O-Level Chemistry

[starlight1968sg]

No need to update science textbooks yet (due to the addition of 4 new elements)

http://www.straitstimes.com/singapore/e ... tbooks-yet

[questionable_i]

Can anyone help me with this question?

https://s.yimg.com/hd/answers/i/782ab88672d145368364af03a7ce7f99_A.jpeg?a=answers&mr=0&x=1452399457&s=8f3b990583b5dec8b7f2d5569c07b4c5\">

The other question is:

Which method can be used to obtain potassium nitrate crystals from a soloution of salt?
A. Chromatography
B. Simple Distillation
C. Evaporation to dryness
D. Crystallisation

:thankyou:

[kennethTutor]

questionable_i:

Can anyone help me with this question?

https://s.yimg.com/hd/answers/i/782ab88672d145368364af03a7ce7f99_A.jpeg?a=answers&mr=0&x=1452399457&s=8f3b990583b5dec8b7f2d5569c07b4c5\">

The other question is:

Which method can be used to obtain potassium nitrate crystals from a soloution of salt?
A. Chromatography
B. Simple Distillation
C. Evaporation to dryness
D. Crystallisation

:thankyou:

Hello. For the first question, it is about Simple Distillation of seawater and the answer should be (A).

Simple Distillation is used to separate a solid-liquid mixture. So for this question, we are separating water from other components in seawater in order to obtain pure water. Firstly in the distillation flask, water is heated to form steam through boiling. Steam then enters the condenser. Subsequently in the condenser, steam is condensed to form pure water. The salt and other compounds in seawater are left behind in the round-bottomed flask.

Let's examine the three points X, Y and Z.

Pure water has a boiling point of 100°C, which means it changes to steam at 100°C. However, due to the presence of impurities in seawater, the boiling point of seawater is higher than 100°C. Therefore the temperature at X is not 100°C.
(Key Concept: Impurities lower melting point and increases boiling point)

Point Y, which is near the opening of the condenser, is the point where pure steam is about to enter the condenser. Hence the temperature at Y is 100°C.
(In textbooks and notes, typically there will be a thermometer at Y with the bulb placed near the opening of the condenser. The purpose of the thermometer is to measure the boiling point of the substance entering the condenser.)

At Z, steam is condensed to form water in the condenser. Therefore the temperature is not 100°C.


For the 2nd question, we use crystallisation (Option D). Potassium nitrate crystals are pure soluble solid. Generally there are two ways to obtain soluble solid from a solution, namely Evaporation to Dryness and Crystallisation. If the solid is heat-stable, we use Evaporation to Dryness. If the solid is not heat-stable, we use Crystallisation.

If we use Evaporation to Dryness to obtain a compound which is not heat stable, we run the risk of decomposing the compound.

Since we are unsure if Potassium Nitrate is heat-stable, we use crystallisation to obtain potassium nitrate crystals.

I am guessing you are Sec 3 this year. You will learn more about Heat Stability of metal compounds in the topic Metals.

[CatMoon]

I'm looking to enroll my dd for practical lab lessons (chemistry) at the Singapore Institute of Science near Boon Keng mrt. Wanted her to get lab experience before taking the O-levels this year.


Does anyone have any insights into SIS?

:please:

[tutor76]

try all

in my Collinphysicschem.blogspot.sg

[CatMoon]

CatMoon:

I'm looking to enroll my dd for practical lab lessons (chemistry) at the Singapore Institute of Science near Boon Keng mrt. Wanted her to get lab experience before taking the O-levels this year.


Does anyone have any insights into SIS?

:please:

Hi, does anyone know of tuition centres offering practical Chemistry lab lessons for O-level students?

Thanks! :?:

[ms_lim]

spa is around the corner, just wondering how other students are preparing for it as there are no past year papers to reply on. only know fr ds its on metals.

[hey.011088joane]

S

[hey.011088joane]

ms_lim:

spa is around the corner, just wondering how other students are preparing for it as there are no past year papers to reply on. only know fr ds its on metals.

Spa questions are set by the examination board but questions are chosen by school teacher. It will be good if your child can look through at what his/her teacher had gone through and listen very attentively in class (some teacher might drop hints). Most of the time, questions doesn't differ too much from what is given in class.

Ms Joane
O Level Physic and chemistry tutor

[tutor76]

edited version