Question

Ramesh mixed some iron filings with sulphur powder in a china dish. In another dish, he mixed iron filings and sulphur powder and heated the mixture. Which of his observations is not correct?
1. Dish X shows a physical change while dishY shows a chemical change.
2. In dish X sulphur powder and iron filings can be seen separately.
3. In dish Y a new substance is formed on heating the mixture.
4. The change which has taken place in dish Y is reversible.

Answer 1

We should first know whether the reaction is chemical or physical and what exactly happens in the reaction so that we can conclude something. In dish X physical change occurs and in dish Y chemical reaction occurs.

Complete step by step solution:
We should first know what kind of reactions take place in both the dishes. Then we can come to some conclusion. Let’s first look at what happens.
When iron filings and sulphur powder are mixed, both retain their properties. It means they have not undergone any chemical reaction. Therefore, mixing of iron and sulphur is a physical change.
When iron filings and sulphur powder are mixed and heated they undergo a chemical reaction and form ferrous sulphide ($FeS$ ). It is a new substance which has properties entirely different from Iron ($Fe$) and Sulphur ($S$) .Therefore, heating of mixture of iron and sulphur powder is a chemical change.
Now let us look at all the options one by one,
So the first option is that “Dish X shows a physical change while dishY shows a chemical change”. This is true according to the above. So it is the correct statement.
Second observation is that “In dish X sulphur powder and iron filings can be seen separately”. Which is true as it is not a chemical reaction.
Third observation is that “In dish Y a new substance is formed on heating the mixture”. This statement is true as we get ferrous sulphide in the reaction.
Fourth statement is that “The change which has taken place in dish Y is reversible”. This is not true as we can not get Iron and Sulphur after the change has occurred. So it is an irreversible reaction. So it is an incorrect observation. So it is the correct option.

So the correct option is 4th.

Note: A reversible reaction is a reaction where the reactants and products react together to give the reactants back. And the opposite is true for irreversible reactions. It must always be remembered that in a reversible reaction the entropy does not change.