Lower Secondary Science

[Verysuperkiasu]

Dr.Daniel:

Sounds like they mean to imply that 350 would be liquid boiling, like water at 100C. One could argue also that the hotter the gas the lower the density (the principle behind hot air balloons).

thanks

[Verysuperkiasu]

Hi I'm using my mom's account to ask a question: Are algae considered plants or protoctista? I know that blue-green algae are considered monera but what about the other types of algae? Thanks in advance

[Dr.033430Daniel]

Verysuperkiasu:

Hi I'm using my mom's account to ask a question: Are algae considered plants or protoctista? I know that blue-green algae are considered monera but what about the other types of algae? Thanks in advance

Classification can be tough sometimes. The term algae refers to a wide range of organisms. I would never create a multiple choice question classifying algae as either a) plants or b) protoctists. If you got such a question though, go with protoctists. Depending on the species, I think you could argue either way for some. To give a simple answer, I think the most common ones, like green algae, red algae and euglenoids are most commonly classified as protoctists.

You can check out this link where they talk about the different kinds of protoctists. By the way \"protist\"means single cell and \"protoctist\"means single or multicellular although very simple multicellular forms (some algae are multicellular)

http://archives.microbeworld.org/microbes/protista/

[Verysuperkiasu]

Dr.Daniel:

Verysuperkiasu:

Hi I'm using my mom's account to ask a question: Are algae considered plants or protoctista? I know that blue-green algae are considered monera but what about the other types of algae? Thanks in advance

Classification can be tough sometimes. The term algae refers to a wide range of organisms. I would never create a multiple choice question classifying algae as either a) plants or b) protoctists. If you got such a question though, go with protoctists. Depending on the species, I think you could argue either way for some. To give a simple answer, I think the most common ones, like green algae, red algae and euglenoids are most commonly classified as protoctists.

You can check out this link where they talk about the different kinds of protoctists. By the way \"protist\"means single cell and \"protoctist\"means single or multicellular although very simple multicellular forms (some algae are multicellular)

http://archives.microbeworld.org/microbes/protista/

Thanks for clarifying my doubts!

[Septima]

Past year papers are better!

[Happylady]

Hi, my child requested for a experienced 1 to 1 maths & sci tutor. Please recommend. He’s in sec 1 this year. I m staying in the east.

[Verysuperkiasu]

I have a chemistry question


In another thread, someone said to get the valency of polyatomic ions, like CO3 just add up the individual valencies of its components, that is, Carbon C has a valency of +4 and each Oxyen atom has a valency of -2. Hence valency of CO3 is +4+(-6)= -2.

If I apply this principle to NO3, it can’t work. N has valency of -3. Hence NO3 will be -3+(-6)= -9! But we know valency of NO3 is -1. Can anyone explain if this is the correct way to derive valency of polyatomic ions?

[Dr.033430Daniel]

Verysuperkiasu:

I have a chemistry question


In another thread, someone said to get the valency of polyatomic ions, like CO3 just add up the individual valencies of its components, that is, Carbon C has a valency of +4 and each Oxyen atom has a valency of -2. Hence valency of CO3 is +4+(-6)= -2.

If I apply this principle to NO3, it can't work. N has valency of -3. Hence NO3 will be -3+(-6)= -9! But we know valency of NO3 is -1. Can anyone explain if this is the correct way to derive valency of polyatomic ions?

Just think of the Nitrogen as +5 (rather than -3) as you thought of the carbon as +4 (rather than -4). Then it works.

For S04, if you count S as +6 and 4 O's at -8 then you get -2. So count the first one as a positive number.

This works for some of the common polyatomic ions, but unfortunately this method cannot be universally applied. It doesn't work for the polyatomic ions that end in \"ite\" rather than \"ate\". For example there is an ion called nitrite which is NO2 -1 and there is also Nitrate which is NO3 -1.

Its good to just know the valencies of the polyatomic ions when moving on in chemistry because you don't want to have to stop and figure them out.

[Verysuperkiasu]

Dr.Daniel:

Its good to just know the valencies of the polyatomic ions when moving on in chemistry because you don't want to have to stop and figure them out.

Thank you Dr Daniel. When my son comes across a Youtube video on the derivation of poly atomic ions, he realises the method doesn't always work on all polyatomic ions. My son is only going to learn atomic structure next year in sec 2. That's why he's reading up in advance and gets all confused about the derivation of valencies for polyatomic ions. (the valencies of single atomic ions are quite straightforward to work out).

Will the exam questions ask the students how these valencies are derived? Or they just have to memorise them? The purpose of knowing these valencies is to write chemical formulae of compounds right? Or are there other uses of knowing the valencies.

Thank you for your clarification!

[Dr.033430Daniel]

Most of the questions I have seen from Singapore school tests require you to know what the valencies are from memory. What you said is right - being able to write chemical equations etc.


But I have seen some of the IP schools going further into drawing diagrams which include ionic compounds. When I teach my Sec 2 class, I go through the logic of why some of the common polyatomic ions have certain valencies. It is interesting to compare CO2 which is neutral to CO3 which has a -2. If you draw out the molecule with all the electrons, it can be made clear why. Some of the schools do want the students to understand that logic in Sec 2.