Q&A - PSLE Science

[James Ang]

atutor2001:

tianzhu:

Thank you for your help.


Adrian bought some ornamental fish from a pet shop. The shop owner told him to remember to place the bag of fish into the tank of water first before releasing the fish into the tank. He also told him to use only water that has been left overnight in a pail if he needs to change the water in the tank.
(a) Why was Adrian told not to release the fish into the tank immediately after he brought them home?
(b) Why is it necessary to use water that has been left overnight ?
(c) Besides these instructions, what else would you advise Adrian to do to improve the environment of the aquarium?

This one I know because I keep fish at home :

(a) Fish is cold-blooded and its body temperature will adapt to the temperature of the surrounding. If the difference in temp. of the water in the bag and that in the tank is very great, the fish will die of shock due to the sudden temperature change.

(b) There is dissolved chlorine in our tap water and they will turn to gas and leave the water if the water if left standing for some time. Chlorine is harmful to fish. Leaving the water overnight helps to remove the chlorine from the tap water.

(c) To improve the evironment in the tank we can add water weeds such as hydrilla because they take in carbon dioxide and give out oxygen when they photosynthesize. Fish needs oxygen to respire.

We can also add an air pump to bubble the water. Most people think that it will increase the amount of dissolved oxygen in the tank. However, actually it is not true. The additional amount of dissolved oxygen from the bubbles is negligible. The real advantage of bubbling is that it will improve circulation of the water in the tank, causing the top layer of water which is richer in dissolved oxygen to move to the bottom and vice versa.

Very good and concise answers. If the student keeps an aquarium, he/she will know how to answer the question better. Science is all around us.

[ChiefKiasu]

James Ang:

atutor2001:

(a) Fish is cold-blooded and its body temperature will adapt to the temperature of the surrounding....

Very good and concise answers. If the student keeps an aquarium, he/she will know how to answer the question better. Science is all around us.

Yes, atutor2001's answers are indeed concise and precise.

Technically speaking, it is nature that surrounds us, and science is merely our way of trying to understand the rules by which nature plays.

[kelly1]

atutor2001:

kelly1:

This is referring to the question about how the stem of a flower is slit in half, then dipped in coloured water on each side. The correct solution says that the xylem tubes in each half will take on the respective colour of the water where that half is dipped. My question is, can the stem also absorb water? The roots have been cut off in this diagram.

Yes, there is a special mechanism in the xylem that enables the water to flow upwards. Sugar cane (a stem) left standing with its cut end resting on dirty wet floor in hawker centre will absorb the dirty water and caused food poisoning. It happened locally before.

Thanks! Now the question makes sense...

[James Ang]

The whole of science is nothing more than a refinement of everyday thinking.

Albert Einstein

[tianzhu]

Thank you for your help.


1) SwimTeam Company wants to make swimming boards and chose styrofoam as a material. Which of the following properties make styrofoam a suitable material for making swimming boards? (a) Styrofoam can float on water (b) Styrofoam does not break or tear easily Styrofoam is light so it can be carried easily (d) Styrofoam is soft so it would not injure anyone. Answer options -> (1) a, b and c only (2) a, c and d only (3) b, c and d only (4) all of the above.


2) What must happen for an egg to hatch? (a) The egg must be fertilized. (b) The egg must be given food and water. The egg must be kept warm by the hen for 21 days. Answer options - > (1) a and b only (2) a and c only (3) b and c only (4) All of the above.

[Tang]

tianzhu:

Thank you for your help.


1) SwimTeam Company wants to make swimming boards and chose styrofoam as a material. Which of the following properties make styrofoam a suitable material for making swimming boards? (a) Styrofoam can float on water (b) Styrofoam does not break or tear easily (c) Styrofoam is light so it can be carried easily (d) Styrofoam is soft so it would not injure anyone. Answer options -> (1) a, b and c only (2) a, c and d only (3) b, c and d only (4) all of the above.

Answer given in the worksheet is not correct. It should be (4). The styroform must not break or tear easily, otherwise the person using it may drown. Styroform used for making swimming boards and boxes can be very robust and will not tear or break easily.


2) What must happen for an egg to hatch? (a) The egg must be fertilized. (b) The egg must be given food and water. (c) The egg must be kept warm by the hen for 21 days. Answer options - > (1) a and b only (2) a and c only (3) b and c only (4) All of the above.

Answer is (2).

[Cando]

[Moderator's note: Topics merged.]


I was asked this question : why do we need to sprinkle water on clothes while ironing? Does it help to straighten the wrinkle? How?

Can anyone help?

Thank you in advance.

[Cando]

I was asked this question : why do we need to sprinkle water to clothes while ironing? Does it help to straighten the wrinkle? How?


Can anyone help?

Thank you in advance.

[TTLauPeh]

Synthetics don’t wrinkle so I assume the question refers to cotton garments. What you spray water on cotton material and then iron it, what happens is:

1) The fibres in the cotton material absorbs the water.
2) The water molecules in the fibres are attracted to one another and the air between the fibres is forced out, flattening out the wrinkle and making the fabric tighter. [Similarly, cotton balls shrink when you wet them and wrinkling in curtains can be reduced by just vacuuming them].
3) Heat from the iron evaporates the water, leaving a wrinkle-free patch of cotton fabric.

[Cando]

tianzhu:

Rate of evaporation


One of the factors affecting the rate of evaporation is temperature.

When we are talking about temperature, does it refer to temperature of the source or temperature of the surroundings? For example a cup of hot water as compared to a cup of warm water in airconditioned/non airconditioned room, clothes left to dry under shade or out in the sun.
Please share your view.

Thank you for your reply