Investigating Science 11–12 · Year 11

Five white solids: identifying chemicals from observations and clues

Module 1: Cause and Effect – Observing

PracticalMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

The idea

Colour changes, fizzing and warming are observations; the name of each powder is an inference, and some pairs cannot be told apart with the tests available.

Safety card

Medium riskA teacher supervises

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

anhydrous calcium chloride heats on dissolving and irritates eyes
sodium carbonate and the acids irritate eyes
no tasting of any chemical

Controls

safety glasses
small quantities only
add solids to water, not water to solids
wash hands at the end

Note

Chemicals are used: follow the NSW Department of Education Chemical Safety in Schools (CSIS) package, Section 1 (general information for all staff, including the CSIS 1.7 risk assessment requirement), read the Safety Data Sheet for each chemical and complete a RiskAssess risk assessment before the lesson.

What you need

about 2 g each of sodium carbonate, anhydrous calcium chloride, citric acid, tartaric acid and non-iodised salt, labelled A to E
bromothymol blue indicator in a dropper bottle, distilled water, spatulas, spotting tile or small beakers
thermometer, stirring rods, a list of clues (for example: bromothymol blue is yellow in acid, green when neutral and blue in base; acid and carbonate give carbon dioxide; calcium carbonate is insoluble)

How to do it

Record the appearance of each solid.
Dissolve one spatula of each in 10 mL of water, recording the temperature before and after.
Add two drops of bromothymol blue to each solution and record the colour.
Mix pairs of solutions and record fizzing, cloudiness or colour change.
Use the clues to infer the identity of each solid, writing the observation that supports each inference.
State which identities remain uncertain and what further test would separate them.

What you should see

Sodium carbonate turns the indicator blue; citric and tartaric acids turn it yellow and each fizzes (carbon dioxide) when mixed with the sodium carbonate solution; salt stays green; calcium chloride gives the largest temperature rise on dissolving and a white precipitate of calcium carbonate with sodium carbonate solution, while its solution stays close to green. Three of the five are identified with certainty; citric and tartaric acid give the same observations in every test available, so the learner must report that inference as undecided, which is the point about the limits of observation.

What changes

This activity lists no variables to change, measure and keep the same.

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

An observation and an inference are the same thing if you are confident.
Every unknown can be identified from a few simple tests.
A white powder that dissolves must be salt.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Investigating Science Stage 6 Syllabus (2017), NESA; currentINS11-8 INS11/12-1 INS11/12-3 INS11/12-4
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

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