Why Sodium Is Kept Under Oil
Learn what happens when metals burn, react with water and react with acids, how to test for hydrogen, and why iron rusts while stainless steel does not.
What happens when metals and non-metals burn in air?
Burning in air means reacting with the oxygen in it, and the product is an oxide. The useful pattern is that metals give basic oxides while non-metals give acidic oxides — a difference you can confirm with litmus paper.
This page covers everything in the CBSE Class 7 Science chapter on the chemical behaviour of metals and non-metals: burning in air, reaction with water, reaction with dilute acids, and corrosion.
This page covers everything in the CBSE Class 7 Science chapter on the chemical behaviour of metals and non-metals: burning in air, reaction with water, reaction with dilute acids, and corrosion.
How do you tell a basic oxide from an acidic oxide?
Dissolve the oxide in water and test the solution with litmus paper.
When magnesium ribbon burns, it gives a bright white flame and leaves a white powder, magnesium oxide. Dissolved in water it gives magnesium hydroxide, which turns red litmus blue — so it is basic.
When sulphur burns, it gives a gas with a sharp choking smell, sulphur dioxide. Dissolved in water it gives sulphurous acid, which turns blue litmus red — so it is acidic.
So the test has two steps: burn the element, then dissolve the oxide and use litmus. Metal oxides are basic; non-metal oxides are acidic.
For example, the white ash left after burning magnesium in a school laboratory is exactly this magnesium oxide.
This is also the link to the previous chapter on acids and bases: burning sulphur produces one of the gases that makes rain acidic, which is why factory smoke is treated before release.
When magnesium ribbon burns, it gives a bright white flame and leaves a white powder, magnesium oxide. Dissolved in water it gives magnesium hydroxide, which turns red litmus blue — so it is basic.
When sulphur burns, it gives a gas with a sharp choking smell, sulphur dioxide. Dissolved in water it gives sulphurous acid, which turns blue litmus red — so it is acidic.
So the test has two steps: burn the element, then dissolve the oxide and use litmus. Metal oxides are basic; non-metal oxides are acidic.
For example, the white ash left after burning magnesium in a school laboratory is exactly this magnesium oxide.
This is also the link to the previous chapter on acids and bases: burning sulphur produces one of the gases that makes rain acidic, which is why factory smoke is treated before release.
How do metals react with water?
Metals differ enormously in how vigorously they react with water, and that difference decides how they must be stored.
Sodium reacts violently even with cold water, fizzing, producing hydrogen and giving out so much heat that the hydrogen can catch fire. Because it also reacts with the moisture and oxygen in air, sodium is stored under kerosene — the oil keeps air and water away from its surface.
Iron reacts only very slowly, and needs both air and moisture over a long period; the result is rust rather than any visible fizzing.
Gold and silver do not react with water at all, which is why ornaments keep their shape and shine for generations.
So storage follows reactivity: the more reactive the metal, the more it must be protected from air and water.
The safety point behind it: highly reactive metals are never handled with bare or wet hands, because the heat released by the reaction is enough to cause burns.
Sodium reacts violently even with cold water, fizzing, producing hydrogen and giving out so much heat that the hydrogen can catch fire. Because it also reacts with the moisture and oxygen in air, sodium is stored under kerosene — the oil keeps air and water away from its surface.
Iron reacts only very slowly, and needs both air and moisture over a long period; the result is rust rather than any visible fizzing.
Gold and silver do not react with water at all, which is why ornaments keep their shape and shine for generations.
So storage follows reactivity: the more reactive the metal, the more it must be protected from air and water.
The safety point behind it: highly reactive metals are never handled with bare or wet hands, because the heat released by the reaction is enough to cause burns.
How do you test the gas released when a metal meets acid?
Most metals react with dilute acids to release hydrogen gas, which you identify with the burning splint or pop test.
Add a few pieces of zinc to dilute hydrochloric acid and bubbles appear immediately. Collect the gas in an inverted test tube and bring a burning splint to its mouth: the gas burns with a pop sound. That pop identifies hydrogen.
The general pattern is:
metal + dilute acid → salt + hydrogen gas
So zinc with hydrochloric acid gives zinc chloride and hydrogen.
Non-metals generally do not react with dilute acids, so no bubbles form when you add sulphur or charcoal — which makes this another useful identification test alongside the physical ones from Part 1.
One caution to state in answers: the test uses dilute acid. Concentrated acids behave differently and are not what the syllabus reaction describes.
Add a few pieces of zinc to dilute hydrochloric acid and bubbles appear immediately. Collect the gas in an inverted test tube and bring a burning splint to its mouth: the gas burns with a pop sound. That pop identifies hydrogen.
The general pattern is:
metal + dilute acid → salt + hydrogen gas
So zinc with hydrochloric acid gives zinc chloride and hydrogen.
Non-metals generally do not react with dilute acids, so no bubbles form when you add sulphur or charcoal — which makes this another useful identification test alongside the physical ones from Part 1.
One caution to state in answers: the test uses dilute acid. Concentrated acids behave differently and are not what the syllabus reaction describes.
What is corrosion, and how do you prevent it?
Corrosion is the slow eating away of a metal's surface by the action of air and moisture over time.
In iron it is called rusting, and it produces a reddish-brown flaky layer that weakens the metal. Rusting needs both air and moisture — which is why iron tools rust quickly in the monsoon and slowly in a dry store.
In silver it is tarnishing: a black layer forms and the shine dulls, which is why silver utensils are polished before festivals. Copper develops a green coating, seen on old copper vessels and statues.
Protection works by keeping air and moisture off the surface:
- painting — used on gates, railings and bridges
- oiling or greasing — used on tools, machine parts and cycle chains
- galvanising — coating iron with a layer of zinc
- using stainless steel, which does not rust at all
Note that corrosion is a chemical change, not dirt. Rust cannot simply be wiped off, because the iron itself has turned into a new substance.
In iron it is called rusting, and it produces a reddish-brown flaky layer that weakens the metal. Rusting needs both air and moisture — which is why iron tools rust quickly in the monsoon and slowly in a dry store.
In silver it is tarnishing: a black layer forms and the shine dulls, which is why silver utensils are polished before festivals. Copper develops a green coating, seen on old copper vessels and statues.
Protection works by keeping air and moisture off the surface:
- painting — used on gates, railings and bridges
- oiling or greasing — used on tools, machine parts and cycle chains
- galvanising — coating iron with a layer of zinc
- using stainless steel, which does not rust at all
Note that corrosion is a chemical change, not dirt. Rust cannot simply be wiped off, because the iron itself has turned into a new substance.
Exam tip
Exam tip: naming the two conditions rusting needs
Asked why iron rusts, students answer "because it is left outside" or "because of water" — and half an answer scores half the marks.
Rusting requires both air (oxygen) and moisture together. Neither alone is enough, which is exactly what the standard three-test-tube experiment demonstrates: a nail in dry air does not rust, a nail in boiled water sealed with oil does not rust, and only the nail exposed to both air and water does.
So answer with both conditions named, and when asked for a prevention method, say what it does: paint or grease forms a barrier that keeps air and moisture away from the iron surface.
Rusting requires both air (oxygen) and moisture together. Neither alone is enough, which is exactly what the standard three-test-tube experiment demonstrates: a nail in dry air does not rust, a nail in boiled water sealed with oil does not rust, and only the nail exposed to both air and water does.
So answer with both conditions named, and when asked for a prevention method, say what it does: paint or grease forms a barrier that keeps air and moisture away from the iron surface.
Did you know
Why does the pop test identify hydrogen?
Hydrogen is itself combustible, so when a flame reaches a small collected volume of it, all of it burns at once.
That sudden burning pushes the air outward quickly, and the sharp sound you hear is that rush — the characteristic pop. A gas that simply put the splint out, rather than popping, would not be hydrogen.
That sudden burning pushes the air outward quickly, and the sharp sound you hear is that rush — the characteristic pop. A gas that simply put the splint out, rather than popping, would not be hydrogen.
Key takeaways
Reactions of metals: quick revision
- Burning in air gives oxides: metal oxides are basic and turn red litmus blue, while non-metal oxides are acidic and turn blue litmus red.
- Sodium reacts violently with water and is stored under kerosene; iron reacts very slowly, and gold and silver not at all.
- Metals react with dilute acids to give a salt and hydrogen, identified by a burning splint giving a pop; non-metals generally do not react.
- Corrosion is air and moisture slowly attacking a metal: rusting in iron, tarnishing in silver, a green coating on copper.
- Rusting needs both air and moisture, and is prevented by painting, oiling, galvanising or using stainless steel.
You will remember all of this far better after answering five questions on it than after reading it twice.
- Sodium reacts violently with water and is stored under kerosene; iron reacts very slowly, and gold and silver not at all.
- Metals react with dilute acids to give a salt and hydrogen, identified by a burning splint giving a pop; non-metals generally do not react.
- Corrosion is air and moisture slowly attacking a metal: rusting in iron, tarnishing in silver, a green coating on copper.
- Rusting needs both air and moisture, and is prevented by painting, oiling, galvanising or using stainless steel.
You will remember all of this far better after answering five questions on it than after reading it twice.