Chemical Reactions and Equations: CBSE Class 10 Chemistry practice questions

In Activity 1.1 a cleaned magnesium ribbon is held in a flame. Which observation shows that a chemical reaction has taken place?

1. The ribbon becomes hot when it is held in the flame
2. The ribbon is cut into a piece 3 to 4 cm long
3. It burns with a dazzling white flame and leaves a new white powder, magnesium oxide
4. The ash is collected in a watch-glass

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C. It burns with a dazzling white flame and leaves a new white powder, magnesium oxide

A chemical change forms new substances. Burning magnesium makes a white powder, magnesium oxide, which is nothing like the shiny metal. Heating alone does not prove a reaction, and cutting the ribbon or collecting the ash are just steps in the procedure.

The skeletal equation for burning magnesium is $\ce{Mg + O2 -> MgO}$. Why is it not balanced?

1. It has 1 magnesium atom on the left but 2 on the right
2. The oxygen molecule should be written as O instead of O₂
3. The product should be written as MgO₂
4. It has 2 oxygen atoms on the left but only 1 on the right

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D. It has 2 oxygen atoms on the left but only 1 on the right

Magnesium has 1 atom on each side, but oxygen has 2 on the left (in O₂) and 1 on the right (in MgO). Never fix this by changing formulae, because O₂ and MgO are correct. Put coefficients instead: $\ce{2Mg + O2 -> 2MgO}$.

Which of these is the correctly balanced equation for iron reacting with steam?

1. $\ce{3Fe + 4H2O -> Fe3O4 + H2}$
2. $\ce{3Fe + 4H2O -> Fe3O4 + 4H2}$
3. $\ce{Fe + 4H2O -> Fe3O4 + 4H2}$
4. $\ce{3Fe + 2H2O -> Fe3O4 + 2H2}$

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B. $\ce{3Fe + 4H2O -> Fe3O4 + 4H2}$

Only $\ce{3Fe + 4H2O -> Fe3O4 + 4H2}$ has 3 Fe, 8 H and 4 O on each side. The first wrong option leaves hydrogen 8 on the left and 2 on the right. The second has 1 Fe against 3. The third has only 2 O against 4.