Tips for Chemistry Practicals (P3)

[sodiumchloride]

how to distinguish between solution that are soluble, insoluble, dissolves?

 

[Mohamed Suhail]

thanks a lot for these tips.also i would advise u do all the procedures and leave the calculations because god forbid if u dont have time to complete the paper, u probably missed up to 5 marks only in calculations while everything went alright.so u can still obtain high marks insha allah

 

[suhaib05]

When they give you a compound with an unknown identity, how do you know whether a gas will be released or not? And if a gas is released, how will you be able to decide which test to use? Especially if it's a colourless gas..

P.S: I'm not used to the smells of different gases, so I would honestly REALLY appreciate if someone could list the gases that they could ask us to test for, along with their colours (if applicable)!

 

[Mohamed Suhail]

if its colorless first try with a lighted splint to test o2 or h2. or else we could do the limewater test to test for co2

 

[Student12]

When they give you a compound with an unknown identity, how do you know whether a gas will be released or not? And if a gas is released, how will you be able to decide which test to use? Especially if it's a colourless gas..

P.S: I'm not used to the smells of different gases, so I would honestly REALLY appreciate if someone could list the gases that they could ask us to test for, along with their colours (if applicable)!

For ANIONS :
Whenever its a carbonate : CO2 is given off which turns lime water milky.
When its Sulphite : SO2 is given off which turns K2Cr2O7 from orange to green
For CATIONS:
Ammonium: NH3 produced which is a pungent colourless gas which even smells like Hair Dye.. on heating which turns moist red litmus Blue.
Nitrite: Colourless NO produces NO2 Its a brown gas formed.

Correct me if i'm wrong.

 

[Student12]

I have a question..
For example we have 0.017 moles of HCl.. how to calculate in 25 cm3 ?

 

[Cesar_malik]

Com

I have a question..
For example we have 0.017 moles of HCl.. how to calculate in 25 cm3 ?

comc x vol (cm3)/1000=mol
Rearrange it

 

[sufyanalizaib]

Enthalpy coming in tommorow's paper /////////////??????????

 

[Gimmick]

What do they mean by "1 cm depth"? Do they mean "1 cm cubed" or "1 cm from a test tube measured with a ruler and not with a syringe"?

 

[Nightmare***]

• Just keep you mind cool and read everything on the paper, mostly students just don't read long paragraphs of text carefully. Practicals are not that hard so try to score most here, the best solution to this is try as many practicals as many possible.
• Don't perform unnecessary titrations, if you did two and got results close to each other then there's no need for a third one.
• Always draw a triangle on the graph to find the gradient, if you are asked to find it.
• Latest salt analysis sections contain a trick with aluminium and lead(II) ions because they have the same results with ammonia and sodium hydroxide. The test that distinguished them is addition of HCL, it will form a precipitate with lead(II) ions but not with aluminum ions. (Lead(II) ions can be distinguished from aluminium ions by the insolubility of lead(II) chloride.)
• Always read carefully the names of the solutions before working with them, 1 small mistake and you might have to start from the beginning.
• Always heat solutions in a boiling tube and not the test tube.
• Make sure the burette is closed before adding solutions to it.
• If you're quick at experiments then I recommend washing the apparatus with distill water before using.(But only if you're sure you can complete the experiment in time.)
• Don't spend more than 45 mins on Salt Analysis.
• When you write the observation for some gas that is released, always include a confirmatory test. You should actually never perform these confirmatory tests (to save time) just use your knowledge of chemistry to find which gas is released but always include the test in the observations. E.g. when a carbonate is reacted with an acid, you should write the observation; "Colorless gas released which turns lime water milky."
• Remember some gases are hard to see when released in a test tube, so where you expect some gases to be released, look carefully for the observation. A good idea is to use the white tile or if you're not provided with it then use your paper as the background when looking at the test tube.
• Try to memorize the smell of ammonia. I know its very disturbing to smell it but you need to identify it from others when they ask you so just smell it enough that you can distinguish it form other gases.
• In the paper you want both speed and accuracy, so do the easy parts fast like pouring solutions and stuff and use the time saved on important parts like observing changes and stuff.
• In the exam if you think some solution has some impurity or you put something into it by mistake then just ask them to replace it, don't use it.
• Remember to not to use the same dropper for two different solutions.
• If some solution involves heating and then heating strongly then you really need to heat it strongly so don't write no change too fast.
• Always wash the thermometer before using, and make sure its working before using, when not dipped into any solution, it should give a constant reading of 25-30 °C.
• Some solutions need shaking to mix them and sometime you will need to shake it really hard to work it out. Just wash your hands and close the top of the test-tube with your thumb and shake.
• Be careful with the amount of solutions you use, you need to finish all the experiments in the amount of solutions provided don't think that you can get more if you're empty because some examiners might be strict.
• Try to work in a clean and objective oriented environment, what I mean is when you're working with 1 part of the paper then just keep the solutions needed for that part to yourself and keep the others away. This helps a lot.
• Always draw tables(similar to how they give you in the questions) to represent results and observations, don't just write them down in lines.
• One last thing and this is really important; understand the question before doing it, if you can't then try drawing diagrams on a paper to help you visualize what you are going to do and if you still don't get it then skip that part you can do it in the end unless it is related to any other part, if so then just try to copy whatever is instructed in the question.

If you have anything else to add feel free to do so. =)

EDIT [24/05/2012]​

Tips for drawing the graphs:

• Think carefully before choosing scales for the axes, you need to keep in mind that the scale needs your values to cover at least half of the page and should is sensible, by that I mean you should be able to easily find values on the axes without needing to calculate what value some point on the axes represent.
• Sharp your pencil, the last thing you want is to lose marks because your line was too thick.
• Double check when marking the points, 1 mistake and in some cases you might mess up the whole graph.
• "Line of best fit." Doesn't necessarily mean that its a straight line. Although rarely, it could be a curve too that fits most of the points.
• In graphs of experiments like heating and then cooling, you need to draw two lines, 1 representing cooling and the other one representing heating instead of a curve.
• Whenever they ask you to find gradient, ALWAYS draw a triangle, and the bigger the triangle, the more accurate your gradient.
• If you have doubt about your gradient, just take any point that is close to the line and find (y/x), this value should be very close to the gradient found using the triangle. But remember this only works when the graph is starting from origin or near it.
• Try to never break your graph but if you do, REMEMBER that the y-intercept needs to be calculated using the equation "y = mx + c" where m is the gradient you found, c is the y-intercept and can be found using the y and x value of any point on the lane.
• Try to keep your graph clean and visible. If you need multiple lines to find gradient and other value, make sure they're distinguishable.
• ALWAYS write what each axis represents with its unit.
• To determine which quantity goes on the x and y axis, you need to find which quantity is dependent and which one is independent. Independent is usually the one you are changing and dependent is the one you measure.
• If you draw multiple graphs, make sure to label them.
• Don't spend more than 15 minutes on the graph. The graph is usually simple, and if its not then you've done probably something wrong and instead of wasting time on it, try to score marks in other parts first and then come in the end to figure out what's the problem with the graph.
• If you're good at salt analysis, then do it first and finish it fast so you can do the graphing without any tension.
• REMEMBER even if you draw the graph wrong due to some wrong measurements or something, don't lose hope on it, you can still workout the rest of the questions for the wrong graph and score most of those marks like gradient and stuff.
• When they ask you to prove some relationship like T=kV or something, write that since the graph is a straight line, the value of the K is constant and therefore the relationship is true...unless emm...your line is not straight.
• If you don't have time to draw the graph, try to do as much as you can like plotting two or three values and then making the line instead of all the points or even writing the quantities on the axes can gain your marks.

Awesome advice ....really helps....but 1 ambiguity !....... how r we supposed to know when to draw a best fit line as straight or when to draw it curved...sometimes u get points that allow u to draw the graph either ways

 

[ShaanSiddiq090909]

For the qualitative analysis, do we have to memorize all the tests for gases, compounds, anions etc?

 

[ShaanSiddiq090909]

For

Can please somebody explain how to identify nitrite and nitrate ion in unknown compounds

For nitrate: 1) Add aq.NaOH and Aluminium powder to the sample; Pungent gas liberates which turns moist red litmus into blue. (NH3)
For Nitrite: 1) Add dilute HCl to the sample; Solution turns blue and a pale brown gas liberates (NO2)
2) Add aq.NaOH and Al powder to sample and heat carefully; Pungent gas evolves (NH3)

 

[Nightmare***]

What do they mean by "1 cm depth"? Do they mean "1 cm cubed" or "1 cm from a test tube measured with a ruler and not with a syringe"?

if ur talking about the salt analysis question it doesnt really matter...u dnt actally need to measure it.....its just an estimate of the depth of solution in test tube u should use each time ...dnt waste ur time measuring the depth of solution in the test tube....ull regret it

 

[Malik777]

if ur talking about the salt analysis question it doesnt really matter...u dnt actally need to measure it.....its just an estimate of the depth of solution in test tube u should use each time ...dnt waste ur time measuring the depth of solution in the test tube....ull regret it

(Y) you 're right dude

 

[Porhour]

if its colorless first try with a lighted splint to test o2 or h2. or else we could do the limewater test to test for co2

But when a gas is released, in order to test for it, we would have to close the test tube, right? So, do we have to close the test tube the whole time? or carry out the experiment again once the 1st one shows that the solution is colorless? Do we have to do the experiments again and again and each time test with litmus papers ,lighted split, glowing splint?

 

[Porhour]

• Just keep you mind cool and read everything on the paper, mostly students just don't read long paragraphs of text carefully. Practicals are not that hard so try to score most here, the best solution to this is try as many practicals as many possible.
• Don't perform unnecessary titrations, if you did two and got results close to each other then there's no need for a third one.
• Always draw a triangle on the graph to find the gradient, if you are asked to find it.
• Latest salt analysis sections contain a trick with aluminium and lead(II) ions because they have the same results with ammonia and sodium hydroxide. The test that distinguished them is addition of HCL, it will form a precipitate with lead(II) ions but not with aluminum ions. (Lead(II) ions can be distinguished from aluminium ions by the insolubility of lead(II) chloride.)
• Always read carefully the names of the solutions before working with them, 1 small mistake and you might have to start from the beginning.
• Always heat solutions in a boiling tube and not the test tube.
• Make sure the burette is closed before adding solutions to it.
• If you're quick at experiments then I recommend washing the apparatus with distill water before using.(But only if you're sure you can complete the experiment in time.)
• Don't spend more than 45 mins on Salt Analysis.
• When you write the observation for some gas that is released, always include a confirmatory test. You should actually never perform these confirmatory tests (to save time) just use your knowledge of chemistry to find which gas is released but always include the test in the observations. E.g. when a carbonate is reacted with an acid, you should write the observation; "Colorless gas released which turns lime water milky."
• Remember some gases are hard to see when released in a test tube, so where you expect some gases to be released, look carefully for the observation. A good idea is to use the white tile or if you're not provided with it then use your paper as the background when looking at the test tube.
• Try to memorize the smell of ammonia. I know its very disturbing to smell it but you need to identify it from others when they ask you so just smell it enough that you can distinguish it form other gases.
• In the paper you want both speed and accuracy, so do the easy parts fast like pouring solutions and stuff and use the time saved on important parts like observing changes and stuff.
• In the exam if you think some solution has some impurity or you put something into it by mistake then just ask them to replace it, don't use it.
• Remember to not to use the same dropper for two different solutions.
• If some solution involves heating and then heating strongly then you really need to heat it strongly so don't write no change too fast.
• Always wash the thermometer before using, and make sure its working before using, when not dipped into any solution, it should give a constant reading of 25-30 °C.
• Some solutions need shaking to mix them and sometime you will need to shake it really hard to work it out. Just wash your hands and close the top of the test-tube with your thumb and shake.
• Be careful with the amount of solutions you use, you need to finish all the experiments in the amount of solutions provided don't think that you can get more if you're empty because some examiners might be strict.
• Try to work in a clean and objective oriented environment, what I mean is when you're working with 1 part of the paper then just keep the solutions needed for that part to yourself and keep the others away. This helps a lot.
• Always draw tables(similar to how they give you in the questions) to represent results and observations, don't just write them down in lines.
• One last thing and this is really important; understand the question before doing it, if you can't then try drawing diagrams on a paper to help you visualize what you are going to do and if you still don't get it then skip that part you can do it in the end unless it is related to any other part, if so then just try to copy whatever is instructed in the question.

If you have anything else to add feel free to do so. =)

EDIT [24/05/2012]​

Tips for drawing the graphs:

• Think carefully before choosing scales for the axes, you need to keep in mind that the scale needs your values to cover at least half of the page and should is sensible, by that I mean you should be able to easily find values on the axes without needing to calculate what value some point on the axes represent.
• Sharp your pencil, the last thing you want is to lose marks because your line was too thick.
• Double check when marking the points, 1 mistake and in some cases you might mess up the whole graph.
• "Line of best fit." Doesn't necessarily mean that its a straight line. Although rarely, it could be a curve too that fits most of the points.
• In graphs of experiments like heating and then cooling, you need to draw two lines, 1 representing cooling and the other one representing heating instead of a curve.
• Whenever they ask you to find gradient, ALWAYS draw a triangle, and the bigger the triangle, the more accurate your gradient.
• If you have doubt about your gradient, just take any point that is close to the line and find (y/x), this value should be very close to the gradient found using the triangle. But remember this only works when the graph is starting from origin or near it.
• Try to never break your graph but if you do, REMEMBER that the y-intercept needs to be calculated using the equation "y = mx + c" where m is the gradient you found, c is the y-intercept and can be found using the y and x value of any point on the lane.
• Try to keep your graph clean and visible. If you need multiple lines to find gradient and other value, make sure they're distinguishable.
• ALWAYS write what each axis represents with its unit.
• To determine which quantity goes on the x and y axis, you need to find which quantity is dependent and which one is independent. Independent is usually the one you are changing and dependent is the one you measure.
• If you draw multiple graphs, make sure to label them.
• Don't spend more than 15 minutes on the graph. The graph is usually simple, and if its not then you've done probably something wrong and instead of wasting time on it, try to score marks in other parts first and then come in the end to figure out what's the problem with the graph.
• If you're good at salt analysis, then do it first and finish it fast so you can do the graphing without any tension.
• REMEMBER even if you draw the graph wrong due to some wrong measurements or something, don't lose hope on it, you can still workout the rest of the questions for the wrong graph and score most of those marks like gradient and stuff.
• When they ask you to prove some relationship like T=kV or something, write that since the graph is a straight line, the value of the K is constant and therefore the relationship is true...unless emm...your line is not straight.
• If you don't have time to draw the graph, try to do as much as you can like plotting two or three values and then making the line instead of all the points or even writing the quantities on the axes can gain your marks.

For gas tests, what if we have no idea about the reactants ( they are unknown) then we really have to carry out the tests right?

 

[MindStealth]

Ok I'm going to write a single post about answering all your questions about testing gases.

Porhour
So first of all, believe it or not, in my 2 years of A Levels, not a single time I had to use any of the tests for gases, no litmus-paper test, no limewater test and neither any other. I solved enough papers in 2 years to confidently tell you that you don't have to do the tests. ALWAYS it would be stated in the question, there will be always some hint to what the resulting gas is going to be. I can't remember of specific examples but trust me they're never going to give you unknown chemicals which you have no idea about and ask you to identify the gas released. One way to familiarize yourself with such hints is by doing this:

1. Find papers that involve testing gases.
2. Check the answer from the marking scheme and write down the gas released on the paper.
3. Try to trace back, i.e. try to figure out what part of the question gave the hint about it. Its almost always a clear hint, not something hidden in words.

An example like I already gave was regarding carbonates, or the reactions from the tests for cations and anions sheet which clearly have ammonia released in them. If there's no hint in the question, then there would be some obvious property of the gas, like the smell of ammonia or something like the color of gas. Unfortunately I cannot give specific answers from past papers because its been a year since I last touched a chemistry book.
Just to give you guys a perspective, my Chemistry teacher didn't even provide us these tests like lime water, litmus papers, etc. even in the exams. Out of all the students in the class, I don't recall a single person ever struggling with that because it's really simple regardless of how good you are at chemistry. This also answers you question about closing the test tube, you don't have to because you're not going to do the tests.

If you want my advice, don't get used to doing these tests. You can't afford to spend more than a few minutes on such parts of the questions because they're usually just a few marks. Try using common sense and this is going to be the least of your problems in the practical. Yes, they've taught all the tests as part of the syllabus but you have to decide for yourself if its worth 2 or 3 marks to do those tests. Just to sum it up, 99% of the times you don't have to perform the tests for gases.
Someone asked about taking one cm depth of solutions, that means just do it approximately, if you really don't understand then just take the width of your thumb or something, it really won't change your results. Taking little makes it faster to find the changes that involve adding extra solution to dissolve or something and also it saves the solutions, which are pretty expensive.
And no you don't have to memorize those tests for cations and anions, they will be given to you; just look at pastpapers -__-

 

[Porhour]

Ok I'm going to write a single post about answering all your questions about testing gases.

Porhour
So first of all, believe it or not, in my 2 years of A Levels, not a single time I had to use any of the tests for gases, no litmus-paper test, no limewater test and neither any other. I solved enough papers in 2 years to confidently tell you that you don't have to do the tests. ALWAYS it would be stated in the question, there will be always some hint to what the resulting gas is going to be. I can't remember of specific examples but trust me they're never going to give you unknown chemicals which you have no idea about and ask you to identify the gas released. One way to familiarize yourself with such hints is by doing this:

1. Find papers that involve testing gases.
2. Check the answer from the marking scheme and write down the gas released on the paper.
3. Try to trace back, i.e. try to figure out what part of the question gave the hint about it. Its almost always a clear hint, not something hidden in words.
   

An example like I already gave was regarding carbonates, or the reactions from the tests for cations and anions sheet which clearly have ammonia released in them. If there's no hint in the question, then there would be some obvious property of the gas, like the smell of ammonia or something like the color of gas. Unfortunately I cannot give specific answers from past papers because its been a year since I last touched a chemistry book.
Just to give you guys a perspective, my Chemistry teacher didn't even provide us these tests like lime water, litmus papers, etc. even in the exams. Out of all the students in the class, I don't recall a single person ever struggling with that because it's really simple regardless of how good you are at chemistry. This also answers you question about closing the test tube, you don't have to because you're not going to do the tests.

If you want my advice, don't get used to doing these tests. You can't afford to spend more than a few minutes on such parts of the questions because they're usually just a few marks. Try using common sense and this is going to be the least of your problems in the practical. Yes, they've taught all the tests as part of the syllabus but you have to decide for yourself if its worth 2 or 3 marks to do those tests. Just to sum it up, 99% of the times you don't have to perform the tests for gases.
Someone asked about taking one cm depth of solutions, that means just do it approximately, if you really don't understand then just take the width of your thumb or something, it really won't change your results. Taking little makes it faster to find the changes that involve adding extra solution to dissolve or something and also it saves the solutions, which are pretty expensive.
And no you don't have to memorize those tests for cations and anions, they will be given to you; just look at pastpapers -__-

Ok I'm going to write a single post about answering all your questions about testing gases.

Porhour
So first of all, believe it or not, in my 2 years of A Levels, not a single time I had to use any of the tests for gases, no litmus-paper test, no limewater test and neither any other. I solved enough papers in 2 years to confidently tell you that you don't have to do the tests. ALWAYS it would be stated in the question, there will be always some hint to what the resulting gas is going to be. I can't remember of specific examples but trust me they're never going to give you unknown chemicals which you have no idea about and ask you to identify the gas released. One way to familiarize yourself with such hints is by doing this:

1. Find papers that involve testing gases.
2. Check the answer from the marking scheme and write down the gas released on the paper.
3. Try to trace back, i.e. try to figure out what part of the question gave the hint about it. Its almost always a clear hint, not something hidden in words.

An example like I already gave was regarding carbonates, or the reactions from the tests for cations and anions sheet which clearly have ammonia released in them. If there's no hint in the question, then there would be some obvious property of the gas, like the smell of ammonia or something like the color of gas. Unfortunately I cannot give specific answers from past papers because its been a year since I last touched a chemistry book.
Just to give you guys a perspective, my Chemistry teacher didn't even provide us these tests like lime water, litmus papers, etc. even in the exams. Out of all the students in the class, I don't recall a single person ever struggling with that because it's really simple regardless of how good you are at chemistry. This also answers you question about closing the test tube, you don't have to because you're not going to do the tests.

If you want my advice, don't get used to doing these tests. You can't afford to spend more than a few minutes on such parts of the questions because they're usually just a few marks. Try using common sense and this is going to be the least of your problems in the practical. Yes, they've taught all the tests as part of the syllabus but you have to decide for yourself if its worth 2 or 3 marks to do those tests. Just to sum it up, 99% of the times you don't have to perform the tests for gases.
Someone asked about taking one cm depth of solutions, that means just do it approximately, if you really don't understand then just take the width of your thumb or something, it really won't change your results. Taking little makes it faster to find the changes that involve adding extra solution to dissolve or something and also it saves the solutions, which are pretty expensive.
And no you don't have to memorize those tests for cations and anions, they will be given to you; just look at pastpapers -__-

Thanks a lot.. This is the very first and last time I am gonna sit for A levels

 

[MindStealth]

I'm not sure when you guys have the paper but BEST OF LUCK!
May Allah help you and us all. I'm giving AICT full A Levels from scratch this month so pray for me

 

[snowbrood]

I'm not sure when you guys have the paper but BEST OF LUCK!
May Allah help you and us all. I'm giving AICT full A Levels from scratch this month so pray for me

i cant figure the test of oxygen with match sticks there is not really much happening i dont see difference when i put the match into the tube can u tell me what to do