A student recently reached out to me via whatsapp asking me to help with this Qualitative analysis question. Since the O Level exams are fast approaching (EOYs for sec 3s), I thought I’d very quickly show again how to approach and solve O Level chemistry QA questions.
Lets start with A.
A turns litmus solution red –> acid.
When Barium Nitrate is added (this is to test for the presence of sulfates), it gives a white ppt, Barium Sulfate (hence sulfates present!)
Hence A must be sulfuric acid!
and white precipitate E must be barium sulfate!
Next, let’s try the gases – should be simple, afterall there are only 5!
When zinc is added to A, a gas D is produced.
Note – whenever a metal (zinc) undergoes a reaction, HYDROGEN gas will be produced! Hence D is hydrogen gas!
Gas C gives a white precipitate with limewater –> gas C must be carbon dioxide gas!
Lets go to B.
B is also an acid. (litmus turns red)
B produces potassium chloride when it reacts with F.
(note: if hydrochloric acid is used… the salt will be a metal chloride. If sulfuric acid is used, the salt will be a sulfate. If nitric acid is used, the salt will be a nitrate!)
Hence B is hydrochloric acid!
Lastly, lets try to figure out what F is.
Since B undergoes a neutralisation reaction with F, F must be a base – since its soluble, it must be an alkali!
There are only 3 commonly used alkalis (sodium hydroxide, potassium hydroxide & aq. ammonia.)
Hence F must be potassium hydroxide! (since potassium chloride is formed!)
I hope this has helped!
Jeremy is one of Singapore’s leading O Level Chemistry coaches and founder of The Classroom. He has coached over 10000 hours, close to a 1000 students from over 60 secondary schools in Singapore. Over the last 7 years, he has worked with students from a wide spectrum of backgrounds, from the F9 “i dont understand what’s going on” student to the A1 “I want to top my class” student. His ultimate goal is to show students that chemistry can be made easy and learning it can be fun.