Tag: chemistry

Questions Related to chemistry

Pollution of lakes and rivers affects aquatic life. Fish cannot live in water that has oxygen less than $4$ milligrams per liter. Other freshwater aquatic life (including anaerobic bacteria) also cannot survive without adequate oxygen. 'BOD' stands for:

  1. Biological oxygen demand

  2. Bacterial oxygen deficit

  3. Basic oxygen demand

  4. Building oxygen deposits


Correct Option: A
Explanation:
BOD stands for Biological Oxygen Demand is the amount of dissolved oxygen needed by aerobic biological organisms to break down organic material present in a given water sample at a certain temperature over a specific time period.

Clean water would have Biochemical oxygen demand (BOD) value of less than:

  1. 17 ppm

  2. 5 ppm

  3. 200,000 ppm

  4. 10 ppm


Correct Option: B
Explanation:
A water supply with a BOD level of 3-5 ppm is considered moderately clean. In water with a BOD level of 6-9 ppm, the water is considered somewhat polluted because there is usually organic matter present and bacteria are decomposing this waste.
Hence, clean water would have a BOD value of less than 5.

Hence, the correct option is $\text{B}$

Biological Oxygen Demand (BOD) can be defined as:

  1. the amount of oxygen required by bacteria to break down the organic matter of a sample of water

  2. the amount of chemicals required to break down the organic matter of a sample of water

  3. the amount of phosphate required to oxidise the organic matter of a sample of water

  4. the amount of organic matter present in the given sample of water


Correct Option: A
Explanation:

BOD refers to the dissolved oxygen that the microorganisms like bacteria need to oxidize, means to break down inorganic and organic matter in water. It is a measure of water quality. 

What does $BO{D} _{5}$ represent?

  1. Biological oxygen demand in five days

  2. Dissolved oxygem left after five days

  3. Dissolved oxygen consumed in five days

  4. Microorganisms killed by ozone in sewage treatment plants in five hours


Correct Option: A
Explanation:

$BO{ D } _{ 5 }$ stands for 5-day Biochemical Oxygen Demand. The BOD test measures, oxygen is used as microbes breathe as they eat the contaminants  in wastewater.

10 mL of water required 1.47 mg of $K _{2}Cr _{2}O _{7}$ (M. wt. = 294) for oxidation of dissolved organic matter in presence of acid. Then the C.O.D of water sample is:

  1. 2.44 ppm

  2. 24 ppm

  3. 32 ppm

  4. 1.6 ppm


Correct Option: B
Explanation:

$ Number \  of   \ milliequivalent $ = $\dfrac{weight \ in \ mg}{Equivalent\  weight}$


= $\dfrac{1.47}{294/6}$ = $0.03meq$

$\therefore$COD of sample is:

$\dfrac{0.03}{10}\times 1000$ =  $3meq/L$

In terms of Oxygen COD is :

$3 meq/L \times  (8 mg O _2/meq)$ = $24 mg/L= 24ppm$

Option B is correct.

Salt, sugar, and many other substances dissolve in water to form :

  1. homogenous mixture

  2. heterogenous mixture

  3. Ionic compound

  4. molecular compound


Correct Option: A
Explanation:

A mixture is a material system made up of two or more different substances which are mixed but are not combined chemically. A homogeneous mixture is a type of mixture in which the composition is uniform and every part of the solution has the same properties. Salt, sugar, and many other substances dissolve in water to form homogeneous mixtures.

Two main constituents of air are :

  1. oxygen and nitrogen

  2. carbon dioxide and nitrogen

  3. rhodium and nitrogen

  4. oxygen and calcium


Correct Option: A
Explanation:

Air is a homogeneous mixture of a number of gases. It's two main constituents are oxygen and nitrogen. Other gases are present in small quantities.

Which of the following is a homogeneous solution?

  1. Muddy water

  2. Bread

  3. Concrete

  4. A solution of sugar in water


Correct Option: D
Explanation:

The solution in which the concentration of solute is the same in every part of the solution is known as a homogeneous solution, sugar is completely soluble in water and forms homogeneous solution.

Air is a ______ mixture.

  1. homogeneous

  2. heterogeneous

  3. both A and B

  4. none of the above


Correct Option: A
Explanation:

Air is a homogeneous mixture and can be separated into its components by fractional distillation.

Aqueous solutions of low molecular mass solutes are :

  1. true solutions

  2. false solutions

  3. saturated solutions

  4. unsaturated solution


Correct Option: A
Explanation:

Aqueous solution of low molecular mass are true solutions. A solution in which size of the solute particles is about $10^{10}$m is called a true solution.