Review draft

Subject: Science · Kind: chapter · Level: medium · Source: AI · 5 questions · draft

Chapters: Chemical Reactions and Equations, Acids, Bases and Salts

Iron reacts with steam to form Fe3O4 and hydrogen gas. Which of the following is the correctly balanced chemical equation with state symbols?

1. A.3Fe(s) + 4H2O(g) → Fe3O4(s) + H2(g)
2. B.Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g)
3. C.3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g)correct
4. D.3Fe(s) + H2O4(g) → Fe3O4(s) + 4H2(g)

Explanation: Three Fe, eight H and four O atoms are present on each side only in 3Fe + 4H2O → Fe3O4 + 4H2. Balancing is done only by changing coefficients, never by altering formulae such as writing H2O4.

1. A.Both A and R are true, and R is the correct explanation of A.
2. B.Both A and R are true, but R is not the correct explanation of A.correct
3. C.A is true, but R is false.
4. D.A is false, but R is true.

Explanation: Magnesium and oxygen combine to form a single product, magnesium oxide, so A is true. R is also true, but the colour of the product does not explain why the reaction is a combination reaction.

Which statement correctly classifies the change taking place on strong heating?

1. A.It is a combination reaction, because iron, sulphur and oxygen combine to form one product.
2. B.It is a neutralisation reaction, because the sulphur oxides formed are acidic in nature.
3. C.It is a physical change, because only the colour of the crystals changes on heating.
4. D.It is a thermal decomposition reaction, because a single reactant breaks down into simpler products on heating.correct

Explanation: A single reactant, FeSO4, breaks down into simpler substances (Fe2O3, SO2 and SO3) when heated, which is a thermal decomposition reaction. The new substances and the smell of burning sulphur show that it is a chemical change.

Carbon dioxide produced by the reaction of sodium carbonate with dilute HCl is passed through lime water for a long time. What is observed, and why?

1. A.The lime water first turns milky due to CaCO3, then becomes clear as excess CO2 forms soluble Ca(HCO3)2.correct
2. B.The lime water first turns milky due to CaCO3, then becomes clear as the CaCO3 decomposes into CaO and CO2.
3. C.The lime water stays clear throughout, because calcium carbonate is soluble in water.
4. D.The lime water turns milky and stays milky, because CaCO3 is insoluble in excess carbon dioxide.

Explanation: CO2 first forms insoluble white CaCO3 with Ca(OH)2, making lime water milky. With excess CO2 and water, CaCO3 changes to soluble calcium hydrogencarbonate, Ca(HCO3)2, and the milkiness disappears.

While diluting concentrated sulphuric acid in the laboratory, the acid must always be added slowly to water with constant stirring, and not water to the acid. The reason is that

1. A.water reacts with the concentrated acid to produce hydrogen gas, which may explode
2. B.the acid becomes weaker if water is poured directly over the concentrated acid
3. C.dissolving the acid in water is highly exothermic, and the heat may make the mixture splash out and cause burnscorrect
4. D.the concentrated acid is lighter than water and floats on it, so it does not mix

Explanation: Dissolving a concentrated acid in water releases a large amount of heat. If water is added to the acid, the local heating can cause splashing and burns, and the glass container may even break.