Q&A - PSLE Science

[Verysuperkiasu]

atutor2001:

The atoms/molecules in a solid are \"bonded\" (held together) more tightly together than that in liquid. Therefore, if equal amount of heat is gained by the same amount liquid and solid of the same material, the atoms/molecules in the liquid will be able to vibrate further away from each other than those atoms/molecules in the solid (because they are not bonded as \"tightly\" as that in solid) and will expand more than solid.

Got it , thanks so much

[atutor2001]

P5G:

Please help.


44. Karen used two set-ups, A and B, for her experiments.She added baking soda to the water in one of the set-ups to increase the concentration of carbon dioxide. She then placed them in the sun.

Pictures of the 2 set-ups given [A beaker of water with a hydrilla in it. An upside funnel is over the hydrilla and a test tube full of water over the funnel. Both set-ups were exactly the same].

The table below shows the number of bubbles produced in the 2 set-ups over 20 minutes.

Time (min) | Number of bubbles produced
-------------| Data set X | Data set Y
(0) | Data set X (0) | Data set Y (0)
(5) | Data set X (3) | Data set Y (4)
(10) | Data set X (9) | Data set Y (11)
(15) | Data set X (13) | Data set Y (17)
(20) | Data set X (19) | Data set Y (25)

a) Which set of data, X or Y, represents the result from the set-up with baking soda added? Explain your answer.

____________

b) Besides the amount of hydrilla, state another variable which can be changed for the set-ups to have similar results as the table shown.

____________.


TIA.

a) Data Y represents the result from the set-up with baking soda because the number of bubbles produced is more.

(Adding baking soda to the water will increase the amount of dissolved carbon dioxide in the water. With more carbon dioxide, the rate of photosynthesis of the hydrilla will increase and will produce greater number of oxygen bubbles.)

b) Changing the amount/intensity of light that each set-up is exposed to (placed under) will produce similar results as shown in the table.

(The hydrilla in the set-up that is exposed to greater amount of light will have faster rate of photosynthesis and will produce greater number of oxygen bubbles.)

[atutor2001]

Verysuperkiasu:

John filled a bottle with water to the brim and placed it in the freezer. Two hours later, he observed that the water had frozen and the bottle had cracked. Which statement has correctly explained what had happened?


(1) The bottle was not able to withstand the low temperature causing it to crack.
(2) The water expanded during freezing pushed outwards and caused the bottle to crack
(3) The water contracted during freezing, pushed outwards and caused the bottle to crack.
(4) When the water froze to ice, it got heavier and the bottle could not withstand the weight, causing the bottle to crack.

My question : According to my son, none of the above 4 obtions seem to fit coz his answer is :
'The bottle contracts faster than the ice inside it therefore it cracked'. What is the answer and why?

The answer is (2).

The volume of water increases when it changes from liquid to solid state.

Do not conclude that \"the volume of water expands when its temperature decreases.\" This is wrong because the increase in volume occurs only when water changes from liquid to solid state. In other word, if we lower the temperature of liquid water to 0 degree without allowing it to freeze, the volume of water actually contracts as compared to its volume at room temperature.

When the temperature of water decreases, the volume of water (like other matters) will contract. However, the moment that water changes from liquid to solid state (ice), its volume will increase. The sequence of event should be visualised as follows :

Volume of water (liquid) at 1 degree C is smaller than its volume at room temperature.
Volume of water (liquid) at 0 degree C is small than its volume at room temperature.
Volume of ice (solid) at 0 degree C is BIGGER than its volume at room temperature.

When water changes from liquid to solid state, the arrangement of its molecules becomes different as compared to when it was in liquid state. The molecules becomes more \"spread out\" causing the volume of ice (solid state) to increase.

[Verysuperkiasu]

atutor2001:

The answer is (2).

The volume of water increases when it changes from liquid to solid state.

Do not conclude that \"the volume of water expands when its temperature decreases.\" This is wrong because the increase in volume occurs only when water changes from liquid to solid state. In other word, if we lower the temperature of liquid water to 0 degree without allowing it to freeze, the volume of water actually contracts as compared to its volume at room temperature.

When the temperature of water decreases, the volume of water (like other matters) will contract. However, the moment that water changes from liquid to solid state (ice), its volume will increase. The sequence of event should be visualised as follows :

Volume of water (liquid) at 1 degree C is smaller than its volume at room temperature.
Volume of water (liquid) at 0 degree C is small than its volume at room temperature.
Volume of ice (solid) at 0 degree C is BIGGER than its volume at room temperature.

When water changes from liquid to solid state, the arrangement of its molecules becomes different as compared to when it was in liquid state. The molecules becomes more \"spread out\" causing the volume of ice (solid state) to increase.

I must say it took me quite a while to digest what you've written. But the important thing is my son understood your explanation.

So, basically temp of liquid drops - like all other matter, vol will drop.
Once it's completely frozen (solid) - will expand rite? That's how the bottle was cracked.

Just one question - the last part of your explanation - molecules becoming more spread out in ice (solid state). I thought solid state, molecules more compact - closely held together not spread out? :?

[atutor2001]

Verysuperkiasu:

Just one question - the last part of your explanation - molecules becoming more spread out in ice (solid state). I thought solid state, molecules more compact - closely held together not spread out? :?

I am glad that your son can make out what I was trying to put across. However, I have made a mistake. Water contracts to a minimum at 4 degree C. After that it will begin to expand even before it solidifies. However, this expansion is very small and will not cause the bottle to crack. It is the conversion to ice that causes a huge increase in volume, resulting in the bottle cracking. Don't bother to find out the reason for this expansion from 4 to 0 degree C. It is a little complex. My old Chem teacher told us God make it this way so that fishes can survive in the ocean because the coldest part in the ocean will never go below 4 degree C.

You are right that for most matter, molecules in the solid state are more compact. However, ice is special. When solidified, the molecules are arranged in a special shape called \"tetrahedral\" which occupies a bigger space than the arrangement of the molecules in the liquid state. This is a common question in Sec Science.

[Verysuperkiasu]

atutor2001:

You are right that for most matter, molecules in the solid state are more compact. However, ice is special. When solidified, the molecules are arranged in a special shape called \"tetrahedral\" which occupies a bigger space than the arrangement of the molecules in the liquid state. This is a common question in Sec Science.

Thank you so much for taking time to explain. Really Appreciate that!!

:thankyou:

[P5G]

Please help.


32. The diagram below shows the grapevine flower.

a). The arrows in the diagram are pointing at three parts of the flower. Name these parts by filling in the boxes next to the arrows.
Ans: Anther, Stigma and Ovary

b. The grapevine flower is able to self pollinate. Explain how this flower is adapted for this process.


c) State one advantage of the self-pollination method.
_


TIA.

[atutor2001]

b) Each flower has both male and female parts. This is a structural adaptation that allows it to self-pollinate.


c) One advantage of self-pollination method is that the quality of the fruit will be maintained / will be the same. Through self-pollination, the new plant will be exactly identical as the parent plant and will produce fruits of the same quality as the parent plant.

[andante]

Hi all,

I would like to find out whether the following is structural or behavioural adaptation.
- an earthworm releases mucus to help it slide through the soil quickly when it is threatened.

My DD thought that it is structural adaptation. Pls advise.

[Pita]

vidhyalog:

Nebbermind:

I thought the bigger vol of air in X will push the ink to Y? Isn't it (1)? :?

Hi Nebbermind

That's what he answered first and I thought too. But then why is there the word \"immediately\". Because 2 things happen - at first - flask expands - and as air occupies space - the air in the flask will occupy the expanded space there causing the ink to momentary move towards X - before the air in X expands and therefore pushes it to the right.

This reasoning was given by my son when he noticed the answer he wrote was wrong. He then guessed it is because of the word immediately.

I find it quite sad that some questions may trip up children who can understand concepts - but are not so careful. But that is life I guess - you should know the concepts and be super-careful in reading the question.

Hi vidhyalog,

Here's what my son said after he read your post.

\"I feel that it is not necessarily true that the air in the flask will occupy the expanded space, because air does not have a definite volume, e.g. pumping 300 cubic centimetres of air into a 500 cubic centimetres container, the air spreads out to take up the entire space, retaining its volume. Therefore, air outside the flask in the plastic tube does not have to enter the flask to fill up the space. Thus, equal forces from both flasks in this case are pushing the water droplet, so it should not move at all :? That's what i think. Although my initial answer was (1), I am seriously beginning to have doubts.\"

Can someone please help to explain to my son?

:imcool: :?: :scratchhead: :idea:

Thank You!!!!