Use the water the protocol names
Three different activities in this unit specify distilled water. Learn what is taken out of it, when that matters, and why refilling the bottle from the tap is worse than leaving it empty.
- Tap water is a mixture: Water from a tap carries dissolved minerals and the chemicals used to treat it. It is a solution, not a blank.
- A blank has to be blank: Whenever water is used as a control or a solvent, anything already dissolved in it can take part in what you are watching.
Prerequisites are inferred: pending teacher review.
Re-learn the skill with worked practice and clear examples.
Now look at two places where the wrong water changes what you can say.
The chromosome spread procedure rinses a stained slide by submerging it in a beaker of distilled water. What would a tap water rinse risk leaving behind?
Reviewed- A.Nothing at all, because a rinse only has to carry the loose stain away
- B.Dissolved minerals that dry as a deposit across the specimen
- C.A chlorine smell that makes the slide unpleasant to work with
- D.Bacteria that would then grow on the slide while it is stored
Show the worked solution ▾
Answer: B. Dissolved minerals that dry as a deposit across the specimen
- Step 1: Follow the film of water: A thin layer stays on the glass after the slide comes out.
- Step 2: Ask what happens as it dries: The water leaves and whatever was dissolved in it does not.
Why it's right: The rinse water dries on the slide rather than being wiped off completely. Anything dissolved in it is left behind on the surface, and that surface is the one you are about to look at through a lens.
- A: A rinse does carry the loose stain away, and it also leaves a film that dries wherever it happens to be sitting.
- C: A smell is a nuisance rather than a defect in the preparation, and it is not what a rinse leaves on the glass.
- D: Growth on a stained slide is not the concern here, and the specimen has already been through heat and stain.
Aligned to Handling, Preparation, Storage and Disposal · reading level ~grade 9
The presumptive blood test uses water as the sample it knows should not react. Why does the choice of water matter more there than in most steps?
Reviewed- A.It does not matter, because a negative control is only there to be compared against
- B.Tap water is warmer, so the reaction would run faster in it
- C.The control has to use exactly the same volume as the samples do
- D.If the water itself reacted, the negative control would stop being a control
Show the worked solution ▾
Answer: D. If the water itself reacted, the negative control would stop being a control
- Step 1: Ask what the negative control proves: That a colourless result is what a non-reacting sample looks like today.
- Step 2: Ask what breaks that proof: A control that reacts. Then you no longer know what colourless means.
Why it's right: The whole value of a negative control is that you are certain it should not react. Water carrying something that could react turns your reference point into just another sample of unknown behaviour.
- A: A control is a comparison, and that comparison only works if you can trust one side of it.
- B: Both bottles sit at room temperature on the same bench, and temperature is not what makes a control reliable.
- C: Matching volumes is sensible technique and it does not address what is dissolved in the liquid.
Aligned to Handling, Preparation, Storage and Disposal · reading level ~grade 9
- A student uses distilled water for the control and tap water for washing up, deliberately.
Fill these in as you work through the lesson.
- Distilled water (boiled and recondensed, so dissolved solids are gone):
- Negative control (a sample you know should not react):
- Residue (what dissolved minerals leave when water dries):
- Substitution (swapping a named reagent, which is a decision, not a shortcut):
Tap water is a , not a blank. Where a step names water, use it, because dissolved substances can with a reagent or dry as a on a slide.
- Say what distillation takes out of water.
- Explain why a negative control has to use water you trust.
- Describe what you do when the distilled water bottle is empty.
Your toxicology data table has a column for the reaction with distilled water. You use the bottle marked distilled, because if the water itself brought something to the reaction you would be recording the water rather than the pill.