Notes

Soaps and Hard Water

Soaps are sodium or potassium salts of higher fatty acids like oleic acid (C₁₇H₃₃COONa) and stearic acid (C₁₇H₃₅COONa).
Hard water does not produce lather with soap.
Soft water reacts with soap to form leather, producing stearic acid (C₁₇H₃₅COOH) and sodium hydroxide (NaOH) from sodium stearate (C₁₇H₃₅COONa).

Soap with Hard Water

When soap comes in contact with hard water, sodium stearate reacts with dissolved calcium and magnesium salts, forming insoluble calcium stearate and magnesium stearate.
These insoluble precipitates prevent the formation of lather.
The chemical reactions for this process are:
- 2C₁₇H₃₅COONa + CaCl₂ -> (C₁₇H₃₅COO)₂Ca + 2NaCl
- 2C₁₇H₃₅COONa + MgSO₄ -> (C₁₇H₃₅COO)₂Mg + Na₂SO₄

Water Hardness

Water hardness is categorized based on the concentration of dissolved calcium and magnesium salts.
Hardness is measured in milligrams per liter (mg/L) or parts per million (ppm) of calcium carbonate equivalents.

Hardness (mg/L)	Name of Water
0-70	Soft water
70-150	Moderate hard water
150-300	Hard water
300 & above	Very hard water

Hardness Salts and Calculation

The total hardness of water is caused by dissolved calcium and magnesium salts, including Ca(HCO₃)₂, MgCl₂, Mg(HCO₃)₂, CaSO₄, MgSO₄, and Ca(NO₃)₂.
Hardness is expressed in terms of calcium carbonate (CaCO₃) equivalents.
The weight of different hardness salts is converted to the equivalent weight of calcium carbonate.

Name of Salt	Molecular Weight
Ca(HCO₃)₂	162
Mg(HCO₃)₂	146
CaCl₂	111
CaSO₄	95
MgSO₄	136
CaCO₃	100

Hardness of water is calculated using the formula:

Hardness of water = (Amount of hardness salt x 100) / Molecular weight of hardness salt

Units of Hardness

** ppm (parts per million)**
- Indicates the parts of CaCO₃ equivalent hardness per 10⁶ parts of water.
- 1 ppm = 1 mg/L
mg per litre (mg/L)
- Represents the number of milligrams of CaCO₃ equivalent hardness in 1 liter of water.
Degree Clark (°Cl)
- Indicates the number of grains (1/7000 lb) of CaCO₃ equivalent hardness per 70,000 parts of water
- 1 °Cl = 1 grain of CaCO₃ per gallon of water.
Degree French (°Fr)
- Represents parts of CaCO₃ equivalent hardness per 10⁵ parts of water
- 1 °Fr = 1 part of CaCO₃ per 10⁵ parts of water
Milliequivalent per litre (meq/L)
- Indicates the number of milliequivalents of hardness per liter of water.

Relation Between Hardness Units

1 ppm = 1 mg/L = 0.1 °Fr = 0.07 °Cl = 0.02 meq/L

Types of Hardness

Temporary Hardness
- Caused by dissolved bicarbonates of calcium and other heavy metals.
- Salts primarily responsible include Ca(HCO₃)₂ and Mg(HCO₃)₂.
- Removed by boiling, which decomposes bicarbonates into insoluble carbonates that can be filtered.
Permanent Hardness
- Caused by dissolved chlorides and sulphates of Ca, Mg, Fe, and other metals.
- Salts responsible for permanent hardness include CaCl₂, MgCl₂, CaSO₄, MgSO₄, FeSO₄, and Al₂(SO₄)₃.
- Not removable by boiling, but can be removed using chemical agents.
- Total hardness of water = Temporary Hardness + Permanent Hardness (T.H + P.H)

Determination of Hardness by Clark's Test

This method removes temporary hardness caused by Ca(HCO₃)₂ and Mg(HCO₃)₂.
Slaked lime (Ca(OH)₂) is added to the hard water, forming insoluble CaCO₃ that is then filtered.
- Ca(HCO₃)₂ + Ca(OH)₂ -> 2CaCO₃ + 2H₂O
- Mg(HCO₃)₂ + Ca(OH)₂ -> CaCO₃ + MgCO₃ + 2H₂O

Estimation of Hardness by EDTA Method (Complexometric Titration)

Hardness in water is due to the presence of dissolved salts of Ca and Mg.
The estimation of hardness is based on complexometric titration using ethylene diamine tetra acetic acid (EDTA).
EDTA is a complexing agent that forms stable complexes with Ca²⁺ and Mg²⁺ ions in alkaline conditions (pH ≥ 9).

Basic Principle of EDTA Method

EDTA forms a stable complex with Ca²⁺ and Mg²⁺ ions, leaving the free Eriochrome Black-T (EBT) indicator in solution, which is steel blue in color.
The titraion is performed using a standard EDTA solution and EBT as an indicator in a basic buffer solution (pH = 10).
The endpoint is reached when the solution changes from a wine red color to a blue color.

Municipal Water Supply

Requirements for Potable Water:
- Clear, odorless, and pleasant in taste.
- Cool temperature.
- Turbidity less than 10 ppm.
- Free from objectionable dissolved gases (e.g., hydrogen sulfide).
- Free from objectionable minerals (e.g., lead, arsenic, copper, and magnesium salts).
- Reasonable alkalinity and pH around 8.0.
- Relatively soft.
- Total dissolved solids less than 500 ppm.
- Free from disease-producing microorganisms.

Water Purification Processes

Sedimentation:
- Water is allowed to stand undisturbed in large tanks, allowing suspended particles to settle to the bottom due to gravity.
- The clear supernatant water is then drawn off.
Coagulation:
- Colloidal impurities are removed by adding coagulants.
- Coagulants form an insoluble, gelatinous substance that entraps fine particles, forming flocs that settle easily.
Zeolite Process (Permutit Process)
- A water softening process that uses zeolites to exchange calcium and magnesium ions for sodium ions.
- This process removes hardness but increases sodium content.
Lime-Soda Process
- Another water softening process that involves adding lime (Ca(OH)₂) and soda ash (Na₂CO₃) to precipitate hardness-causing ions.
- The process removes both temporary and permanent hardness and produces a softened water with a lower sodium content compared to Zeolite.

Comparison of Zeolite and Lime-Soda Processes

Permutit Method	Lime - Soda Method
Produces water with 10-15 ppm hardness.	Produces water with 15-50 ppm hardness.
Treated water has higher Na⁺ salts than original water.	Treated water has lower Na⁺ salts.
Higher plant and material cost.	Lower capital cost.
Lower operational expenses.	Higher operational expenses.
Cannot treat acidic water.	Can treat acidic water.
Plant occupies less space.	Plant occupies more space.
Requires clear water (free of suspended matter).	Works with turbid water.

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Description

This quiz explores the interactions between soaps and hard water, detailing the chemical reactions involved and the consequences of water hardness on soap efficacy. Understand how hard water prevents lather formation and the underlying chemistry of these processes.