Chemical reaction signs

Overview

This is lesson 3 of 3 in “Introduction to Chemistry”. Students build on earlier ideas about mixtures and changes by identifying chemical reactions using observable indicators (including qualitative and quantitative observations) and deciding whether changes are reversible or irreversible.

Learning intentions

• WALT identify chemical reactions by their observable indicators.
• WALT describe changes using both qualitative and quantitative observations.
• WALT evaluate whether a reaction is likely to be reversible or irreversible based on evidence.
• WALT carry out and record results from safe investigations into chemical reactions.

Success criteria

• I can say what a chemical reaction is in my own words.
• I can list at least three observable signs that a chemical reaction has happened.
• I can record observations clearly (what I saw/smelled/heard and any measurements).
• I can give evidence-based reasons for whether a change is likely reversible or irreversible.

Curriculum links

• Physical Science: Chemical change — designing and carrying out investigations that distinguish chemical change from physical changes using observable indicators, and evaluating whether changes are reversible or irreversible.
• Physical Science: Chemical change — observations can be qualitative and quantitative at this level.
• Scientific inquiry: students use a fair approach (same conditions, careful measuring, and systematic recording) to support conclusions.

Lesson structure (60 minutes)

1. 0–7 min · Starter: “Reacting or not?” Teacher shows 3 quick scenario cards/images (e.g., rusting iron, dissolving salt, baking a bread roll) and asks: “What evidence would tell you something new has formed?” Students write a one-sentence prediction and one observable indicator they would look for.

2. 7–15 min · Mini-teach: What to notice in reactions Teacher uses a simple chart: indicators such as colour change, gas production, temperature change, precipitate/solid forming, and irreversible change (new substances). Students complete a “Qualitative vs quantitative” sorting task for prompts like “bubbles appear” and “mass changes by 2 g”.

3. 15–27 min · Demonstration carousel (teacher-led) Teacher runs three short, safe demonstrations (explains safety before each):

• Colour change (e.g., indicators reacting with acids/alkalis in a controlled way)
• Gas production (e.g., reaction that visibly produces bubbles)
• Solid formation/precipitate (e.g., mixing solutions that form a new cloudy solid) Students record in a table: indicator observed, what it looked like, and any measurement taken (e.g., time to start, temperature before/after if appropriate, or mass of container before/after where safe).

4. 27–35 min · Predict, plan, and set up Teacher hands out the investigation sheet for the hands-on activity and reviews the variables for a fair test: same volumes, same order of mixing, and careful timing. Students in groups assign roles (materials manager, measurer/recorder, reporter) and complete the prediction section: “I expect a chemical reaction because…”

5. 35–53 min · Hands-on investigation: “Mix and observe” Students mix common, classroom-safe substances provided by the teacher (pre-set combinations known to produce clear indicators). They:

• Follow the procedure exactly (including order and timing)
• Measure and record at least two indicators (e.g., bubbles/gas + colour change; or cloudiness + temperature/time)
• Record quantitative data where possible (time to reaction start, number of bubbles if observable, thermometer reading before/after, or mass change if supplied and safe) Teacher circulates, checks for accurate recording, and prompts evidence language: “What did you observe, not what do you think?”

6. 53–58 min · Share and reconcile Teacher selects two groups with different outcomes (e.g., reaction vs no clear reaction) and asks the class: “Which evidence supports a chemical reaction?” Students do a brief gallery share and refine their indicator lists.

7. 58–60 min · Exit ticket (quick assessment) Students answer:

• “Name one indicator of a chemical reaction from today.”
• “Is chemical change usually reversible? Give one reason from evidence today.”

Resources

• Student investigation worksheet (observation table with qualitative and quantitative prompts)
• PPE: safety glasses for each student, disposable gloves if required by materials
• Thermometers or temperature probes (if temperature change is included)
• Measuring cylinders/spoons, digital balance (if mass changes are used)
• Provided reaction materials in labelled containers (pre-selected safe pairs)
• Stopwatch/timers
• Heat-safe trays or stands, paper towels, and spill kits
• Lab mats and waste disposal containers (clearly labelled)
• Whiteboard/marker and chart paper for indicators

Assessment

• Formative: teacher checks group recording during demonstrations and the hands-on investigation (evidence vs opinion).
• Formative: students complete the qualitative/quantitative sorting task and uses indicator language correctly.
• Summative-in-mini: exit ticket evaluates understanding of observable signs and evidence-based reasoning about reversibility.

Differentiation

• Support: provide sentence starters on the investigation sheet (e.g., “A chemical reaction occurred because…” “I observed…” “The quantitative data shows…”).
• Support: pre-teach measuring/timing expectations with one practice run before the hands-on mixing.
• Extension: challenge students to compare two reactions they observe and state which indicators are most reliable and why (using evidence).
• EAL/SEN: offer visual word banks (colour change, gas, precipitate/solid, temperature change, irreversible) and allow oral responses to be recorded by a scribe if needed.