Reactions of acids and using Universal Indicator at school PXL 20220121

High School Chemistry
Using Universal Indicator paper to measure the pH of four solutions and two reactions with dilute hydrochloric acid.
Questions
Can you identify the four solutions whose labels have fallen off in the video?
Can you identify the names of the gases produced during the two reactions shown for dilute hydrochloric acid?
What are the tests for hydrogen gas and carbon dioxide gas on the GCSE Chemistry specification?
Further questions
What would you expect to see when dilute hydrochloric acid is added to dilute sodium hydroxide solution? How could you show that a reaction occurs? Is there a heat change associated with this type of reaction? What is it?
Answers at the end of the description.
Universal Indicator paper was used to measure the pH of four colourless solutions. These were known to be either hydrochloric acid, sodium hydroxide, ethanoic acid or ammonia solution. All of the solutions were approximately 0.1M concentration.
Dilute hydrochloric acid was reacted with a small piece of magnesium ribbon in a test-tube. The gas produced was tested by trapping it in the test-tube and then introducing it to an ethanol spirit burner flame.
The gas should be tested with a burning wooden splint.
Dilute hydrochloric acid was reacted with a few small marble chips and bubbles of colourless gas were produced.
I was unable to bubble the gas through limewater which is the diagnostic test at GCSE level Chemistry. However, the gas did extinguish a burning splint very rapidly.
These reactions are typical of those carried out at Key Stage 3 and Key Stage 4 in Chemistry at school and are presented here in portrait orientation for viewing on a smartphone.
Answers to the questions posed above
Universal Indicator
Solution A = dark red, pH 1, hydrochloric acid
Solution B = orange, pH 4, ethanoic acid
Solution C = green, pH 8, ammonia solution
Solution D = blue-purple, pH 12 - 13, sodium hydroxide solution
Hydrochloric acid + magnesium metal produced hydrogen gas.
Bubbles of colourless gas were observed. Remember, you cannot claim to see hydrogen gas when writing answers to examination questions.
Hydrogen gas burns with a pop when tested with a lit splint or burning wooden splint.
Hydrochloric acid + calcium carbonate produced carbon dioxide gas.
Bubbles of colourless gas were observed.
Carbon dioxide gas produces a white precipitate when bubbled through limewater. We often say that limewater turns milky when carbon dioxide is bubbled through it.
Carbon dioxide will extinguish a burning wooden splint, but so will other gases on the GCSE Chemistry specification such as nitrogen gas and chlorine gas, so this is not a diagnostic test.
Answers to further questions
Hydrochloric acid reacts with sodium hydroxide to produce sodium chloride and water. The sodium chloride product is soluble in water and no visible change is observed with our eyes during the reaction. However, the reaction can be monitored using Universal Indicator.
In the above example scenario when hydrochloric acid is added to sodium hydroxide the colours observed would change from purple-blue to blue, then green, then yellow, then orange, then finally red when the acid would be in excess.
The neutralisation reaction could also be monitored by taking temperature readings since heat is given out during the reaction. This is due to the formation of water molecules. The neutralisation reaction is exothermic. However, this is very difficult to show in a video clip using dilute 0.1M solutions of hydrochloric acid and sodium hydroxide and an alcohol thermometer.