Unit 01

Question Bank

Q-1Explain Chemical Weathering and Physical Weathering.

(a) Chemical Weathering: Chemical weathering occurs by the chemical between minerals of rocks and external agents like air or water. Oxygen oxidizes minerals to alteration products whereas water can convert minerals to clays or dissolve minerals completely.

(b) Physical Weathering: Physical weathering occurs when rocks are broken apart by mechanical processes such as rock fracturing, freezing and thawing, or breakage during transport by rivers or glaciers.

Q-2What are the factors that control the rate of weathering?

Factors Which Control the Rates of Weathering

1. The mineralogy and structure of a rock affects weathering.

2. Different minerals weather at different rates.

3. A rock’s structure also affects its susceptibility to weathering.

Q-3Explain different types of water and the range of hardness level.

Q-4What are hard water?

Hard water: is water that contains an required quantity of dissolved minerals (like calcium and magnesium) As rainwater falls, it is naturally soft. However, as water flows through the ground and into waterways, it picks up minerals like chalk, lime and mostly calcium and magnesium and becomes hard water. Since hard water contains essential minerals, it is essentially used as drinking water. Not only because of the health benefits, but also the flavour.  Water that does not produce lather with soap readily is called hard water. Water hardness is usually measured as calcium hardness in milligrams per litre (mg/l) OR parts per million (ppm) OR in grains per gallon (GPG).             

Q-5Mention the disadvantage of using hard water.

Disadvantages of using hard water are-

1. Hard water is unfit for washing as it is difficult to form lather with soap.

2. Scum may form in a reaction with soap, wasting the soap.

3. Furring of tea kettles will take place due to the formation of carbonates of calcium and magnesium.

4. Hard blocks hot water pipes. This is due to the formation of layers of carbonates of calcium and magnesium.

Q-6Discuss about scale and sludge formation.

The water evaporates continuously and the dissolved salts concentration increases progressively. When their concentrations reach at saturation point, they are thrown out of water in precipitate form which get stick in inner walls of boiler. If the precipitation takes place in the form of loose or slimy precipitate it is called sludge. While if the precipitated matter forms a hard adhering coating on inner walls of boiler, then it is called as scale. Eg- MgCO3, MgCl2, MgSO4 etc.

Q-7Discuss about priming and foaming.

The duration at which boiler is producing steam rapidly, some particles of the condensed liquid are carried along with the steam. The process of wet steam formation is called priming. Priming is mainly caused by the presence of large amounts of dissolved solids, high steam velocities, sudden boiling etc. Whereas the continuous production of foam or bubbles in boilers which do not break easily are called as the foaming. This is caused due to the presence of substance like oils in water that reduce the surface tension in water.

Q-8What is caustic embrittlement?

The use of high alkaline water in the boiler cause rust in the boiler which is called as Caustic Embrittelment. The presence of sodium carbonate plays a major role during the softening process.

Na2CO3 + H2O → NaOH + CO2
the caustic embrittelment is caused by using sodium phosphate as a softening agent instead of sodium carbonate.

Q-9Explain boiler corrosion.

The decay of the boiler material by attack of chemicals or electro-chemicals at its environment is called as boiler corrosion.

Reasons of boiler corrosion:

(i)                Dissolved Oxygen

(ii)              Dissolved Carbon di oxide

(iii)           Acid from dissolved salts

Sludge, scale, priming and foaming, caustic embrittlement, boiler corrosion are collectively known as boiler troubles.

Q-10Describe Lime Soda softening process.

Surface water hardly exceed hardness level above 200 mg/1 and softening is not at all required in most of the cases, unless the water is being polluted by some effluent sources. In case of groundwater, hardness level  of more than 1000 mg/1 are quite common. Since, soft water is corrosive, therefore public water supply are usually not softened below 30 to 50 mg/1. The most accepted and commonly used water softening methods are cat ion exchange and precipitation method. In order to obtain maximum profit, the factors to be considered are a good choice of a softening process, quality of the raw water, the cost of softening chemicals and the cost of disposing of waste streams.

Precipitation methods

The principle that follows the precipitation method is to bind calcium cations Ca and magnesium cations Mg , with ions of CO3 and OH . The precipitate CaC03 and Mg (OH)2 formed are removed from the water. Slake lime Ca(OH)2, Quick lime CaO , soda ash NaC03 and sodium hydroxide (caustic soda) NaOH, are reagents that are commonly used in water softening. Depending upon the quality of initial water, the following main precipitation methods are determined. a) Lime softening b) Lime - Soda softening c) Sodium Hydroxide softening Lime affects the carbonate hardness (alkalinity) and therefore can be used in order to decrease the carbonate hardness present in the initial water. This method however does not result in deep softening. Magnesium is removed from water if there is excess of OH” present. Water dissolved carbon dioxide is removed, total solids in the treated water diminishes and the total hardness in the lime treated water also reduces. But the pH increases to 10 or beyond. When lime is added to the hard water following reactions occurs, In the above reactions, 

Lime Addition:-

Hardness Lime Precipitate

CO2 + Ca(OH)2 -- > CaCO3 + H2O Ca(HCO3)2 + Ca(OH)2 -- > 2CaCO3 + 2H2O Mg(HCO3)2 + Ca(OH)2 -- > CaCO3 + MgCO3 + 2H2O MgCO3 + Ca(OH)2 -- > CaCO3 + Mg(OH)2 CO2  the insoluble products do not contribute to the hardness, but It reacts with the lime, and thereby uses up some lime before the lime can start removing the hardness in water. Lime - Soda softening method is commonly practiced in most of the Public water supply. (Belan1984) The method is universal as water of almost any composition is treated with lime and soda. In this treatment, two reagents are used namely lime and soda ash. Lime as  earlier discussed , decreases the carbonate hardness, (Mg2+) and removes C02 from the water.

Soda therefore reduces the non - carbonate hardness, mainly due to Ca2+, that shows after reaction with lime and the reaction occurs after the addition of soda ash is as follows.

Lime and Soda ash Addition:- Lime Precipitate MgSO4 + Ca(OH)2 -- > Mg(OH)2 + CaSO4 Soda ash Precipitate CaSO4 + Na2CO3 -- > CaCO3 + Na2SO4.

Q-11Explain Zeolite process.

Zeolite process is a process of softening hard water through ion exchange technique using a chemical compound zeolite. It possesses a chemical compound that has hydrated sodium alumina silicate. Thus, the name of the process is called as zeolite process. Zeolite compound can exchange its sodium cations reversibly with calcium and magnesium ions in the water softening process.

There are two types of zeolite used in this process they include natural and synthetic zeolite. The natural form is found to be porous and synthetic form is a non-porous zeolite. However synthetic form possesses a high exchange capacity per unit weight than the natural form.

Figure 01: Cylinder Containing the Zeolite Bed

Process

In the water softening process, hard water is passed through a bed of zeolite (inside a cylinder) at a specified rate. Then the cations that cause the water-hardening will remain on the zeolite bed because these cations exchange with the sodium cations of zeolite. Therefore, the water coming out of this cylinder contains sodium cations rather than calcium and magnesium cations.

After some time, the zeolite bed gets exhausted, the water flow is stopped and treat the bed  is treated with concentrated brine solution (10%) in order to regenerate the zeolite.   The bed is treated with a brine solution, as  it washes away all the calcium and magnesium ions, by exchanging them with sodium ions in a brine solution. Hence, this treatment regenerates the zeolite.

Q-12Explain specification of potable water and drinking water.

“Potable water” simply means water that is safe to drink, and it is becoming scarcer in the world. Increasing use is stressing freshwater resources worldwide, and a seemingly endless list of contaminants can turn once potable water into a health hazard or simply make it unacceptable aesthetically.

Of the more than 2 billion people who lack potable water at home, 844 million don’t have even basic drinking water service, including 263 million who must travel 30 minutes per trip to collect water. Unsafe drinking water is a major cause of diarrheal disease, which kills about 800,000 children under the age of 5 a year, usually in developing countries, but 90 countries are expected to fail to reach the goal of universal coverage by 2030. Physical requirement of potable water are-

Q-13Describe permanent hardness and its unit.

It occurs due to the presence of chlorides and sulphates of calcium, magnesium, iron, and other heavy metals. Unlike temporary hardness, permanent hardness is not destroyed on boiling.

The degree of hardness of drinking water has been classified in terms of the equivalent CaCO3 concentration as follows: 

In a hard water sample, the total hardness can be determined by titrating the Ca2+ and Mg2+ present in an aliquot of the sample with Na2 EDTA solution, using NH4Cl-NH4OH buffer solution of pH 10 and Eriochrome Black-T as the metal indicator.

Na2H2Y (Disodium EDTA solution) → 2Na+ + H2Y-

Mg2+ + HD2- (blue) → MgD (wine red) + H+

D (metal-indicator complex, wine red colour) + H2Y- →Y- (metal EDTA complex colourless) + HD- (blue colour) + H+

Ethylenediamine tetra-acetic acid (EDTA) and its sodium salts form a chelated soluble complex when added to a solution of certain metal cations. If a small amount of a dye such as Eriochrome black T is added to an aqueous solution containing calcium and magnesium ions at a pH of 10 ± 0.1, the solution will become wine red. If EDTA is then added as a titrant, the calcium and magnesium will be complexed. After sufficient EDTA has been added to complex all the magnesium and calcium, the solution will turn from wine red to blue. This is the end point of the titration.

Units of Hardness:

1. Parts per million (ppm): Is the parts of calcium carbonate equivalent hardness per 106 parts of water, i.e, 1 ppm = 1 part of CaCO3 eq hardness in 106 parts of water.

2. Milligram per litre (mg/L): Is the number of milligrams of CaCO3 equivalent hardness present per litre of water. Thus:

1 mg/L = 1 mg of CaCO3 eq hardness per L of water.

3. Clarke’s degree (oCl): Is number of grains (1/7000 lb) of CaCO3 equivalent hardness per gallon (10 lb) of water. Or it is parts of CaCO3equivalent hardness per 70,000 parts of water. Thus,

1oClarke = 1 grain of CaCO3 eq hardness per gallon of water.

4. Degree French (oFr): Is the parts of CaCO3 equivalent hardness per 105 parts of water. Thus,

1o Fr = 1 part of CaCO3 hardness eq per 105 parts of water.