Question 1

It is important to determine the freshness of cooking oil, since the long-term used one will contain harmful substances, e.g., toxic aldehydes and acrolein, which pose health risk to human. Which of the following parameters can be applied to reveal whether a cooking oil is fresh or used?

(a) Colour
(b) Saponification value
(c) Acid value
(d) All of the above

Question 2

Adulteration is an important issue in food. Which of the followings is NOT a method to determine the identity of a food?

(a) Solid fat index
(b) Starch profile
(c) Type of proteins present
(d) DNA profile

Question 3

Comet Neowise (C/2020 F3) could be seen in the evening sky from Hong Kong in July 2020 (see photo). Why did its orbital period change from about 4,400 years to 6,700 years after this trip to the inner solar system?

(a) Outgassing from the comet when it is close to the sun
(b) Changing of its orbit by the gravity of Jupiter and other planets
(c) Solar wind forcing on the comet when it comes near the sun
(d) All of the above

Question 4

Every year we enjoy the view of the moon during the Moon Festival. How come we on Earth always see the same face of the moon?

(a) The moon does not rotate
(b) Tidal locking between Earth and Moon
(c) The other side of Moon faces Earth only during New Moon
(d) None of the above

Question 5

Why does your voice vary when you speak as compared with when you listen from a voice recorder?

(a) Vibration in air
(b) Vibration in your head
(c) Vibration of your neck
(d) All of the above

Question 6

When does Solar Eclipse happen?

(a) When moon comes in between Earth and Sun
(b) When Earth comes in between Moon and Sun
(c) When Sun comes in between Earth and Moon
(d) When Sun rays does not reach Earth


Question 1: (d) All of the above
Fresh cooking oil is of yellow colour. The colour of the cooking oil changes to brown, then red, and finally black upon using.

Fresh cooking oil mainly consists of triglycerides. The triglycerides will decompose to glycerol and free fatty acids upon using. Saponification value determines the amount of alkali required to saponify a lipid, and thus the amount of triglycerides, i.e., fresh oil has a large saponification value, and vice versa.

Acid value measures the amount of alkali required to neutralise a lipid, and thus the amount of free fatty acids, i.e., fresh oil has a small acid value, and vice versa.

Question 2: (a) Solid fat index
Solid fat index is used to measure of the amount of solids in a lipid. The proportion of solid to liquid and how quick the solid melts only have an impact on the respective functional properties, e.g., mouth-feel.

The ratio of the amylose and amylopectin, i.e., starch profile, in food is dependent of the type of food that can be applied for the identification of a food.

Each food will have some unique proteins present, as well as a unique set of DNA sequence (profile), which can be used for the identification of a food.

Question 3: (d) All of the above
Comet Neowise (C/2020 F3) is a long-period comet that has a highly eccentric orbit. Any small changes in its eccentricity would result in a large change in its orbit. When the comet is near the sun, it gets warmed up and some materials are vaporised to form the tail. Since this outgassing is asymmetric, it produces a net force on the comet. Similarly, the solar wind from the sun would exert a force on the comet. The gravitational pull of giant planets such as Jupiter on the comet is probably the largest effect to change its orbit.

Question 4: (b) Tidal locking between Earth and Moon
Gravity force between two objects is inversely proportional to the distance between them squared. So Earth exerts a stronger gravitational attraction on the side of Moon facing us than the far side. It is as if Earth grabs the nearest point of Moon with a firm grip. This so-called tidal force tends to slow down Moon’s rotation until its same side always faces Earth. Moon is still rotating, but its self-rotation period is the same as its orbital period.

Question 5: (d) All of the above
When you hear your voice from a voice recorder (same as hearing external sounds):
It is a signal created by converting sound waves into vibrations at your eardrum, which then transmits the vibration into your inner ear and followed by vibration-signal conversion.

When you hear your own voice:
Apart from the eardrum, the sound is also conducted through your bone. When you speak with vocal cord and tongue, the extra vibrations created at your soft tissue of head, neck and facial bones make your voice sounds at a lower pitch.

Question 6: (a) When moon comes in between Earth and Sun
When the moon orbits Earth, it moves between the sun and Earth. When this happens, the moon blocks the light of the sun from reaching Earth. During a solar eclipse, the moon casts a shadow onto Earth.