Top 29 Most Asked Chemistry Interview Questions and Answers
1) What is an atom?
An atom is the smallest unit of matter that has the characteristic properties of a chemical element. It is also called the basic building block of chemistry. Every solid, liquid, gas, and plasma is made of neutral or ionized atoms. Most of the atoms are empty space. The rest are made of a nucleus and one or more electrons around the nucleus. The nucleus is made of one or more protons and some neutrons. The electrons are the lightest and negatively charged particles. The nucleus is small and dense compared to the electrons.
2) What do you understand by electrons, protons, and neutrons?
Electrons, protons, and neutrons are tiny subatomic particles that build an atom. Every atom has a central nucleus that contains neutrons and protons. Electrons roam around the nucleus. Protons are the type of subatomic particles which have a positive electric charge. The electrons are a type of subatomic particle with a negative electric charge, and the neutrons are a type of subatomic particle with no electric charge (they are neutral).
If an atom has an equal number of protons and electrons, it is called an electrically neutral atom. If an atom has more electrons than protons, it is called a negatively charged atom. If an atom has fewer electrons than protons, it is an overall positively charged atom. Such atoms are called ions.
3) How were the electron, proton, and neutron discovered? Who discovered the electron, proton, and neutron, respectively?
Electron, proton, and neutron are the building block of an atom. The discovery of electrons, protons, and neutron is very interesting. Let’s see the history of their discovery in detail.
Electron
J.J. Thomson discovered electrons in 1897. At that time, J. Thomson was experimenting with cathode ray tubes, and he saw that all atoms contain some tiny negatively charged subatomic particles or electrons. The electrons are the lightest and negatively charged particles within an atom. He saw that the velocity of cathode rays was much less than the speed of light.
So, he devised a new way to measure cathode rays’ charge to mass ratio. He concluded that the cathode rays are the 11000th mass of hydrogen ion (which is a proton).
Characteristics of an Electron
- An electron has a negligible mass. The relative mass of an electron is 11840 u.
- The absolute charge is -1.6 × 10-19 coulomb of negative charge.
Proton
A proton is a stable subatomic particle, denoted by p, p+, H+, or 1H+ and has a positive electric charge. E. Goldstein saw the first sign of proton in an atom in 1886. Goldstein experimented, and he passed electricity at high voltage through gas at very low pressure. He saw a stream of heavy particles by the anode (positive electrode). These streams of particles were called canal rays or anode rays. Later, Ernest Rutherford discovered protons and named them. Protons reside inside the nucleus along with the neutron. So, he is known as the inventor of the proton.
Characteristics of a Proton
- The mass of a proton is slightly less than a neutron and 1836 times the mass of an electron. The relative mass of a proton is 1 u.
- The charge of a proton is equal to and opposite to the charge of an electron. The absolute charge is 1.6 × 10-19 coulomb of positive charge.
- Protons and neutrons, each with masses of approximately one atomic mass unit, are jointly referred to as “nucleons”.
- Every atom has one or more protons in the nucleus, which must provide an attractive electrostatic central force to bind atomic electrons.
Neutron
A neutron is a stable subatomic particle, denoted by n or n0, with a neutral (not positive or negative) charge. The mass of a neutron is slightly greater than a proton. Protons and neutrons both make the nuclei of atoms. Sir James Chadwick, the British Physicist, is credited as the inventor of neutrons. He discovered the neutron in 1932.
Characteristics of a neutron
- The relative mass of a neutron is almost 1 u.
- Neutron is a neutral particle and has no charge.
4) What is the atomic number of a chemical element?
The atomic number of a chemical element is the charge number of an atomic nucleus. In other words, the number of protons in the nucleus is called the atomic number. It is the defining property of an element. It is also called the nuclear charge number and is denoted by the symbol Z.
The atomic number is a very important property of a chemical element and can be used to identify the chemical elements uniquely. In the case of a normal uncharged atom, the atomic number is also equal to the number of electrons.
5) What do you understand by the mass number of an element?
The mass number of an element can be defined as the total number of protons and neutrons present in the nucleus of a single atom of the element. The mass number is represented by the symbol A.
6) What do you understand by isotopes? What are their usages?
Isotopes are two or more atoms of an element with the same atomic number and position in the periodic table, but they have different numbers of neutrons in their nucleus. The number of protons in a nucleus determines the element’s atomic number on the Periodic Table. In other words, we can say that Isotopes are the members of an element’s family that all have the same number of protons but different numbers of neutrons. They have the same atomic number and position in the periodic table but different mass numbers.
Isotopes are mainly represented by writing the element’s name, followed by a hyphen and the isotope’s mass number. For example, uranium-235 and uranium-239 are two different isotopes of uranium.
Some examples of Isotopes are given below:
- Hydrogen has three isotopes, Protium, Deuterium, and Tritium.
- Uranium has two isotopes, uranium-235, and uranium-239.
- Carbon has two isotopes, Carbon-12 and Carbon-14.
Usage of Isotopes
Following are some important usages of isotopes:
- In medicine, an isotope of cobalt is used to treat cancer.
- Industries as an isotope of uranium are used in nuclear power plants to generate electricity.
- In Agriculture.
- In modern discovery and inventions etc.
7) In which case can an atom be called neutral?
An atom can be called neutral if the number of electrons in the extra nuclear part equals the number of protons in the nucleus.
8) What do you understand by an acid?
If a substance has a pH value less than 7, it is called acid by nature. The pH scale measures how acidic or basic a substance is. The pH of an acid is determined by the concentration of Hydrogen ions (H+) the substance contains when it is in solution. The higher the concentration of H+ ions, makes the pH lower and the substance more acidic. All acids contain hydrogen ions when they are in solution.
In Chemistry, pH stands for “potential of hydrogen”. It is a measuring scale used to measure the acidity or basicity of an aqueous solution. Acidic solutions are measured to have lower pH values, and the basic or alkaline solutions have higher pH values.
| pH Value | Nature of the substance |
|---|---|
| 0-3 | The pH value within 0-3 specifies that the substance is a strong acid. In this case, the UI turns red. |
| 4-6 weak acid (UI turns orange/yellow) | The pH value within 4-6 specifies that the substance is a weak acid. In this case, the UI turns orange/yellow. |
| 7 neutral (UI turns green) | The pH value of 7 specifies that the substance is neutral. It means it is not acidic or basic. In this case, the UI turns green. |
| 8-10 weak alkali (UI turns blue) | The pH value within 4-6 specifies that the substance is weak alkali. It means it is basic but not strong. In this case, the UI turns blue. |
| 11-14 strong alkali (UI turns purple) | The pH value within 4-6 specifies that the substance is strong alkali. In this case, the UI turns purple. |
Here, UI means Universal Indicator. It is a solution that changes color according to the pH of the substance.
9) What are solute, solvent, and solution in chemistry?
Solute, solvent, and solution are the terms used in chemistry. Let’s understand them in detail:
- Solution: A solution is a homogeneous mixture of one or more solutes dissolved in a solvent.
- Solvent: Solvent is the substance in which a solute dissolves and produces a homogeneous mixture. This mixture is called a solution. The material of the solvent usually decides the physical state of the solution, i.e., solid, liquid, or gas.
- Solute: Solute is the substance that dissolves in a solvent and produces a homogeneous mixture.
For example: A solution of salt and water. Here, water is the solvent, and salt is the solute.
Note: In every solution, the solvent is the substance present in the greatest amount, and the solute decides the concentration of the mixture. There may be different kinds of solutions, and a solute can be a gas, a liquid, or a solid. Solvents can also be gases, liquids, or solids.
10) What is concentration in chemistry?
Concentration is a common concept used in chemistry and chemical-related fields. The term concentration is derived from the word concentrate, which comes from the French word concentrer. It is made from con + center, which means “to put at the center”.
Concentration is used to measure how much of a given substance there is mixed with another substance. It can be used in any type of chemical mixture, but most frequently, it is used with solutions, where it refers to the amount of solute dissolved in a solvent. If you want to concentrate a solution, you must add more solute or reduce the amount of solvent. On the other hand, if you want to dilute a solution, you must add more solvent or reduce the amount of solute.
There are several types of concentration for a solution or a mixture. Those are:
- Mass concentration
- Molar concentration
- Number concentration
- Volume concentration etc.
Note: The molar concentration is further divided into normal and osmotic concentrations.
11) What is saturation in a chemical solution?
In a chemical solution, a point occurs where no further solute dissolves in a solution. At this point, the solution is said to be saturated, and this process is called saturation. In this condition, if you add additional solute to a saturated solution, it will not dissolve. Instead, you will see phase separation that leads to either coexisting phases or a suspension.
The point of saturation depends on many variables, such as the temperature of the solution and the precise chemical nature of the solvent and solute.
12) What do you understand by the term titration?
Titration is a process or a technique used to determine the concentration of an unknown solution using a solution of known concentration. We can determine the concentration of an unknown solution only by knowing the volume of titrant used.
In other words, we can say that titration is a process to determine the molarity of a base or an acid of a solution. In this process, we react between the known volumes of a solution with a known concentration against the known volume of a solution with an unknown concentration.
13) What do you understand by the term molality?
In chemistry, the term molality specifies the number of solutes present in 1 kg of a solvent. It is used to measure the number of moles of solute in a solution corresponding to 1 kg or 1000 g of solvent. The unit for molality in chemistry is mol/kg. If a solution has a concentration of 1 mol/kg, it is denoted as 1 molal.
14) What do you understand by the term Aliquot and Diluent in Chemistry?
- Aliquot: The term Aliquot specifies the measured sub-volume of the original sample.
- Diluent: The term Diluent specifies the material with which the sample is diluted.
15) What do you understand by the valency of an element?
In Chemistry, the atoms of every element tend to combine with the atoms of another element to form a molecule. An atom of each element has a definite combining capacity known as its valence or valency. So, we can say that the valence or valency of an element is the measurement of its combining capacity with other atoms in forming a chemical compound or molecule.
The number of hydrogen atoms determines the combining capacity of an atom of a specific element that it combines with. For example, in water, oxygen has a valency of 2. That’s why it is called H2O. Let’s see the valency of different atoms in different compounds.
- In methane, carbon has a valency of 4. That’s why the chemical formula of methane is CH4.
- In ammonia, nitrogen has a valency of 3. That’s why the chemical formula of ammonia is NH3.
- In hydrogen chloride, chlorine has a valency of 1. That’s why the chemical formula of hydrogen chloride is HCl.
- In phosphorus pentachloride, phosphorus has a valence of 5. That’s why the chemical formula of phosphorus pentachloride is PCl5.
To represent the valency in pictorial format, we use valence diagrams. The valence diagram of a compound represents the connectivity of the elements. It consists of lines drawn between two elements, sometimes called bonds, representing a saturated valency for each element.
16) In which condition is an atom called neutral?
An atom is called neutral when the number of electrons in the extranuclear part is equal to the number of protons in the nucleus of that atom. In this condition, the atom is neutral.
17) What do you understand by a buffer or a buffer solution? What are its different types? Give an example of a buffer solution.
A buffer or a buffer solution is an aqueous solution (a solution containing water or a watery solution) with a highly stable pH. It means the pH of a buffer is hardly changed when we add a small amount of base or acid to the buffer. A buffer is a blend of a weak acid and its conjugate base or a weak base and its conjugate acid.
An example of a buffer solution is bicarbonate in the blood, which maintains the body’s internal pH.
Different types of Buffers
Buffers can be mainly divided into two types:
- Acidic buffer solutions: Acidic buffer solutions are the solutions that have a pH below 7 and contain a weak acid and one of its salts. For example, a mixture of acetic acid and sodium acetate acts as a buffer solution with a pH of about 4.75. It is an example of an acidic buffer solution.
- Alkaline buffer solutions: Alkaline buffer solutions are the solutions that have a pH greater than 7 and are made from a weak base and one of its salts. For example, a mixture of ammonia and ammonium chloride solution is a very common example of an alkaline buffer solution. A mixture of ammonia and ammonium chloride solution in equal molar proportions has a pH of 9.25. So, it is an example of an alkaline buffer solution.
18) What is Buffer capacity?
Buffer capacity specifies the minimum amount of acid or base that we have added before the pH of a buffer changes.
19) How does buffer work in Chemistry?
In Chemistry, a buffer is a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid. Buffers work by reacting with any added acid or base to control the pH of the solution. It neutralizes any added acid (H+ ions) or base (OH- ions) to maintain the moderate pH, making them a weaker acid or base. When we add a hydrogen ion to the buffer, it is neutralized by the base in the buffer, and the acid neutralizes the hydroxide ion. On the overall pH of the buffer solution, these neutralization reactions will not show much effect.
Example: Let’s take a buffer made up of the weak base ammonia, NH3, and its conjugate acid, NH4+. When we add a strong acid (HCl) to this buffer system, the extra H+ ions added to the system are consumed by the NH3 and form NH4+. Now, because all the extra H+ ions are locked up and have formed a weaker acid, NH4+, thus the pH of the system does not change significantly.
20) What do you understand by isotopes and their different types? What are the applications of isotopes in our daily life?
Isotopes are the different variants of a single chemical element that possess the same number of protons and electrons but a different number of neutrons. In other words, we can say that atoms have the same atomic number, but different mass numbers are called isotopes.
For example, Carbon has three isotopes, carbon-14, carbon-13, and carbon-12. Carbon-14 contains a total of 8 neutrons, carbon-13 contains 7 neutrons, and carbon-12 contains a total of 6 neutrons.
There are mainly three types of isotopes:
1. Stable Isotopes
The isotopes with extremely long half-lives (Hundreds of millions of years) are called stable nuclides or stable isotopes.
Some common examples of stable nuclides are carbon-12, carbon-13, oxygen-16, oxygen-17, oxygen-18 etc.
2. Primordial Isotopes
Primordial isotopes are the types of isotopes that have existed since the solar system’s formation. There are a total of 339 naturally occurring isotopes on Earth, of which 286 isotopes are primordial.
3. Radioactive Isotopes
The isotopes which are radioactive in nature are called radioactive isotopes. These isotopes have unstable atomic nuclei that undergo radioactive decay continuously.
Some examples of radioactive isotopes are carbon-14, hydrogen-3 (also known as tritium), chlorine-36, uranium-235, uranium-238, etc.
Most of the chemical element has one or more radioactive isotopes. For example, hydrogen, the lightest element on the periodic chart, has three isotopes with mass numbers 1, 2, and 3. Here, only hydrogen-3 is a radioactive isotope, and the other two are stable.
There are more than 1000 radioactive isotopes known of the different elements. Of these 1000 radioactive isotopes, only approximately 50 are found in nature; and the rest are produced artificially as the direct products of nuclear reactions or created indirectly as the radioactive descendants of these products.
Application of Isotopes
Isotopes have many useful applications in our daily life. Mainly radioactive isotopes are used in medical cases. For example, cobalt-60 is used as a radiation source to determine the development of cancer. Other radioactive isotopes are also used for diagnosis and research on metabolic processes. Let’s see the use of isotopes in daily life:
- Isotopes are used to determine the isotopic signature of element samples via isotope analysis.
- Isotopes can also be used to determine the chemical reaction with the help of isotopic substitution.
- Isotopes are also used to determine the concentration of many elements/substances via isotope dilution.
- Isotope of uranium is used as a fuel in nuclear reactors.
- An isotope of iodine is used in the treatment of goiter.
- Isotope of sodium is used to detect blood cells.
- An isotope of cobalt is used in the treatment of cancer.
21) What do you understand by the photoelectric effect?
When the emission of electrons by electromagnetic radiation strikes a metal surface, this process is called the photoelectric effect. In the photoelectric effect, the electrons emitted in this manner are called photoelectrons.
22) What is a raincoat made of?
There are mainly two types of raincoats available in the market made of water-resistant or waterproof materials. So, according to the functionality of the raincoats, there are two types of raincoats: water-resistant and waterproof.
Most people think that water-resistant and waterproof raincoats are the same, but there is a difference between them. Water-resistant raincoats mean that they are not 100% waterproof. These kinds of raincoats can be used in light rain, but they are not good enough to go through a downpour and return dry. These water-resistant raincoats will soak under heavy rain.
On the other hand, waterproof raincoats are the best in every condition. They will keep you dry no matter what amount of rain or water is pouring on you.
Materials used in Raincoat
There are many different materials used to prepare raincoats:
Water-resistant materials used for raincoats are:
- Laminated cotton: This material is used for raincoats to provide an extra layer to the usual material. The advantage of this material is that it’s breathable.
- Nylon and polyester are chemical-formed materials, and raincoats made of these artificial materials are water-resistant.
Waterproof materials used for raincoats are:
- Polyurethane laminate: This material is a blend of polyester and cotton. It is a very durable material, and the raincoats made of this are completely waterproof and breathable.
- Gore-Tex: This is one of the best materials for waterproof jackets and coats.
- Microfiber: This material has a special waterproof coating perfect for raincoats.
23) Why do bee stings cause irritation and pain?
Bee stings are acidic in nature. When a bee stings, it injects methanoic acid into the skin, which mainly causes immense pain and irritation. It can be neutralized by using baking soda or baking powder which contains sodium hydrogen carbonate. When we rub baking soda on the sting area, it gives relief because baking soda is alkaline in nature. It neutralizes the effect of methanoic acid and gives relief.
24) How does a glow stick glow?
We see the glow in a glow stick due to two chemicals reacting together and giving off light energy. This process is called chemiluminescence.
A glow stick is a glass vial or a tube that contains different chemicals (usually phenyl oxalate and a fluorescent dye). When we mix it with other chemicals (usually hydrogen peroxide), the two chemicals react. When the chemicals mix, electrons in the constituent atoms are raised to a higher energy level, and they release light energy when they return to their normal state. This process is called chemiluminescence. Glow sticks are mainly used in the military, diving, night-time fishing lures, etc. The world’s largest glow stick was 8ft 4 inches tall.
25) What are the most common acids used in Chemistry?
Acids are solutions that contain hydrogen ions. That’s why the pH of all acids is less than 7. The most common acids used in Chemistry are:
- Hydrochloric Acid (HCl)
- Sulphuric Acid (H2SO4)
- Nitric Acid (HNO3)
- Phosphoric Acid (H3PO4)
- Ethanoic Acid (Vinegar) (CH3COOH) etc.
26) Which chemicals are used in fireworks to provide different colors?
Fireworks use different chemicals and chemical reactions to provide different colors. Two things happen in fireworks incandescence (light created through heat) and luminescence (light without heat). Here, one thing is notable orange, red and white colors are usually made through incandescence, and blues and green colors are usually made through luminescence.
Note: Japan witnessed the largest single firework in 1988. The burst was over 1 kilometer across.
Following are the chemicals used to provide different colors:
| Chemical | Color |
|---|---|
| Calcium | It produces orange color in fireworks. |
| Strontium and Lithium | These chemicals produce red color in fireworks. |
| Iron | It produces gold color in fireworks. |
| Sodium | It produces yellow color in fireworks. |
| Magnesium or Aluminium | These chemicals produce white color in fireworks. |
| Barium plus a Chlorine producing chemical | These chemicals produce green color in fireworks. |
| Copper plus a Chlorine producing chemical | These chemicals produce a blue color in fireworks. |
| Strontium plus Copper | These chemicals produce purple color in fireworks. |
| Aluminum or Magnesium powder | These chemicals produce a silver color in fireworks. |
27) What are the most important features of atoms?
The word atom is derived from the Greek word “Atomos,” which means indivisible or non-divisible. In Greek literature, the prefix “a” means “not,” and the word “tomos” means cut. But nowadays, we know that atoms are made of even smaller subatomic particles known as electrons, protons, and neutrons. Following is the list of the most important features of atoms:
- An atom is the smallest recognized component of a chemical element.
- An atom comprises three particles: the proton, neutron, and electron.
- Most of the mass (almost 99% of the mass) of the atom is held in the central nucleus, consisting of the protons and neutrons. The negatively charged electrons move around the nucleus in orbital shells of different energies.
- The number of protons in a nucleus is known as its atomic number.
- Usually, the number of electrons in an atom equals the number of protons. It means the atoms are generally neutralized in nature and have no overall charge.
- When an atom gains or losses electrons, it is called an ion.
28) What is a mole in Chemistry?
In Chemistry, a mole is a standard scientific SI base unit of the amount of substance. It is used for measuring large quantities of very small entities such as atoms, molecules, or other specified particles. It defines the number of chemical substances present in an element. It is the amount of substance with the same number of chemical units as there are atoms in exactly 12 grams of pure carbon-12.
The mole consists of an extremely large number of chemical units, 6.02214076 × 1023.
29) What is the ozone layer? What is its usage for humanity?
Ozone is a special molecule of oxygen, and its chemical formula is O3. The ozone layer is a massive shield 50 kilometers above the surface of the planet earth. Its thickness is up to 20 kilometers, and most of this gas is found in the stratosphere. It surrounds the Earth and protects us against UVB radiation.
UVB radiation is extremely dangerous radiation emitted by the Sun. The ozone layer absorbs around 99% of this harmful radiation and protects humanity.