Hardness of Water - Temporary and Permanent Hardness in Water
It is often seen that no lather forms while we wash our hands with soap when the water is hard. So, what makes the water hard?
Hard water owes its unpleasantness to the presence of dissolved calcium (Ca²⁺) and magnesium (Mg²⁺) ions. These ions, particularly their bicarbonates, chlorides, and sulfates, wreak havoc on our daily tasks.
But why are these ions unwanted guests? Well, they react with soap, forming slimy curds instead of the desired lather. This reduces cleaning efficiency, wastes soap, and leaves surfaces feeling rough and unpleasant.
Types of Hardness
Temporary Hardness: Caused by bicarbonates of calcium and magnesium. This hardness can be easily removed by boiling, as the bicarbonates decompose to insoluble carbonates that can be filtered out.
Permanent Hardness: A tougher nut to crack, arising from chlorides and sulfates of calcium and magnesium. Boiling won't budge these ionic bonds, necessitating chemical treatments like lime soda or ion exchange to soften the water.
Measuring Hardness
To assess the severity of your hard water battle, you need to understand its degree of hardness. This is expressed in parts per million (ppm) of calcium carbonate (CaCO₃) equivalents. Here's a helpful scale:
Soft water: Less than 75 ppm
Moderately hard water: 75–150 ppm
Hard water: 150–300 ppm
Very hard water: Above 300 ppm
Knowing the degree of hardness will guide you in choosing the appropriate softening method.
The Compounds that may Form During the Reaction are:
Ca(HCO3)2,
Mg(HCO3)2,
CaCl2,
MgC2l,
CaSO4, or
MgSO4
These appear as a precipitate on the surface of the water, which is scum.
Temporary Hardness of Water
When Ca2+ and Mg2+ ions are present in the form of Ca(HCO3)2, Mg(HCO3)2 in water, such type of hardness is temporary. This hardness can be removed by boiling the water. When we boil the water, the reaction takes place in the following manner:
Ca(HCO3)2 → CaCO3 + H2O + CO2
Calcium Bicarbonate Calcium Carbonate Water Carbon Dioxide
And,
Mg(HCO3)2 → Mg(OH)2 + CO2
Calcium Bicarbonate → Magnesium Hydroxide Carbon Dioxide
CO2 escapes the surroundings. The resulting salts CaCO3 and Mg(OH)2 do not dissolve in water and form precipitates.
Removal of the Temporary Hardness of Water
Let us discuss the methods of removing the temporary hardness of water:
Filtration
We remove the precipitates by filtration. This is how we can get rid of Ca2+ and Mg2+ ions.
Clark Method
In this method, we add some amount of lime (Slaked lime) to the water. The reaction between Ca(HCO3)2 and Ca(OH)2 takes place in this way:
Ca(HCO3)2 + Ca(OH)2 → CaCO3 + H2O
Calcium Bicarbonate Slaked lime Calcium Carbonate Water
The lime reacts with Mg(HCO3)2:
Mg(HCO3)3 + Ca(OH)2 → CaCO3 + Mg(OH)2 + H2O
We already know that CaCO3 and Mg(OH)2 are precipitates, which we can remove by filtering. This is how we can get rid of these precipitates and remove the temporary hardness of the water.
Now, let us study the permanent hardness of the water.
Permanent Hardness of Water
You might have seen a hard white coating on electric kettles, showers, or any bath asset or utensils in which we boil the water. This white coating is due to the permanent hardness of the water.
So, What is the Permanent Hardness of the Water?
The Permanent Hardness Occurs because of the Following Compounds (or Salts):
CaCl2,
MgCl2,
CaSO4, and
MgSO4.
Thus, the white coating we notice on the utensils is the deposit of these salts. This hardness cannot be removed by boiling.
So, How to Remove the Permanent Hardness of Water?
We use certain chemical methods to remove the permanent hardness of water, which are:
1. Treating the Water with Washing Soda
In this method, we add washing soda, i.e., Na2CO3 to the hard water. It combines with chloride salts of calcium and magnesium present in the water to form compounds. The reaction is:
CaCl2 + Na2CO3 → CaCO3 + 2NaCl
Calcium Chloride Sodium Carbonate Calcium Carbonate Sodium Chloride
It reacts with MgCl2 to form MgCO3 and 2NaCl. It also reacts with sulfate salts of calcium and magnesium to form carbonates and Sodium sulfate.
The reaction takes place in this way:
CaSO4 + Na2CO3 → CaCO3 + Na2SO4 , and
MgSO4 + Na2CO3 → MgCO3 + Na2SO4
Now, the precipitates of CaCO3 and MgCO3 are removed by filtration, which frees the water from Ca2+ and Mg2+ ions. After these ions are removed, the water becomes soft (just like rainwater).
2. Calgon’s Method
In this method, when Calgon, i.e Sodium Hexametaphosphate (Na6P6O18) is added to the water, each of its molecules ionizes to give two Na+ ions and one complex anion.
(Image will be Uploaded Soon)
This complex anion further releases Na+ ions and captures all the Ca2+ or Mg2+ ions.
These ions become a part of the complex anion. In this manner, water is freed from Ca2+ or Mg2+ ions. Now, it contains only Na+ ions only. Therefore, it no longer remains hard, i.e., becomes soft.
3. Gan’s Permutit Method
Gan's Permutit Method uses a substance called permutit or zeolite, which is a type of sodium aluminum ortho silicate. This substance helps get rid of the permanent hardness in water. The process involves a reaction where permutit reacts with calcium ions in water, resulting in the removal of those calcium ions and making the water less hard.
The reaction takes place in this way:
$Na_2 Al_2 Si_2 O8.KH_2O + Ca^{++} \to 2Na^{++} Ca Al_2 Si_2 O8.xH_2O$
4. Ion-Exchange Method
In this method, we add a Permutit called zeolite (Sodium Aluminum Silicate) or (AlNa12SiO5) to the water, which is insoluble in water. On adding this to water, the ion-exchange process starts between it and the Ca2+ or Mg2+ ions.
In this way, water becomes free from Ca2+ or Mg2+ ions, and no longer remains hard.
(Image will be Uploaded Soon)
5. Synthetic Resins Method
In this method, we use synthetic ion-exchange resins (RNa+), which is insoluble in water. When it is added to water, an exchange between RNa+ and Ca2+ or Mg2+ ions occurs. This process frees water from Ca2+ or Mg2+ ions. So, water no longer remains hard, and it becomes soft.
Applications of Hard Water
While hard water may be a domestic nuisance, it has its uses:
Agriculture: Calcium and magnesium are essential plant nutrients, making hard water beneficial for irrigation.
Construction: The setting properties of calcium ions make hard water useful for mortar and concrete production.
Medicine: Hard water can provide important dietary minerals like calcium and magnesium.
Effects of Hard Water in Daily Life
Some of the effects of hard water are as follows:
Hard water has an unpleasant taste.
Hard water (due to high quantities of calcium and magnesium) dries out your skin and hair.
Clothes and fabrics that are washed in hard water can look dull as their colour fades faster, and they begin to feel rough to touch. It can also shorten their life.
It leaves a chalky, white residue on dishes, and causes spots to appear on them.
It causes stains on sinks, bath tubs, bathroom floors, glassware, utensils, porcelain materials and other material in the house that comes under frequent contact with the water.
It leads to scale build-up and soap scum on plumbing fixtures like taps and shower heads, bath tubs etc, leaving stains.
Steel pipes don’t handle hard water very well. They can get easily clogged or damaged due to the buildup of minerals, impacting the water flow. This can lead to corrosion, low water pressure, and drainage problems.
Heaters, coffee pots/machines, dishwashers and other appliances may require frequent repairs due to build up of minerals.
Conclusion
Understanding the hardness of water is crucial for daily life. Hard water, with high mineral content, can create issues like soap scum and scaling. Learning about it is essential for JEE Main as it connects chemistry concepts to real-world problems. Solving related problems sharpens analytical skills. Knowing how to soften water becomes practical knowledge, as it aids in improving water quality for domestic use. Overall, the topic bridges theoretical learning with everyday applications, showcasing the relevance of scientific knowledge in addressing common challenges. So, grasping the concept of water hardness not only aids exam preparation but also enriches our ability to tackle real-life scenarios.
FAQs on Hardness of Water
1. What are the Units of Water Hardness?
We measure water hardness in the following units:
Grains per gallon
Milligrams of Ca per liter
Parts per million (ppm)
2. Why doesn’t the Hard Water Get along with Soap?
When hard water is treated with soap solution, the Ca2+ or Mg2+ ions present in the hard water react with the anions of fatty acids present in soap and form a curdy white precipitate, i.e., scum. So, hard water doesn’t get along with soap.
The reaction is:
C17H35COO- Na+ + CaCl2 → (CHCOO)2Ca ↓+ 2 NaCl
(soap) (Scum)
C17H35COO- Na+ + MgSO4 → (CHCOO)2Mg↓+ Na2SO4
In making the precipitate of these ions, a lot of soap is wasted. That’s why it is not suitable for washing.
3. Which Performs better in Hard Water, Soap, or Detergent?
Detergents can lather well in the hard water. Detergent is the sodium salt of a long chain benzene sulphonic acid (or the sodium salt of a long chain of alkyl hydrogen sulfate), which repels the Ca2+ or Mg2+ ions.
When added to water, they don’t produce insoluble precipitates, and therefore dissolve easily in the hard water. That’s why they have a stronger cleansing action than soaps.
4. What are the Advantages and Disadvantages of Soft Water?
Advantages
Soft water allows soap and detergents to work effectively.
It has a pleasant taste.
It is more comfortable on most people’s skin and hair because it’s stripped of the minerals that can make the skin and hair dry.
It does not cause any scaling on electrical equipment, like heaters, washing machines, dishwashers and others, prolonging their life.
It does not leave stains and soap scum on your sinks, shower heads, taps, glassware, dishes etc.
Disadvantages
Most experts consider soft water to be harmful if consumed on a regular basis. As it is stripped of the minerals, and replaced with sodium ions (salt). So drinking soft water regularly increases a person's sodium levels, which can lead to multiple health problems, including blood pressure.
If the softened water comes in contact with the environment it can cause environmental harm. For instance, if it flows into plants and trees, it will increase the acidity levels of the soil, making the plants less productive.
Drinking soft water could cause an imbalance in the human body, with reduced content of essential minerals and increased levels of sodium in diet.
The soft water contains a lot of sodium ions. Therefore, regular drinking of soft water may lead to serious health issues like an increase in blood pressure.
5. Why is groundwater hard?
As water moves through soil, and rocks like limestone, chalk and dolomite, small amounts of naturally-occurring minerals get dissolved in it. Water is a great solvent for calcium and magnesium. And if these are present in the rocks and soil from where the water is being drawn for supply, the water will be hard.
Industries depending on this water will have to take some measures to soften it, as hard water can damage equipment in a shorter amount of time.
Hardness in groundwater can also be as a result of mining and chemical industries in the area, or from excessive use of lime in agricultural areas.
Many households may rely on wells and other groundwater sources for domestic use.
The acceptable range of hardness in drinking water is 80 -100 mg/L. For domestic purposes, water with over 200 mg/L hardness is regarded as poor, and hardness higher than 500 mg/L is considered unacceptable.
Hard water is unpleasant to taste thanks to the high concentration of calcium and other ions. It can also shorten the life of fabrics.
6. What are the sources of hard and soft water?
Hard water
Groundwater is the biggest source of hard water. Groundwater passes through layers of mineral-rich rock (especially limestone, chalk and dolomite) and soils, absorbing the minerals like calcium and magnesium that cause the hardness.
Well water also becomes hard because of its prolonged exposure to the earth, and contact with minerals.
Spring water is also rich in metals and hence is hard.
Medium hard
A lot of the water from the public water supply system is medium hard as at least some, if not all, of the impurities are removed from it, through some amount of purification.
Soft water
Rain water amounts to the most of the soft water we have. Surface water like large lakes and reservoirs do not come in contact with mineral content as their water supply comes mainly from rain water.
Natural sources thus are rainwater, lakes, reservoirs, drainage basins of rivers and melted snow.
Bottled water is naturally soft, thanks to low levels of calcium and magnesium. Purified water can be said, is a mix of spring and distilled water. Impurities like magnesium and calcium are removed from it, and replaced with salt.
Very soft water
Distilled water is very soft as the minerals and impurities are removed to make it.
Here are the levels of hardness based on the water type (dH stands for general hardness):
0–4°dH: Very soft
4–8°dH: Soft
8–18°dH: Medium hard
18–30°dH: Hard
Over 30°dH: Very hard
