Exercises: Chemistry in Practice Investigation: skills step by step

Your class in Ennis is starting Leaving Certificate Chemistry. A classmate makes the statements below about the Chemistry in Practice Investigation.

1. The investigation is worth 40% of the available marks in Leaving Certificate Chemistry.

2. Each teacher writes the brief so that it matches the class's favourite topic.

3. Students submit a report of their investigation to the State Examinations Commission (SEC) in Year 2.

4. The investigation is only secondary research from books and websites; no laboratory work is needed.

5. Students lose marks unless their experiment gives a "perfect" answer.

6. The report must be no longer than 12 pages.

7. The SEC publishes the Investigation Brief in term two of Year 1.

1. a) For each statement, write true, false or cannot tell from the specification.
2. b) For every false statement, write a corrected version.
3. c) For every "cannot tell" statement, say where you should look to find out the real rule.
4. d) Name the other assessment component for Leaving Certificate Chemistry and state its weighting.

Four students in Tralee each write the first line of their investigation:

• Student A: "Vinegar is the best cleaner."

• Student B: "If the temperature of the acid is increased, the reaction will be faster."

• Student C: "If the particle size of calcium carbonate is decreased, the volume of carbon dioxide collected in the first 60 s from $1.0\,\text{g}$ of solid and $50\,\text{cm}^3$ of $0.50\,\text{mol/L}$ hydrochloric acid will increase, because smaller particles have a larger surface area, so there are more frequent collisions between particles."

• Student D: "I will investigate antacids."

1. a) Classify each line as a topic, an opinion, a hypothesis that is testable but weak, or a strong testable hypothesis underpinned by chemistry.
2. b) For Student B, say exactly what is missing. Then rewrite the hypothesis so that it names the acid and a metal or carbonate, gives a measurable quantity, and includes a chemical reason.
3. c) Turn Student D's topic into a focused research question that could be answered with a laboratory experiment.

A student at a school in Sligo investigates the research question: "How does the concentration of hydrochloric acid affect the rate at which magnesium ribbon reacts?" She uses a gas syringe to measure the volume of hydrogen gas collected in the first 60 s.

1. a) Write the balanced equation for the reaction, with state symbols.
2. b) State the independent variable and the dependent variable.
3. c) List four variables that must be kept constant. For each, say how the student can control it in practice.
4. d) Another student changes the acid concentration and uses magnesium ribbon of different lengths each time. Explain why this makes the conclusion unreliable.

For a practice investigation on "Does hard water form more limescale?", you collect five sources. All of them are invented for this exercise.

Source	Description
A	A review article in a peer-reviewed chemistry journal (2021), written by three university chemists. It has 48 references and tables of measured calcium ion concentrations in tap water.
B	A web page from "ScaleAway", a company that sells kettle descaler. It is undated and says "Removes 100% of limescale in seconds." It has no data and no references.
C	A short video by a lifestyle influencer (2023): "Hard water ruins your hair and your kettle." It has 2 million views and gives no sources.
D	A drinking-water quality report for a supply in Cork, published last year by the local authority. It lists measured calcium concentrations and the analytical methods used.
E	A chapter on water chemistry in a school textbook (2019 edition), written by teachers. It has no original data but gives standard explanations.

1. a) Copy and complete a table that judges each source on: author's expertise, purpose or possible bias, evidence offered, and date and ability to be checked.
2. b) Rank the sources from most to least reliable for your research, with a one-line reason for each position.
3. c) Choose two sources you would use in your report and say what each would be used for. Name one claim from a source you would cross-check against another source.
4. d) Self-assess your answer. Give yourself a level (1, 2 or 3) for each criterion and quote a phrase from your answer as evidence.

Criterion	Level 3	Level 2	Level 1
Criteria applied	All four criteria are used for every source	Some criteria are used for some sources	Judgements are given with no criteria
Justification	Each judgement is backed by a detail from the description	Some judgements are backed by details	Judgements are only opinion ("seems good")
Awareness of limits	Limits are noted even for good sources (for example, E has no original data)	Limits are noted for weak sources only	No limits are noted
Cross-checking	A specific claim and a second source to check it against	A general statement that sources should be checked	No mention of cross-checking

A student in Waterford wants to find out whether a supermarket vinegar contains the $5.0\%$ ethanoic acid stated on its label. She will use titration with standard sodium hydroxide solution, $0.100\,\text{mol/L}$, and phenolphthalein as indicator. She will first dilute $10.0\,\text{cm}^3$ of vinegar to $250\,\text{cm}^3$.

1. a) Write a research question and a testable hypothesis for this investigation. The hypothesis must include a chemical reason or a quantitative prediction.
2. b) Write the method as numbered steps (at least eight). Include rinsing the apparatus with the correct solutions, the dilution, a trial titration, the repeated titrations and how you decide when to stop repeating.
3. c) Copy and complete a risk assessment table with at least three hazards, the risk each one creates, and a control measure.

Hazard	Risk	Control measure

1. d) Explain what is meant by concordant titres and why a plan should include a trial titration.
2. e) State one thing you would do before starting practical work to be sure that your method is safe and acceptable in your school laboratory.

The student from the previous task carried out her titration. She diluted $10.0\,\text{cm}^3$ of vinegar to $250\,\text{cm}^3$ and titrated $25.0\,\text{cm}^3$ portions of the diluted solution with $0.100\,\text{mol/L}$ NaOH.

Titration	Trial	1	2	3
Final burette reading / $\text{cm}^3$	9.4	17.8	26.1	34.5
Initial burette reading / $\text{cm}^3$	0.3	9.4	17.8	26.1
Titre / $\text{cm}^3$

The equation is $\mathrm{CH_3COOH + NaOH \rightarrow CH_3COONa + H_2O}$. Take $M_r(\mathrm{CH_3COOH}) = 60.0$.

1. a) Complete the table by calculating each titre. State which titres you use to calculate the mean and why. Give the mean titre.
2. b) Calculate the moles of NaOH in the mean titre, then the moles of ethanoic acid in the $25.0\,\text{cm}^3$ portion.
3. c) Calculate the concentration of ethanoic acid in the original vinegar in $\text{mol/L}$ and in $\text{g}$ per $100\,\text{cm}^3$.
4. d) Treat the label value as $5.0\,\text{g}$ per $100\,\text{cm}^3$. Calculate the percentage difference between your value and the label.
5. e) Write a conclusion of two or three sentences. It must refer to the hypothesis, quote your numerical result and avoid claiming more than the data show.

Practice brief (invented for this exercise): "Water hardness in Irish homes." You have found two sources.

Extract 1 (council leaflet): "In parts of the midlands and east of Ireland, water passes through limestone before it reaches the tap. It contains more dissolved calcium and hydrogencarbonate ions and is called hard water. Heating hard water produces limescale on kettles and heating elements."

Extract 2 (website of a plumbing company): "Hard water increases your energy bills by up to 30%. Buy our water softener today."

You plan to compare the hardness of tap water from two different areas by finding the volume of a standard soap solution needed to produce a lasting lather in $50.0\,\text{cm}^3$ of each sample.

Write the following parts of the report. Use your own words.

1. a) Purpose of the investigation (3 or 4 sentences) that links the brief to your experiment.
2. b) Summary of the research (5 or 6 sentences). Include the ions responsible for hardness and a balanced equation showing how limescale forms when hydrogencarbonate-containing water is heated.
3. c) Evaluation of the two extracts (two sentences each): reliability and how you would use each one.
4. d) Reflection on society (4 or 5 sentences). Relate possible outcomes to households, water suppliers or the environment, for example the cost of appliance damage or the environmental cost of softening.
5. e) Self-assess parts a) to d) with this checklist. Give yourself a level for each criterion and quote evidence.

Criterion	High	Moderate	Low
Purpose	Clearly describes the purpose and links it to the brief	Describes the purpose with some link to the brief	Purpose is unclear
Chemical concepts	Explains the chemistry accurately and in detail	Describes the chemistry with minor gaps	Outlines the chemistry with errors or gaps
Sources	Evaluates sources and says how each is used	Evaluates some sources	Lists or uses sources without evaluation
Society	Considered reflection that locates the outcome in a broader issue	Some link to a broader issue	Few or no links

A student in Limerick tests the hypothesis: "The rate of the reaction between sodium thiosulfate and hydrochloric acid doubles for every $10\,^{\circ}\text{C}$ rise in temperature." She mixes $50.0\,\text{cm}^3$ of $0.040\,\text{mol/L}$ sodium thiosulfate with $5.0\,\text{cm}^3$ of $2.0\,\text{mol/L}$ HCl, heated to the stated temperature, in a beaker sitting on a cross. She records the time for the cross to disappear. Each temperature was tested once. The temperature was measured before mixing.

$$\mathrm{Na_2S_2O_3(aq) + 2HCl(aq) \rightarrow 2NaCl(aq) + S(s) + SO_2(g) + H_2O(l)}$$

Temperature / $^{\circ}\text{C}$	Time $t$ / s	$1000/t$ / $\text{s}^{-1}$
15	142
25	81
35	47
45	28
55	17

1. a) Complete the third column, to two decimal places. Explain why $1/t$ is used as a measure of reaction rate.
2. b) Describe the shape of a graph of $1000/t$ against temperature. State whether it is a straight line.
3. c) Calculate the ratio of the rates for each $10\,^{\circ}\text{C}$ step. Say whether the data support, partly support or do not support the hypothesis, and justify your answer with the ratios.
4. d) Identify two systematic and two random sources of error in this design. For each, say its likely effect on the results.
5. e) Explain one safety reason why this experiment is done with small volumes and good ventilation.
6. f) Suggest three improvements to the method. For each, say how it would improve data quality.
7. g) Write a conclusion of three or four sentences that states what the data show, refers to the hypothesis and avoids overclaiming.

Cian from Kilkenny compared three antacid brands. For each brand he measured the volume of $0.100\,\text{mol/L}$ HCl that one tablet neutralised, using one tablet per brand.

Brand	Volume of $0.100\,\text{mol/L}$ HCl neutralised / $\text{cm}^3$
A	31.5
B	24.0
C	28.0

His draft conclusion reads:

"My results prove that brand A is the best antacid because it neutralised the most acid. The experiment went well and there were no problems apart from human error. In conclusion, antacids are very important for society."

Use this simplified practice rubric, based on the descriptors of quality for the investigation. The real marking is done by the SEC.

Strand	High	Moderate	Low
Knowledge and understanding	Explains the chemistry accurately and clearly	Describes the chemistry	Outlines some of the chemistry
Investigative skills: design and method	Thorough design and appropriate methods that give high-quality data	Somewhat structured design and good-quality data	Some experimental methods used
Analysis and conclusions	Valid conclusions justified by the data; thorough evaluation of limitations	Some conclusions linked to the hypothesis; some limitations identified	Some conclusions; limitations not identified
Relation to society	Considered reflection on the broader societal and scientific issues	Some reflection on broader issues	Few reflections

1. a) Judge Cian's draft against each strand. Give a level and quote the words that justify it.
2. b) List at least three specific weaknesses in the draft.
3. c) Calculate the moles of HCl neutralised per tablet for each brand.
4. d) Rewrite the conclusion in about 120 to 150 words so that it reaches the high level. It must use your numbers from c), state at least three limitations (for example one tablet per brand, tablets of different mass, judging the end point, stomach acid is not simply $0.100\,\text{mol/L}$ HCl) and link to a wider issue such as how people choose and use antacids.
5. e) Assess your own rewrite with the same rubric. State your level for each strand and one more thing you would improve.