The aim of this series “Secrets to Score in Acids, Bases, Salts & Qualitative Analysis” is to help students to the understand the basic concepts surrounding these 2 topics: Acids, Bases & Salts and Qualitative Analysis, and to take a look at the type of questions that are usually associated with it in exams.

This blog-post will be the last post of this typical series,  and how about checking another typical type of exam-based questions in exams.

Question:
The following excerpts were taken from a student’s chemistry journal.

“A colourless gas P produced a white precipitate when bubbled into aqueous calcium hydroxide. Gas P was dissolved in water to form a colourless solution Q, which registers a pH of range of 5-6.

Solution Q was then added to a colourless barium solution, known as R. A white precipitate, S, in a colourless solution, T, was observed. Upon filtering, the colourless filtrate, T, was added into a test tube containing aqueous sodium hydroxide and Devarda’s Alloy. The gas evolved turns moist red litmus paper blue.”

Identify the following unknowns – P, Q, R, S and T. (5 marks)

PS: Leave your suggested answers in the “Comments Section” and we will come in to discuss on it real soon…

This will be the second last blogpost on under the series of “Secrets of Acids, Bases & Salts and Qualitative Analysis revealed”, and it will be the section where either a student loves its or hates it totally.

Qualitative Analysis (QA) is essentially a chapter that is associated with the Practicals or SPAs that you do your in Chemistry Laboratories. It involves your skills in the Identification of Ions (Cations & Anions) and Gases.

Here is a screen shot on the learning outcomes that students need to know to do well in this topic:

(Click image to see properly)

Let’s get started with a typical exam-based questions that you will normally see – A Flow-Chart asking you to identify the unknowns.

(i) Identify P, Q, R , S and T. (5 marks)

(ii) In colourless solution R, identify the:
Cation Present _______________
Anion Prsent ________________
(2 marks)

(iii) Identify the compound A and element B (2 marks)

(Click image to see properly. You can then save it to your computer, analyze and solve the question step-by-step)

PS: Leave me your suggested answers (as many parts as possible that you can solve) in the Comments Section below, and i will get back to you all with a Last Blogpost under this series.

“Wanting Leads to Wanting, Doing Leads to Having”

Following is a question that was sent to me recently by a local students.
This is a question based on the Properties of Acids and is getting popular in many of the Preliminary Exams in local Top Secondary Schools.

Q )Sulphuric acid is a dibasic acid because:
A. each molecule contain 2 hydroxide ions
B. each molecule can produce 2 hydrogen ions
C. one mole of the acid contains two atoms of hydrogen that can be replaced by a metal
D. sulphuric acid can only be neutralized by 2 bases
E. sulphuric acid has the usual properties of an acid, but is also a dehydrating agent

Try working out the question before checking the answer and reasoning below. (more…)

 (Image by Aim and Shoot)

During the process of making cake, there is an important chemical reaction that is involved that determines if your cake will rise, aka, if your cake will look really like a “cake” – all puff up!

This is due to the chemical reaction between the Baking Soda (which contains Sodium Hydrogencarbonate) and the acid, to form carbon dioxide (CO2):

Sodium Hydrogencarbonate + Acid -> Sodium Salt + Carbon Dioxide + Water

Baking soda and the acidic component such as youghurt, lemon juice, vinegar or sour cream must be added and mixed into the pastry mix. Carbon dioxide is then released by the reaction, but gets trapped in the mix, and causes the cake to rise. This reaction produces your light and delicious pastries.

Many students have problem in this fundamental chapter on Acids, Bases & Salts. One of the basic concepts lacking is the determination & differentiation of Strong Acid Vs Weak Acid & Strong Base Vs Weak Base.

First, we need to understand that:

• Acid –> pH < 7
• Pure Water –> pH = 7
• Base/Alkali –> pH > 7

Next, we need to differentiate between Strong vs Weak of Acids/Alkalis using examples. Examples given in the chart below will help you to learn better. Copy & print the chart out and paste it in your note if necessary.
(Click image to see clearly)

A great question posted by a student earlier: 

Q) Use your knowledge of the nature of acids, bases and salts to explain the following observations or practices.

“A farmer tells his workers to correct the pH of soil by using solid calcium hydroxide. They should add fertilizers such as ammonium nitrate(V) to improve the soil fertility only after a lapse of several days.”

Try it out!

This is the answer given by Jennifer, the student that posted the question, and i agree totally with her. Well Done Jennifer!

“Adding solid calcium hydroxide is to lime the soil so as to reduce the acidity of the soil caused by acid rain. Ammonium nitrate(V) can only be added after a few days and not immediately because ammonium nitrate, an alkali salt would react with calcium hydroxide, an alkali to produce ammonia gas, calcium nitrate and water. Hence, it would not improve the soil fertility.”

PS: Please check the “Comments” section below for suggested answers

I had recevied a couple of emails from Secondary 5 Normal (Academic Students) asking for help to cope for their ‘O’ Level this year. Transiting from completing the N Levels to preparing for the O Levels in Sec 5 is not an easy journey indeed. This is also mentioned by Beetsma, the Singapore Top O-Level Performer among the Sec 5 cohort for 2007 GCE ‘O’ Level examinations.

As mentioned by Beetsma, “the gap is very wide because they teach us more in a shorter amount of time. The classes were getting harder and harder and I had difficulties.” In recent weeks, they has been controversies of cases that Sec 5 students attempting O levels are being “discouraged” by their school principal to transfer to the Institute of Technical Education (ITE) as they are unlikely to do well at ‘O’ Levels.

Even though the road to ‘O’ Level from ‘N’ Level is not easy, i personally felt the principal should have been more tactful in his speech. Perhaps, the principal (and thus the school) should have a meeting/breifing with parents and students together and let them know the options that are available. I remembered in my early school days, such “talks” on Options that are available, have always been very smooth and positive.

Having said that, i strongly urge Normal (Academic) Students to keep their focus and study hard. Starting from now, go learn all the fundamental concepts in Chemistry that you are not strong with. It is important to have a good understanding of the basic Chemistry Concepts in order to appreciate the Sec 5 chapters.

Following are some fundamental chapters In Chemistry that you need to master:

• Elements, Compounds & Mixtures
• Atomic Structure
• Bonding (Ionic, Covalent & Metallic)
• Writing Chemical Equations (including Ionic Equations) –> Most Students have problems
• Mole Concepts & Chemical Calculations –> Most Students have problems
• Acids, Bases & Salts
• The Periodic Table & Its Elements

You can do it! Start your revision early, so that you can find help. Do not revise on the 11th hour.

PS: Go download your Free Video on Tips for Balancing Chemical Equations on the right side of this webpage.

PSS: I will be sharing with you on questions that emphasize on Essential Chemistry Concepts in this post. Stay Tuned!