In this lab session, our objective was to find out the properties of acids, bases and neutral solutions through several experiments.
Apparatus and Materials
- 6 test tubes
- Small beaker
- Glass rod
- Droppers
- Electrical conductivity tester
- Distilled water
- Litmus papers (both red and blue)
- Universal Indicator
- Methyl Orange
- Phenolphthalein
- 'Mysterious' aqueous solutions named A, B, C, D, E and F.
Procedures:
Using a glass rod,, place a drop of the solution onto a piece of:
(i) Blue litmus paper
Observations:
A and B turned the litmus paper red, while the litmus paper remained the same colour for all the rest of the solutions.
(ii) Red litmus paper
Observations:
E and F turned the litmus paper blue, whereas the litmus paper remained blue for all the rest of the solutions.
Inference:
A and B are acids, while E and F are alkalis, because acids turn litmus paper red while alkalis turn litmus paper blue. C and D are probably neutral solutions as no change in colour was observed when tested with litmus paper.
Electrical Conductivity Test
Observations:
C did not light up the bulb, A and F lit it only dimly while B, D and E made the bulb shine brightly.
Inference:
B, D and E are good conductors of electricity, A and F are poor conductors of electricity and C is somewhat of an electrical insulator.
This suggests that B is a strong acid and E is a strong alkali as strong acids and alkalis dissociate completely in water to form H+ and OH- ions respectively, making them good electrical conductors.
The rest of the procedures were to test the solutions using several pH indicators. The pH scale is used to determine whether a substance is a strong/weak acid, neutral solution or a strong/weak alkali.
 |
| pH scale with examples of substances which fall into each of the pH categories. Note that pH 0 represents the most acidic substances while pH 14 represents the most alkaline ones, with neutral substances falling in the middle (pH 7). |
pH indicators we used in the practical:
|
Indicator
|
pH Range
|
Acid
|
Base
|
|
Methyl orange
|
3.1-4.4
|
red
|
orange
|
|
Phenolphthalein
|
8.0-10.0
|
colorless
|
red
|
|
Bromothymol blue
|
6.0-7.6
|
yellow
|
blue
|
I felt that this lab session was rather fascinating as I could learn about properties of acids and alkalis in a hands-on approach to the extent that I got the opportunity to taste ethanoic acid (which is actually found in vinegar) and feel dilute sodium hydroxide (it had a soapy texture). I also found out that many acids and alkalis can be found in the household, such as milk and orange juice, which are apparently acids, and soap and toothpaste, which are alkalis. This is an example of how Science can be relevant even in our everyday lives, as we rely on the type of alkali metal used to make soap to determine whether the soaps are firm or have a liquid-like feel. All in all, an enjoyable lab session which I learnt quite a bit from.
2P02: When Dilute Acids Meets Metals
The aim of this practical was to investigate the action of dilute acids on metals.
Apparatus and Materials:
- 4 test tubes
- Test tube rack
- Bunsen burner
- Lighter
- Dilute hydrochloric acid
- Magnesium ribbon
- Zinc powder
- Iron filings
- Copper turnings
- Wooden splint
Procedure:
- Using the dropper, add dilute hydrochloric acid into a clean test tube to a depth of 3 cm. Take care to ensure that the acid does not touch the rim.
- Drop a piece of magnesium ribbon into the test tube of dilute acid.
- Quickly close the mouth of the test tube with your thumb for a few minutes.
- Light a wooden splint and hold it at the mouth of the test tube. Observe what happens. Place a piece of damp red litmus paper and a piece of damp blue litmus paper over the mouth of test-tube.
- Repeat steps 1 to 4 using iron filings, zinc powder and copper turnings.
Observations:
When the metal was added to the acid, effervescence of a colourless, odourless gas was released for magnesium, iron and zinc. No visible change was observed for copper.
When the gas was tested on a burning splint, a 'pop' sound was heard. The colour of the damp litmus paper (both red and blue) remained the same when tested with the gas.
Inference:
The gas is hydrogen gas. I can infer this from the fact that the burning splint emitted a 'pop' sound, which suggests the presence of hydrogen gas as it is very flammable. The reason behind the colour of the litmus papers not changing is because the hydrogen gas produced is neutral.
Conclusion:
Dilute acids react with reactive metals to produce hydrogen gas. (Acid + Metal --> Salt + Hydrogen)
Less reactive metals like copper will not react with dilute acids. For instance, metals like gold, silver and platinum are very nonreactive and thus very suitable for jewellery as they do not rust/corrode easily.
Reactivity table:
 |
| Metals above copper can react with dilute acids to form a salt and hydrogen gas, whereas those below do not even react with dilute acids at all. |
I found this practical to be a very enriching one as I learnt about the reactivity table and how to test for hydrogen gas. I also learnt about the reaction between dilute acids and reactive metals. From this practical I realised the reason behind several 'ordinary' things like why people select silver, gold and platinum: Not just because of their attractive appearance but also because of their lack of reactivity being a large factor. This shows that the knowledge of Science aren't just confined to the laboratory, but can be applied on 'normal' objects as well.
2P03: When Dilute Acid Meets Carbonates...
In this practical, we were to investigate the action of dilute acids on carbonates.
Apparatus and Materials:
- 4 test tubes
- Test tube rack
- Stopper with a delivery tube attached
- Spatula
- Dilute hydrochloric acid
- Dilute sulfuric acid
- Calcium carbonate
- Potassium carbonate
- Limewater
Procedure:
- Put 1-2cm of limewater into a clean test-tube. Place the delivery tube into the limewater.
- Pour dilute hydrochloric acid into another clean test tube to a depth of 3cm.
- Add 1 flat spatula of calcium carbonate into the acid and quickly close the mouth of the test tube with a stopper carrying a delivery tube. Avoid getting acid on the rim.
- Observe what happens.
- Remove the stopper carrying the delivery tube and test the gas with damp litmus paper.
Observations (Reaction between hydrochloric acid and calcium carbonate):
When the carbonate is added to the acid, effervescence of a colourless, odourless gas is released. When the gas is exposed to limewater, a white precipitate is formed. When exposed to moist blue litmus paper, the paper turned red, and when exposed to red litmus paper, no change in colour was observed.
Conclusion:
When dilute acid reacts with a carbonate, salt, water and carbon dioxide are produced.
Acid + Carbonate --> Salt + carbon dioxide + water
Carbon dioxide is acidic (turned blue litmus paper red).
This practical reminded me of my primary school days when there were a lot of questions concerning limewater. However, in secondary school, when I used the word 'chalky' to describe the appearance of the limewater, I was penalised as the appropriate term was that a white precipitate would be formed. This showed me that in Secondary School, it is very essential to use the scientific terminology lest marks be deducted. From this practical I learnt how to remove carbon dioxide gas from air, which is to simply bubble the sample of air through limewater and trap the carbon dioxide in the air, mainly leaving behind nitrogen and oxygen. In addition, I had the opportunity to practise writing word and chemical equations which was rather tricky for me as I was not very good at balancing the equations, yet I enjoyed the challenge. A fun practical which also taught me several new things :)
2P04: When Acid Meets Alkali
Investigating the action of an acid on an alkali was the objective for this practical.
Apparatus and Materials:
- Evaporating dish
- Dropper
- Glass rod
- Measuring cylinder
- Wire gauze
- Tripod stand
- Bunsen burner
- Dilute hydrochloric acid
- Dilute sodium hydroxide solution
- Red litmus paper
Procedure:
- Using a measuring cylinder, pour 10ml dilute hydrochloric acid into an evaporating dish.
- Dip a piece of red litmus paper into the acid by the side of the dish.
- Using a dropper, add about 10ml dilute sodium hydroxide solution, drop by drop, to the acid until the red litmus paper just turns blue. Stir the mixture with a glass rod as you add the alkali.
- Remove the litmus paper. Evaporate the mixture to the point where crystals can be seen forming over a Bunsen flame.
- Allow the evaporating dish to cool. Then observe the colour and taste of the solid left in the dish.
Observations:
The solution was colourless. The litmus paper turned from red to blue. The crystals formed were white in colour.
Conclusion:
When acid is added to an alkali, water and salt are formed.
Acid + Alkali --> Water + Salt
In the previous practicals I had learnt about the properties of acids and alkalis, and was curious about what would happen if one put them together. In this practical, my questions were answered. Apparently, acid-alkali reactions are classified under a type of neutralisation reaction, and this has many practical applications. For instance, alkalis like calcium hydroxide (or slaked lime) are commonly used by farmers when the soil on the farms used for crops gets too acidic, and they use an alkali to reduce the acidity level so as to allow the crops to grow better. Once again, a proof of the more 'everyday' applications of Science outside the laboratory. Like the previous practical, we had to practise writing chemical equations, but this time it was for reactions between an acid and an alkali. The mental exercise was stimulating and overall I found this practical to be an enlightening experience.
2P05: When Alkali Meets Ammonium Salts...
The purpose of this practical was to investigate the action of alkalis on ammounium salts.
Apparatus and Materials:
- 3 test tubes
- Test tube holder
- Test tube rack
- Spatula
- Bunsen burner
- Lighter
- Dilute sodium hydroxide
- Dilute calcium hydroxide
- Aqueous ammonia
- Ammonium chloride
- Ammounium nitrate
- Red and blue litmus papers
Procedure:
- Pour dilute sodium hydroxide into a clean test tube to a depth of 3cm.
- Add a spatula of ammonium chloride into the test tube of the alkali.
- Hold both pieces of damp red and blue litmus papers over the mouth of the test tube. Warm the mixture gently over a Bunsen flame. Observe (i) the colour change of the litmus papers and (ii) the colour and odour of the gas given off.
- Repeat steps 1 to 3 using ammonium nitrate and calcium hydroxide.
- Record your observations.
Observations:
Sodium hydroxide + ammonium chloride
Colourless, pungent gas given off, which turned red litmus paper blue and remained blue when exposed to blue litmus paper.
The same observations were recorded for calcium hydroxide added to ammonium nitrate.
Conclusion:
Alkali reacts with ammonium salt to produce salt, ammonia and water.
Ammonium salt + alkali --> Salt + Ammonium + Water
Ammonia gas is alkaline (turns red litmus paper blue).
An alkali only shows its alkaline properties in the presence of water (hence the need for moistening the litmus paper).
I found this practical to be engaging despite the stench of ammonia during the experiment (which caused many students to recoil in disgust). From this practical I learnt how to confirm if a substance was an ammonium salt (dissolving it in sodium hydroxide/calcium hydroxide, then observing the gas released to see whether it was ammonia), the negative effects of nitrogenous fertilizers with calcium hydroxide (drastic rise in pH level disallowing plants from growing well) and once again did some chemical equations.
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