Lower Secondary Science
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Past year papers are better!
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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.
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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? -
Verysuperkiasu:
Just think of the Nitrogen as +5 (rather than -3) as you thought of the carbon as +4 (rather than -4). Then it works.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?
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. -
Dr.Daniel:
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).
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.
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! -
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. -
Dr.Daniel:
Noted and thank you, Dr Daniels!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.
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Does anyone know if there will be a change in the syllabus for lower and upper secondary science for next year?
I am thinking of buying books in advance for my kids.
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Yes the Secondary 1 Syllabus is being updated for the year 2013. One major textbook that will be in Popular is Publisher: Pearson, Title: All About Science Volume A & B. I have just purchased these two volumes and in reading through them, my first impression is a good one. They are not very wordy. There are lots of pictures, so that students who are very visual should enjoy them. In terms of content, they provide a good foundation in physics, chemistry and biology, although some of the IP and top schools go further than the content of these books. Also, for problems involving mathematics, I have been seeing harder problems than the examples in these books. But as a starting point for a new Sec 1 student, they look good to me.
Another book that will be in use by some schools is Publisher: Marshall Cavendish, Title: Lower Secondary Science Matters (2nd edition). This book was not available yet today in Popular.
My understanding is that for most schools, the 2013 textbooks apply to Sec 1, while Sec 2 continues with the textbooks they have used last year. This is because most of the lower secondary books are written as a two year series covering both Sec 1 and 2.
For Sec 3 there will be some new books out as well. The Physics Matters from Marshall Cavendish isn’t out yet but there will be a new book for 2013.
So you may want to check with your school on which textbook they intend to use for the 2013 year, but for Sec 1, I will be recommending the Pearson books during our December holiday classes for students to read to get ahead.
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