Free Scientific Reasoning MCQs with Answers

281. Why does the burning of a tire yield black smoke while burning stoves in homes rarely produce smoke?

• A. Burning tires cause parts of the tires to break off and fly off in the air, causing the appearance of black smoke
• B. The heat of the fire causes tires to melt and eventually evaporate, as a result, the vapors in the smoke appear black
• C. Combustion of heavy fuels such as tires causes incomplete combustion which yields elemental carbon which is black
• D. None of the above options are correct

Explanation: Thick, black smoke indicates heavy fuels that are not being fully consumed. In general, a hotter fire will convert more fuel into elemental carbon, which forms into tiny particles that absorb light and appear in the sky as black smoke.

282. The figure shows a table where some cutlery is placed on top of a table cloth. When the table cloth is pulled quickly, it is possible that the cutlery lands directly on the same spot on the table and does not get carried away with the cloth. Which of the following serves as a reasonable explanation?

• A. The rapid pulling of the table cloth causes the plates and cloth to be charged. The repulsion of like charges then causes a levitation force to lift up the cutlery subtly
• B. The instant nature of the movement coupled with the inertia of the cutlery prevents any movement of the cutlery
• C. The cloth is slippery and offers no friction hence can be slipped out
• D. The cutlery is held permanently in the same position by gravitational force

Explanation: If the tablecloth is pulled slowly, friction (rubbing) between the cutlery and the tablecloth will cause the tablecloth to pull the cutlery with it (Hence, options C and D can both be rejected). But, if the tablecloth is pulled quickly, the cutlery does not move. Inertia keeps it in place because friction is not as strong when you pull fast. The trick works no matter how heavy an object is since friction is weaker for lighter objects. Provided you pull quickly enough, this trick can even be done with glasses or plates.

283. The carpet moves while the dust particles remain stationary due to inertia, causing a separation of dust from the carpet. Why does this happen?

• A. The beating loosens the fibers of the carpet causing the dust trapped in them to come loose
• B. The beating movement charges the carpet by friction and repels the dust off of it
• C. The beating causes a reaction force by the carpet which also applies to the dust particles, causing their expulsion
• D. None of these.

Explanation: The inertia of the dust particles residing on the carpet resists the change in the carpet's motion. Therefore, the carpet's forward motion exerts an apparent backward force (no real force is exerted, this is just apparent due to inertia) on the dust particles, setting them in motion in the opposite direction. This is why the dust comes out of the carpet when beaten.

284. Why do satellites not come falling down to Earth due to gravity?

• A. The gravitational force due to the Moon prevents this
• B. The reaction force to gravity keeps them in a balanced state and prevents them from falling
• C. They are actually falling due to gravity but their horizontal velocity is so large that they end up orbiting instead of hitting the surface
• D. They have thrusters to prevent them from falling down

Explanation: In actuality, satellites are indeed "falling" due to gravity. However, their horizontal velocity is so high that they keep "falling" sideways. We can depict this using the concept of projectile motion. If we were to have an imaginary canon on top of a mountain with limitless power to shoot horizontally. The force of gravity keeps acting towards the center of the earth. However, the horizontal component of velocity never changes (ignoring air resistance). As we keep increasing the power of the canon and hence the horizontal velocity, it keeps landing farther and farther. Eventually, there comes a point when the cannonball never touches the surface of the Earth and instead forms an orbit around Earth (pathway D and E).

285. Why is color blindness more common in males than in females?

• A. Men do not take care of their eyes well enough
• B. Men spend more time in front of digital screens than women
• C. Men usually aren't able to distinguish between colors due to some circuitry in their right hemisphere of the brain
• D. Color blindness is a sex-linked genetic disease

Explanation: Since it's passed down on the X chromosome, red-green color blindness is more common in men. This is because: Males have only 1 X chromosome, from their mother. If that X chromosome has the gene for red-green color blindness (instead of a normal X chromosome), they will have red-green color blindness.

286. Which bacteria are responsible for the formation of curd?

• A. Lactic acid bacteria
• B. Lactobacillus Acidophillus
• C. Lactobacillus aureus
• D. Bacillus radicicola

Explanation: Curd may contain a wide variety of bacteria like Lactobacillus acidophilus, Lactococcus lactis, Lactococcus lactis cremoris, etc. These bacteria then carry out fermentation of lactose present in the milk to form lactic acid which lowers the pH and causes agglutination of milk proteins and hence curd formation.

287. Why does the Sun appear red at sunset and sunrise?

• A. At the time of sunset, fusion reactions of the Sun are so intense that the color of the Sun's flame changes from yellow to red
• B. The Sun has sides of different colors similar to the Moon's dark and bright side. Hence, at sunset, we are facing the red side of the Sun
• C. The sun's rays have to pass through a larger distance in the atmosphere. Most of the blue and other shorter wavelengths are removed by scattering and red light enters our eye
• D. Plants release red chemicals at the time of sunset which diffuses into the sky and fluoresces, giving the sky a red color

Explanation: When the Sun is setting, the light that reaches you has had to go through lots more atmosphere than when the sun is overhead, hence the only color light that is not scattered away is the long-wavelength light, the red.

288. What would be the total weight of all humans on Earth combined approximately?

• A. 7 billion kg
• B. 320 billion kg
• C. 320 million kg
• D. 2 billion kg

Explanation: There are approximately 8 billion people on Earth. If we approximate the average weight to be 40 kg, we come to the approximate value of 320 billion kgs which is close to the actual value of 316 billion kg.

289. It takes longer to cook meat on mountainous regions. Why?

• A. The boiling point of water is lower at higher altitudes.
• B. Meat from mountainous animals is stronger than animals on the ground level
• C. Pathogens and pollutants in urban cities located on ground levels make already cause the meat to rot a bit before we begin to cook it
• D. None of the above

Explanation: Because water boils at a lower temperature at higher elevations, foods that are prepared by boiling or simmering will cook at a lower temperature, and it will take longer to cook. High altitude areas are also prone to low humidity, which can cause the moisture in foods to evaporate more quickly during cooking

290. Why are Air-Conditioning units (ACs) always installed on the upper side of a room?

• A. To prevent someone from bumping into them and hurting themselves
• B. So that the cool wind directly hits the person's face, causing them instant relief
• C. To set up convection currents to cool the room since the cool air is denser and settles down
• D. It's just a standard, you can place the units anywhere you like

Explanation: Warmer air at the bottom is displaced and due to it being less dense, it rises. This process continues and sets up a convection current that will allow thermal transfer to occur. Since cold air is emitted from the AC unit blowers, the unit has to be placed at the top of a room to allow a convection current to form.