Hold up two clear cups of water (prepared in advance — one is pure distilled water, one has ~25 g of dissolved salt per 100 mL, but do not tell students). Drop a fresh chicken egg (or a small grape) into each: it sinks in the pure water and floats in the saltwater. Ask: 'Same water, same egg — what changed, and how could we PROVE it with numbers, not just eyeballs?' Take 2-3 student guesses. Then reveal today's investigative question: 'Does dissolving more salt in water increase its density, and by how much per gram?' Tell students that in 30 minutes they will design and run the experiment themselves, and tomorrow's test will ask them to spot variables and design flaws in exactly this kind of setup.

1. 5m

   Variables and controls in a fair test

   Content

   A controlled experiment isolates ONE cause. The independent variable is the one thing you deliberately change — today, grams of table salt dissolved (0 g, 5 g, 10 g, 20 g per 100 mL). The dependent variable is what you measure in response — today, density in g/mL, calculated from mass ÷ volume. Everything else must be a controlled variable: same 100 mL of distilled water, same temperature, same graduated cylinder read at the meniscus, same balance zeroed with the same weigh boat. The 0 g setup is the control group — it establishes the baseline density of pure water (~1.00 g/mL) against which the treated samples are compared. A hypothesis for today might read: 'If more salt is dissolved in a fixed volume of water, then the density will increase, because dissolved salt adds mass without significantly changing volume.' That is a testable prediction grounded in prior knowledge — not a guess.

   Delivery

   Walk the variables table on the slide row by row and have students call out which column each factor belongs in. Emphasize that 'controlled variable' and 'control group' are DIFFERENT things — students confuse these constantly and the test will exploit it. Ask: 'If I used a different graduated cylinder for the 20 g trial, which category did I break?' (controlled variable). Pre-empt the misconception that a hypothesis is a guess by rewriting a weak one ('I think salt does something') into a strong one on the fly.

2. 5m

   Data, error, and what counts as a conclusion

   Content

   Density is calculated as mass of solution ÷ volume of solution, in g/mL. Students will collect quantitative data (mass on the balance, volume in the graduated cylinder) and qualitative data (did all the salt dissolve? is the solution cloudy?). A source of error is a specific, identifiable reason a measurement could be off — for example: reading the meniscus from above instead of at eye level (overestimates volume, underestimates density), failing to tare the weigh boat (overestimates mass), or salt that didn't fully dissolve (mass counted but volume not fully mixed). A valid conclusion uses the actual numbers to answer the question — even if the data do NOT support the hypothesis, that is a real scientific result, not a failed experiment. More trials help only if the design is sound; ten sloppy trials are worse than three careful ones.

   Delivery

   Work one density calculation aloud: 105.2 g ÷ 100 mL = 1.052 g/mL. Ask students to predict whether the 20 g trial's density will be higher or lower than the 5 g trial and WHY. Directly address the misconception that disconfirming data means the experiment failed — restate: 'If your data show no change, that IS the answer, and you write it up.' Also head off 'more data is always better' — remind them one well-designed trial beats ten sloppy ones.

1. 32m

   Saltwater Density InvestigationLab

   Group students in pairs or trios at 8 stations. Each station gets: 1 balance, 1 graduated cylinder, 4 labeled beakers (A/B/C/D), spatula, weigh boats, paper towels, container of salt, wash bottle of distilled water. Solution prep (teacher, before class): You are NOT pre-making solutions — students make them. Just have on each bench: ~50 g table salt in a labeled container, ~500 mL distilled water, and the four empty labeled beakers. Optional demo cups for the hook: 100 mL distilled water (Cup 1); dissolve 25 g salt in 100 mL distilled water (Cup 2). Run the lab (30 min of student work): 1. Goggles on before any water or salt is handled. 2. Students copy the variables table and hypothesis onto their handout (below) before touching materials. 3. They measure 100 mL of distilled water into each of the four beakers (A, B, C, D) using the SAME graduated cylinder, read at eye level. 4. Using a tared weigh boat on the balance, they mass out: 0 g salt for A (control), 5 g for B, 10 g for C, 20 g for D. Add each to its beaker and stir with the spatula until fully dissolved. 5. For each solution, pour it back into the graduated cylinder and record the final volume (it will be close to but not exactly 100 mL). Then pour it into a weigh boat/beaker on the tared balance and record total mass of solution. 6. Calculate density = mass ÷ volume for each. Plot density (y-axis) vs grams of salt (x-axis) on the grid provided. 7. Write the conclusion and error analysis paragraphs. Walk around and check: are they reading the meniscus at eye level? Did they tare the weigh boat? Are they writing units? Redirect groups who ask 'is this right?' — instead ask 'what does your control tell you?' Student handout — Saltwater Density Investigation Research question: Does dissolving more salt in a fixed volume of water increase its density, and by how much per gram of salt added? My hypothesis (If… then… because…): __________________________________________ Part 1 — Variables table (fill in before you start) - Independent variable: __________ (units: __________) - Dependent variable: __________ (units: __________) - Three controlled variables you will hold constant: 1. __________ 2. __________ 3. __________ - Control group (which beaker?): __________ Part 2 — Data table - Beaker A: 0 g salt · volume ___ mL · mass ___ g · density ___ g/mL · dissolved fully? ___ - Beaker B: 5 g salt · volume ___ mL · mass ___ g · density ___ g/mL · dissolved fully? ___ - Beaker C: 10 g salt · volume ___ mL · mass ___ g · density ___ g/mL · dissolved fully? ___ - Beaker D: 20 g salt · volume ___ mL · mass ___ g · density ___ g/mL · dissolved fully? ___ Part 3 — Graph. On the grid, plot grams of salt (x-axis, 0-25) vs density in g/mL (y-axis, 0.95-1.15). Label axes with units. Draw a best-fit line. Part 4 — Conclusion (2-3 sentences). State whether your data support your hypothesis, cite at least two specific density values as evidence, and give the approximate change in density per gram of salt from your graph. Part 5 — Error analysis. Name TWO specific sources of error in YOUR procedure (not 'human error') and state, for each, whether it would make your calculated density too HIGH or too LOW. 1. __________ — effect on density: __________ 2. __________ — effect on density: __________

   Materials

   • electronic balance
   • 100 mL graduated cylinder
   • 50 mL beakers (4 per group)
   • metric ruler
   • spatula
   • table salt (sodium chloride)
   • distilled water
   • paper towels
   • weigh boats
   • safety goggles

   Example outputs

   • Beaker A: 100 mL, 100.1 g, 1.001 g/mL, dissolved N/A. Beaker B: 100 mL, 105.0 g, 1.050 g/mL, yes. Beaker C: 101 mL, 109.8 g, 1.087 g/mL, yes. Beaker D: 102 mL, 119.6 g, 1.173 g/mL, mostly (trace crystals). Conclusion: 'The data support my hypothesis — density rose from 1.001 g/mL with no salt to 1.173 g/mL with 20 g salt, an increase of about 0.009 g/mL per gram of salt added.'
   • Error analysis example: '(1) I read the meniscus from above the first time, which made my volume reading too large and my calculated density too LOW. (2) Some undissolved salt sat on the bottom of Beaker D, so mass was counted but that salt wasn't really in solution — this made the reported density of the SOLUTION too HIGH.'
2. 10m

   Test-Review: Spot the Design Flaw

   Whole-class review activity — closes the lesson and previews test-style items. Project each scenario one at a time (they are also on the handout below). Give students 60 seconds to write their answer, then cold-call and discuss. Correct the misconception explicitly each time. Student handout — Spot the Flaw (Test Review) Scenario 1. A student wants to know if temperature affects how fast sugar dissolves. She uses 10 g of sugar in 100 mL of cold tap water, and 15 g of sugar in 100 mL of hot distilled water. She stirs the hot one with a spoon and the cold one with a straw. - What is the intended independent variable? __________ - Name TWO controlled variables she FAILED to control: __________ - Why can she NOT conclude anything about temperature from this data? Scenario 2. A student tests whether salt raises water's density. He runs the 20 g trial ten times and gets very consistent results, but only runs the 0 g and 5 g trials once each. He concludes salt has a huge effect. - What's wrong with his design? (Hint: more trials of one condition ≠ better experiment) Scenario 3. A student predicts that adding salt will DECREASE density. Her data show density clearly INCREASES. She writes: 'My experiment failed.' - Rewrite her conclusion correctly in one sentence. Scenario 4 (rapid-fire vocabulary). In today's saltwater lab, name: - one qualitative observation you made: __________ - one quantitative measurement you made: __________ - the difference between a controlled variable and the control group: __________

   Example outputs

   • Scenario 1: IV = temperature. Uncontrolled: mass of sugar (10 vs 15 g), water type (tap vs distilled), stirring implement. Can't conclude anything about temperature because any of the other differences could explain the result.
   • Scenario 3: 'The data do not support my hypothesis — density increased rather than decreased as salt was added. This is a valid result showing that dissolved salt raises the density of water.'

1. In the saltwater density lab, identify the (a) independent variable, (b) dependent variable, and (c) one controlled variable.

   short answer(a) Grams of salt dissolved (0, 5, 10, 20 g). (b) Density of the solution in g/mL. (c) Any ONE of: volume of distilled water (100 mL), temperature, same graduated cylinder, same balance, same reader's meniscus technique.

2. A student's data show that density did NOT change when salt was added. Which statement is the BEST scientific response? A) The experiment failed and must be redone. B) The student should change the hypothesis to match the data before writing it up. C) The data do not support the hypothesis, which is itself a valid result to report. D) The student needs at least 30 more trials before concluding anything.

   multiple choiceC. Disconfirming data are a valid scientific outcome — you report what the data show, you don't rewrite the hypothesis after the fact, and more trials of a flawed design won't fix it.

3. A student measured 100 mL of water plus 10 g of salt and calculated a density of 0.98 g/mL — LOWER than pure water, which doesn't make sense. Give ONE specific source of error that could cause density to be measured too low, and explain why.

   short answerAny ONE clearly explained: (1) Read the meniscus from above → recorded volume too large → density (mass ÷ volume) comes out too low. (2) Did not tare the weigh boat, then subtracted too much → mass too low → density too low. (3) Spilled some solution before massing → mass too low → density too low.

• ☐Safety goggles (one per student)
• ☐Electronic balances (1 per station, 8 stations)
• ☐100 mL graduated cylinders (1 per station)
• ☐50 mL beakers, 4 per station, labeled A/B/C/D
• ☐Metric rulers
• ☐Spatulas
• ☐Table salt (sodium chloride) — ~50 g per station
• ☐Distilled water — ~500 mL per station
• ☐Weigh boats
• ☐Paper towels
• ☐2 clear cups + 1 fresh egg (or grape) for the hook demo
• ☐Student handouts (lab sheet + Spot-the-Flaw review sheet)