Rain is slightly acidic before anything human happens to it. Wet concrete is unpleasantly basic and will burn skin given long enough. Both facts come from the same short reaction — an oxide meeting water — and which way it goes appears to depend on which side of the periodic table the oxide came from.
Today you find out whether that is really the pattern, and then you run the reaction that undoes it.
What you are trying to find out
Two questions, in order.
First: does an oxide’s effect on water depend on whether it came from a metal or a non-metal? You have one of each and water with an indicator in it. This is a small experiment with a large claim behind it, and the claim is testable in fifteen minutes.
Second: if one oxide makes water acidic and the other makes it basic, what happens when you combine the two solutions? Something is consumed — but is anything made? Watching an indicator change colour tells you the acid went away. It does not tell you where it went. The only way to answer that is to recover the product and put it on a balance.
What you have to work with
Part A — the two oxides
- Distilled water and universal indicator (or bromothymol blue).
- A drinking straw, for the non-metal oxide. Your breath contains carbon dioxide; bubbling it through water is the cleanest source of a non-metal oxide a school lab has.
- Magnesium oxide powder, a spatula and a stopper for shaking.
- Test tubes, rack, and a pH scale card.
Part B — neutralisation, and recovering the product
- Sodium hydroxide solution, 0.500 mol/L and hydrochloric acid, 0.500 mol/L — both school dilutions.
- Buret or graduated pipette with a bulb, conical flask, indicator.
- Evaporating basin, water bath or hot plate, tongs, and the balance.
The design question, and it is genuinely yours
You need to recover enough sodium chloride to weigh well, and the balance has a resolution written on it — probably 0.01 g.
Work out, before the lab, what volume of 0.500 mol/L sodium hydroxide you need so that the mass of sodium chloride you could recover is at least a hundred times the balance’s resolution. You will need , the balanced equation, and a molar mass.
Then answer the other half of the question: what stops you simply scaling up until the mass is enormous? Name at least two limits. One of them is on the safety list below and the other is about how long an evaporating basin takes.
Bring the number and the reasoning to the start of the lab. Nobody dispenses anything before I have seen it.
Your other design decisions:
- How you will know you have reached neutrality, and what your method’s resolution is. An indicator changes over a range, not at a point.
- How you will remove the water without destroying what you made or hurting anybody, and how you will know when to stop.
- Whether the indicator you used in Part B ends up in your recovered solid, and what that does to your mass. Decide what to do about it before it is a problem.
Sodium hydroxide is the one that does not warn you
Read this paragraph even if you skim the rest.
- Dilute hydrochloric acid stings on contact, so you notice it and wash it off. Sodium hydroxide does not sting at first. It feels soapy or slippery — that sensation is the solution reacting with the fats in your skin — and by the time it hurts it has gone deeper than an acid burn of the same strength would have. Rinse any suspected splash under running water for at least 15 minutes, whether or not it hurts, and tell me. In an eye, this is an emergency: eyewash immediately, keep the eye open, and somebody else comes for me.
- Eye protection is not optional for one second of this lab, including while you are waiting for a basin to cool.
- Add acid to water, never water to acid, if any further dilution is needed. The heat of dilution has somewhere to go when the water is already there; poured the other way it concentrates in a few drops that can boil and throw acid back out of the container.
- Never pipette by mouth. Bulb or pump, every time, including for the water.
- Evaporating: gentle heat, and stop before dryness. A basin taken to complete dryness over strong heat spits solid and hot liquid several centimetres. Use a water bath or the lowest useful hot plate setting, take it off while the solid is still damp, and let the last of the water go in air.
- A hot evaporating basin looks exactly like a cold one. Tongs only. Never place a hot basin on the balance — you will damage the balance, and rising air off a hot object makes the reading drift low anyway.
- Your recovered solid is not table salt. It has been in lab glassware, it may contain indicator, and it is not food. Nothing is tasted, today or ever.
- Magnesium oxide is a fine powder: do not raise dust, and waft rather than sniff anything.
- Waste to the labelled containers. Nothing returns to a stock bottle.
The prediction you write first
Before the first drop:
- Part A. For each oxide, predict the approximate pH of the resulting solution, and say whether you expect acidic, neutral, or basic. Give a reason that is about the oxide, not about a memory.
- The general rule you expect to be able to state at the end, in one sentence, in the form “oxides of ______ make water ______“.
- Part B. State the balanced equation you expect, and calculate the theoretical mass of solid you should recover from the volumes you chose. This is a number, in grams, to the right number of significant figures, written down before you have any data.
- How close you expect to get, as a percentage, and which direction you expect to miss in. Commit to a direction.
What to collect
Part A
| Sample | Colour of indicator before | Colour after | Approximate pH | Acidic, neutral, or basic |
|---|---|---|---|---|
| Distilled water alone (control) | ||||
| Water with carbon dioxide bubbled through | ||||
| Water shaken with magnesium oxide |
The control is not a formality. Distilled water that has been standing in an open bottle has already absorbed carbon dioxide from the air, and if your control is not neutral you have found that out and it changes how you read the second row.
Part B
| Quantity | Value | Unit | Resolution of the instrument |
|---|---|---|---|
| Concentration of sodium hydroxide | mol/L | ||
| Volume of sodium hydroxide used | mL | ||
| Concentration of acid | mol/L | ||
| Volume of acid to reach the endpoint | mL | ||
| Mass of empty evaporating basin | g | ||
| Mass of basin plus recovered solid | g | ||
| Mass of recovered solid | g |
Every row carries a unit and every measurement carries the resolution of the thing that made it. A volume from a buret and a volume from a beaker are not the same measurement even when they are the same number.
What to bring to the consolidation discussion
- Your Part A table, including the control, and your one-sentence rule.
- The balanced equations for both oxides meeting water, and for the neutralisation. Written out, states included.
- Your theoretical mass, your recovered mass, and the percentage recovery, calculated to a defensible number of significant figures.
- Which way you missed, and the mechanism. Not “we lost some”. The specific route by which mass left or arrived, and roughly how much of the discrepancy it could account for.
- One sentence on what the recovered solid proves that the colour change alone did not.
What you should not claim
- An indicator colour is not a pH measurement. Universal indicator gives you a band a unit or two wide, judged by eye against a printed card under classroom lighting. Report it as a range, and do not write a decimal place you did not measure.
- Two oxides are not “metals versus non-metals”. You tested one of each. The pattern you are proposing covers most of the periodic table from a sample of two, which is a hypothesis worth having and not a result you have established.
- Reaching the endpoint does not mean equal volumes were used, and it does not mean the reaction stopped. It means the indicator changed, which is a statement about the indicator.
- Indicator left in the solution ends up in your solid, and it pushes the mass up. So does any water that had not finished leaving. So does a basin that picked up moisture from the air while cooling. If your recovery came out above 100%, these are the first three places to look — and a recovery over 100% is information about the procedure, not an embarrassment to be rounded away.
- Solid left clinging to the flask or the stirring rod pushes the mass down, and so does spitting during evaporation. Both of these are bigger than most students expect. Estimate how much, rather than listing them.
- You have not shown the product is sodium chloride. You have shown that a white solid of roughly the predicted mass was left behind. What would you need to do to establish its identity? That question is worth a paragraph, and answering it properly is a whole other lab.
Curriculum connection
C2.8
plan and conduct an inquiry to compare the properties of non-metal oxide solutions and metal oxide solutions (e.g., carbon dioxide reacts with water to make water acidic; magnesium oxide reacts with water to make water basic) [IP, PR, AI]
Link to original
C2.9
investigate neutralization reactions (e.g., neutralize a dilute solution of sodium hydroxide with a dilute solution of hydrochloric acid, and isolate the sodium chloride produced) [PR]
Link to original
C3.3
explain the chemical reactions that result in the formation of acids and bases from metal oxides and non-metal oxides (e.g., calcium oxide reacts with water to produce a basic solution; carbon dioxide reacts with water to produce an acidic solution)
Link to original