Q&A - PSLE Science

[atutor2001]

vidhyalog:

Need some help with the following OE Qn http://i52.tinypic.com/2v0bcxx.jpg\">

(a) The x-axis of the graph is \"Distance from the farm\" and the graph shows decreasing number of duckweed as the distance from farm increases.

Answer : The number of duckweed at a point in the lake increases as the distance from from that point to the farm decreases. OR The number of duckweed at a point in the lake decreases as the distance from that point to the farm increases.


(b) Need logical think. Increases in the duckweed must be from something discharged by the farm - fertiliser.

Answer : Some of the fertiliser from the farm will be washed into the lake during rain which helps the duckweed to thrive (standard keyword)/ to reproduce faster.

[Alchemist]

atutor2001:

vidhyalog:

Need some help with the following OE Qn http://i52.tinypic.com/2v0bcxx.jpg\">

(a) The x-axis of the graph is \"Distance from the farm\" and the graph shows decreasing number of duckweed as the distance from farm increases.

Answer : The number of duckweed at a point in the lake increases as the distance from from that point to the farm decreases. OR The number of duckweed at a point in the lake decreases as the distance from that point to the farm increases.


(b) Need logical think. Increases in the duckweed must be from something discharged by the farm - fertiliser.

Answer : Some of the fertiliser from the farm will be washed into the lake during rain which helps the duckweed to thrive (standard keyword)/ to reproduce faster.

Thanks atutor - its a pleasure to read your answers. Wish I had discovered this earlier - but as they say - better late than never.

[Verysuperkiasu]

atutor2001:

Yes, I agree with your answer. However, it is safer to use the standard key words :\"force that act at a distance\".

Magnetic force can act a distance while frictional force cannot.

Yah now that u mentioned, think I remember coming across the phrase 'act at a distance' in the PSLE guide book under chapter on forces.

Thanks for reminding us (our chn) to use the key words, coz sometimes even if answer convey the same meaning, no keywords also minus marks.

So once again, to all parents, pls remind your chn to write down the relevant key words if they want full marks.

[Verysuperkiasu]

Which one of the following is not a source of light?


1. stars
2. firefly
3. burning torch
4. fluorescent light bulb

ans key is 4. My question : I thought all the 4 choices above are sources of light - can emit light. So why is fluorescent light bulb not a source of light?

[Cockroach]

Verysuperkiasu:

Which one of the following is not a source of light?


1. stars
2. firefly
3. burning torch
4. fluorescent light bulb

ans key is 4. My question : I thought all the 4 choices above are sources of light - can emit light. So why is fluorescent light bulb not a source of light?

I think that it is because the light bulb needs to paired with electricity to emit light, hence, i think that the answer is correct? Ha ha no confidence. The burning torch emit light as fire is present for combustion to take place.

[Cockroach]

Can someone explain to me why dry ice will be seen with white smoke when it is in contact with water?

[Verysuperkiasu]

Cockroach:

Verysuperkiasu:

Which one of the following is not a source of light?


1. stars
2. firefly
3. burning torch
4. fluorescent light bulb

ans key is 4. My question : I thought all the 4 choices above are sources of light - can emit light. So why is fluorescent light bulb not a source of light?

I think that it is because the light bulb needs to paired with electricity to emit light, hence, i think that the answer is correct? Ha ha no confidence. The burning torch emit light as fire is present for combustion to take place.

hmm....guess it makes sense. fluorescent bulb - different from the other 3 choices in that they are already lighted up. thanks!

[Cockroach]

Can someone explain to me why dry ice will be seen with white smoke when it is in contact with water?

[atutor2001]

Verysuperkiasu:

Which one of the following is not a source of light?


1. stars
2. firefly
3. burning torch
4. fluorescent light bulb

ans key is 4. My question : I thought all the 4 choices above are sources of light - can emit light. So why is fluorescent light bulb not a source of light?

In science, the definition of light source is an object that can produce light on its own. Unfortunately, the meaning of \"on its own\" is very vague.

A clearer way to identify a light source is to first find out, \"where did the energy that is used to produce the light comes from\". If potential energy from an object is used to produce (convert to) the light then that object is a light source.

For example, a paper with a silver shining surface is not a light source when it is reflecting light from a torch because no energy from the paper is being used to produce the light.

However, if we burn this same paper, it becomes a light source because the chemical potential energy of the paper is now being converted to light.

With the same reasoning, the lighted bulb in the question is not a light source because no energy from the bulb (itself) is being used to produce the light. The light comes from electrical energy which is provided by either batteries or power plant.

[atutor2001]

Cockroach:

Can someone explain to me why dry ice will be seen with white smoke when it is in contact with water?

Carbon dioxide, water vapour (i.e. air), are colourless and cannot be seen.

In Pr science, students are thought \"smoke\" is usually either water droplets or dust particles. Therefore, my deduction is that the white smoke from dry ice must be water droplets.

When the carbon dioxide sublimates, the gaseous molecules are very cold and water vapour will condense on them into tiny water droplets. These water droplets will appear as white smoke.