The periodic table · Contrast
Metal and non-metal oxides
Burn a metal and burn a non-metal, drop each product into water, and the two solutions go opposite ways on the pH scale. The side of the table the element came from decides which way.
Start here
A ribbon of magnesium and a spoonful of sulfur, burned in the same flame.
Both catch. Both leave a product behind, and both products go into water and give a colourless solution you could not tell apart by looking. Then the universal indicator goes in: one beaker turns purple, the other turns red.
What decided which way each one went?
Magnesium is a metal, and its oxide takes the water above 7. Sulfur is a non-metal, and its oxide takes the water below 7. Swap the flame for a hotter one, burn ten times as much, use twice the water — the direction does not move. What sets it is which side of the periodic table the element started on.
Burn almost any element in air and it joins with oxygen. What is left behind is an oxide.
magnesium + oxygen magnesium oxide, MgO
sulfur + oxygen sulfur dioxide, SO2
Both of those are oxides and neither looks anything like the element that made it. Put each one in water and test the solution, and the two results sit at opposite ends of the pH scale.
Reference · keep this one open
The two kinds of oxide
- What it isA base — it reacts with an acid to make a salt and waterAcidic — it reacts with an alkali to make a salt and water
- Shaken with waterAbove pH 7, if it dissolves at allBelow pH 7
- Universal indicatorBlue, or purpleOrange, or red
- Where its element sitsLeft and middle of the periodic tableTop right of the periodic table
- On the bench belowcalcium oxide, magnesium oxide, copper oxidecarbon dioxide, sulfur dioxide, water
Two words that are not the same word
A base does not have to dissolve. A base that has dissolved is an alkali.
Base
Reacts with an acid to make a salt and water.
It does not have to dissolve to be one.
Alkali
A base that has dissolved in water.
Only a dissolved base can move a pH reading.
The second row of the table is the one to read twice. A metal oxide only shows up as an alkaline solution if it dissolves, and not all of them do.
Your turn · six oxides, two beakers
Two beakers of the same water.
Beaker 1
water only
Water, nothing added
pH of the solution
7
Universal indicator: green.
The baseline. Both beakers start here.
Beaker 1
metal oxide
Calcium oxide, stirred in
pH of the solution
12
Universal indicator: purple.
All of it dissolved. Nothing left on the bottom.
Calcium is a metal, so calcium oxide is a base. This one dissolves freely, so there is plenty of it in solution and the reading goes a long way up the scale.
Beaker 1
metal oxide
Magnesium oxide, stirred in
pH of the solution
10
Universal indicator: blue.
A little dissolved. Most of the white powder is still on the bottom.
Magnesium is a metal, so magnesium oxide is a base. Only a little of it dissolves, so the solution is alkaline but the reading does not climb to the top of the scale.
Beaker 1
metal oxide
Copper oxide, stirred in
pH of the solution
7
Universal indicator: green.
None of it dissolved. The black powder is sitting on the bottom exactly as it went in.
Copper is a metal, so copper oxide is a base — and this reading cannot show that. Almost nothing went into solution, so there is nothing in the water for the indicator to report.
Beaker 1
non-metal oxide
Water — the oxide is the water
pH of the solution
7
Universal indicator: green.
Nothing to dissolve. This oxide is the water.
Hydrogen is a non-metal, so water is a non-metal oxide: hydrogen oxide. It is also the liquid already in the beaker. Not every non-metal oxide is acidic — this one is the definition of neutral.
Beaker 1
non-metal oxide
Carbon dioxide, bubbled through
pH of the solution
5
Universal indicator: orange.
Bubbled in. The gas dissolves as it goes.
Carbon is a non-metal, so carbon dioxide is acidic. Dissolved in water it makes carbonic acid, which is why rain is slightly acidic before it has touched anything at all.
Beaker 1
non-metal oxide
Sulfur dioxide, bubbled through
pH of the solution
3
Universal indicator: red.
Bubbled in. The gas dissolves as it goes.
Sulfur is a non-metal, so sulfur dioxide is acidic. It dissolves readily, and the reading drops well down the scale.
Beaker 2
water only
Water, nothing added
pH of the solution
7
Universal indicator: green.
The baseline. Both beakers start here.
Beaker 2
metal oxide
Calcium oxide, stirred in
pH of the solution
12
Universal indicator: purple.
All of it dissolved. Nothing left on the bottom.
Calcium is a metal, so calcium oxide is a base. This one dissolves freely, so there is plenty of it in solution and the reading goes a long way up the scale.
Beaker 2
metal oxide
Magnesium oxide, stirred in
pH of the solution
10
Universal indicator: blue.
A little dissolved. Most of the white powder is still on the bottom.
Magnesium is a metal, so magnesium oxide is a base. Only a little of it dissolves, so the solution is alkaline but the reading does not climb to the top of the scale.
Beaker 2
metal oxide
Copper oxide, stirred in
pH of the solution
7
Universal indicator: green.
None of it dissolved. The black powder is sitting on the bottom exactly as it went in.
Copper is a metal, so copper oxide is a base — and this reading cannot show that. Almost nothing went into solution, so there is nothing in the water for the indicator to report.
Beaker 2
non-metal oxide
Water — the oxide is the water
pH of the solution
7
Universal indicator: green.
Nothing to dissolve. This oxide is the water.
Hydrogen is a non-metal, so water is a non-metal oxide: hydrogen oxide. It is also the liquid already in the beaker. Not every non-metal oxide is acidic — this one is the definition of neutral.
Beaker 2
non-metal oxide
Carbon dioxide, bubbled through
pH of the solution
5
Universal indicator: orange.
Bubbled in. The gas dissolves as it goes.
Carbon is a non-metal, so carbon dioxide is acidic. Dissolved in water it makes carbonic acid, which is why rain is slightly acidic before it has touched anything at all.
Beaker 2
non-metal oxide
Sulfur dioxide, bubbled through
pH of the solution
3
Universal indicator: red.
Bubbled in. The gas dissolves as it goes.
Sulfur is a non-metal, so sulfur dioxide is acidic. It dissolves readily, and the reading drops well down the scale.
Side by side
Both readings are above 7. Calcium oxide at pH 12 is the more alkaline of the two. Both are metal oxides.
Calcium oxide took the reading up to 12. Copper oxide left it at 7. Both are metal oxides. The 7 beside copper oxide is not a verdict on it: almost none of it dissolved, so the water had nothing to report.
Calcium oxide took the reading up to 12. Water left it at 7. One of them is a metal oxide and one is not.
Carbon dioxide read pH 5, which is acidic. Calcium oxide read pH 12, which is alkaline. One of them is a metal oxide and one is not.
Sulfur dioxide read pH 3, which is acidic. Calcium oxide read pH 12, which is alkaline. One of them is a metal oxide and one is not.
Both readings are above 7. Calcium oxide at pH 12 is the more alkaline of the two. Both are metal oxides.
Magnesium oxide took the reading up to 10. Copper oxide left it at 7. Both are metal oxides. The 7 beside copper oxide is not a verdict on it: almost none of it dissolved, so the water had nothing to report.
Magnesium oxide took the reading up to 10. Water left it at 7. One of them is a metal oxide and one is not.
Carbon dioxide read pH 5, which is acidic. Magnesium oxide read pH 10, which is alkaline. One of them is a metal oxide and one is not.
Sulfur dioxide read pH 3, which is acidic. Magnesium oxide read pH 10, which is alkaline. One of them is a metal oxide and one is not.
Calcium oxide took the reading up to 12. Copper oxide left it at 7. Both are metal oxides. The 7 beside copper oxide is not a verdict on it: almost none of it dissolved, so the water had nothing to report.
Magnesium oxide took the reading up to 10. Copper oxide left it at 7. Both are metal oxides. The 7 beside copper oxide is not a verdict on it: almost none of it dissolved, so the water had nothing to report.
Copper oxide and water both read pH 7. One of them is a metal oxide and one is not. The 7 beside copper oxide is not a verdict on it: almost none of it dissolved, so the water had nothing to report.
Carbon dioxide took the reading down to 5. Copper oxide left it at 7. One of them is a metal oxide and one is not. The 7 beside copper oxide is not a verdict on it: almost none of it dissolved, so the water had nothing to report.
Sulfur dioxide took the reading down to 3. Copper oxide left it at 7. One of them is a metal oxide and one is not. The 7 beside copper oxide is not a verdict on it: almost none of it dissolved, so the water had nothing to report.
Calcium oxide took the reading up to 12. Water left it at 7. One of them is a metal oxide and one is not.
Magnesium oxide took the reading up to 10. Water left it at 7. One of them is a metal oxide and one is not.
Water and copper oxide both read pH 7. One of them is a metal oxide and one is not. The 7 beside copper oxide is not a verdict on it: almost none of it dissolved, so the water had nothing to report.
Carbon dioxide took the reading down to 5. Water left it at 7. Both are non-metal oxides.
Sulfur dioxide took the reading down to 3. Water left it at 7. Both are non-metal oxides.
Carbon dioxide read pH 5, which is acidic. Calcium oxide read pH 12, which is alkaline. One of them is a metal oxide and one is not.
Carbon dioxide read pH 5, which is acidic. Magnesium oxide read pH 10, which is alkaline. One of them is a metal oxide and one is not.
Carbon dioxide took the reading down to 5. Copper oxide left it at 7. One of them is a metal oxide and one is not. The 7 beside copper oxide is not a verdict on it: almost none of it dissolved, so the water had nothing to report.
Carbon dioxide took the reading down to 5. Water left it at 7. Both are non-metal oxides.
Both readings are below 7. Sulfur dioxide at pH 3 is the more acidic of the two. Both are non-metal oxides.
Sulfur dioxide read pH 3, which is acidic. Calcium oxide read pH 12, which is alkaline. One of them is a metal oxide and one is not.
Sulfur dioxide read pH 3, which is acidic. Magnesium oxide read pH 10, which is alkaline. One of them is a metal oxide and one is not.
Sulfur dioxide took the reading down to 3. Copper oxide left it at 7. One of them is a metal oxide and one is not. The 7 beside copper oxide is not a verdict on it: almost none of it dissolved, so the water had nothing to report.
Sulfur dioxide took the reading down to 3. Water left it at 7. Both are non-metal oxides.
Both readings are below 7. Sulfur dioxide at pH 3 is the more acidic of the two. Both are non-metal oxides.
Not yet in a beaker: calcium oxide, magnesium oxide, copper oxide, water, carbon dioxide, sulfur dioxide.
All six have been in a beaker.
Two went above 7. Two went below. Two did not move at all.
Calcium oxide and magnesium oxide are oxides of metals, and both took the water above 7. Carbon dioxide and sulfur dioxide are oxides of non-metals, and both took it below. Then there is copper oxide, a metal oxide that left the reading at 7 because it would not dissolve — and water, a non-metal oxide that left the reading at 7 because it is neutral. Same number, two different reasons, and neither of them is a hole in the pattern.
Key fact
An oxide is an element joined with oxygen. Metal oxides are bases; non-metal oxides are acidic. A base that dissolves is an alkali and takes the pH above 7 — a base that will not dissolve is still a base.
Think again
“The copper oxide left the pH at 7, so it cannot be a base.”
It is the reading everyone trusts. Commit before you read on.
Pour warm dilute sulfuric acid onto that black powder and it disappears. The liquid turns blue, and what has formed is copper sulfate.
copper oxide + sulfuric acid copper sulfate + water
A salt and water, out of an acid. That is what a base does, and the copper oxide has just done it.
The pH test was never asking the right question. It reads what is dissolved, and almost no copper oxide dissolves. Basic is what a substance does to an acid. Alkaline is a description of a solution. Copper oxide is the first and never the second.
Five words
Say your answer out loud before you turn each card over. If you cannot say it, you do not know it yet.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
Sulfur is burned in air and the product is shaken with water. What will that solution do to universal indicator?
Rung 2 · The one that catches people
Copper oxide is stirred into water. The black powder sits on the bottom and the pH reads 7. What does that tell you?
Rung 3 · Explain
Copper oxide and magnesium oxide are both metal oxides. Stirred into water, magnesium oxide takes the pH to 10 while copper oxide leaves it at 7. Explain how both can still be bases.
Rung 4 · Take it somewhere new
A grower finds the soil in one field is too acidic for the crop. Using the oxides on the bench above, say what could be spread on that field and why. Then explain why burning a heap of sulfur at the edge of the field would make the problem worse.
Key note
An oxide is an element joined with oxygen. Metal oxides are bases: they react with acids to give a salt and water, and the ones that dissolve give alkaline solutions above pH 7. Non-metal oxides are acidic: dissolved in water they give solutions below pH 7. Two cases show why the words have to be exact. Copper oxide is a base that barely dissolves, so its reading stays at 7 and the acid test is the only one that finds it. Water is itself a non-metal oxide, and it is neutral.
Going further
Coal and oil contain sulfur, so burning them sends sulfur dioxide up the chimney, and an engine running hot enough will make nitrogen and oxygen from the air combine into nitrogen oxides. Both are non-metal oxides and both dissolve in rain. Clean rain is already slightly acidic at about pH 6, because carbon dioxide from the air dissolves in it; rain measured downwind of heavy industry has come in nearer pH 4. Limestone is calcium carbonate and reacts with acid, which is why carved stone weathers faster in it, and why powdered limestone is the thing tipped into an acidified lake.
The staircase of elements that will not commit to being metals or non-metals has oxides to match. Aluminium oxide reacts with acids the way a metal oxide should, and with alkalis the way a non-metal oxide should. An element in between gives an oxide in between — which is a useful reminder that the rule in this lesson is a summary of behaviour rather than a law that behaviour obeys.
Before this lesson
Next in this unit
At GCSE this becomes
- Amphoteric oxides, the trend in oxide acidity across a period, and neutralisation written as an ionic equation.
Where to next
- Next: The reactivity series
Metals and materials
- Previous: Group 0 and why groups exist
Ask Mr Badmus AI
Still unsure how copper oxide can be a base at pH 7?
Teacher demonstration only. Burning sulfur must be done in a fume cupboard — the gas it gives off irritates your lungs. Do not look straight at burning magnesium; the light is bright enough to hurt your eyes. Eye protection for everyone in the room.
Lesson content © MrBadmusAI.