Chemical Reactions and Equations – important questions for Boards 2027
CBSE Class 10 Science
Which is the skeletal chemical equation for the burning of magnesium in air? (A) Mg + O₂ → MgO (B) 2Mg + O₂ → 2MgO (C) Mg + O → MgO (D) MgO → Mg + O₂
Answer
(A) Mg + O₂ → MgO — it is the skeletal chemical equation for the burning of magnesium in air.
How do chemical formulae make a chemical equation a more concise representation of a reaction than a word equation?
Answer
Chemical formulae replace substance names with symbols, making chemical equations more concise and useful than word-equations. For example, magnesium + oxygen → magnesium oxide becomes Mg + O₂ → MgO.
For Mg + O₂ → MgO, state the number of oxygen atoms on each side and explain why these counts show that the equation is unbalanced.
Answer
1. In Mg + O₂ → MgO, oxygen is present as O₂ on the left-hand side, so there are two oxygen atoms among the reactants. 2. On the right-hand side, the formula MgO contains one oxygen atom in the product. The oxygen-atom counts are therefore unequal, so the equation is unbalanced and does not show equal mass on both sides.
Consider the skeletal equation Mg + O₂ → MgO. Count the atoms of each element on both sides and state whether the equation is balanced. Explain what the term ‘skeletal chemical equation’ means, and write the balanced equation for the burning of magnesium in air.
Answer
2Mg(s) + O₂(g) → (heat) 2MgO(s) This is a combination reaction because magnesium and oxygen form one product, magnesium oxide. Magnesium ribbon burns with a dazzling white flame and changes into a white powder, magnesium oxide. A skeletal chemical equation is an unbalanced chemical equation that does not show equal numbers of atoms of each element on both sides. In Mg + O₂ → MgO, the LHS has one magnesium atom and two oxygen atoms; the RHS has one magnesium atom and one oxygen atom. Therefore, magnesium atoms are balanced, but oxygen atoms are not balanced, so the equation is skeletal. A balanced equation has equal numbers of each type of atom on both sides, in accordance with the law of conservation of mass.
What is a skeletal chemical equation?
Answer
A skeletal chemical equation is an unbalanced chemical equation for a reaction, in which the mass is not the same on both sides.
State the law of conservation of mass as it applies to a chemical reaction.
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When aqueous sodium sulphate and barium chloride are mixed, a white insoluble substance forms. Write the balanced chemical equation and identify the precipitate. State its colour and whether it dissolves in water.
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(a) Write the reaction for respiration and explain why it is exothermic. (b) State why the decomposition of vegetable matter into compost is exothermic. (c) Write the reaction of slaked lime with carbon dioxide in air and explain how it affects whitewashed walls.
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A reaction in which two or more reactants form a single product is called a: (A) decomposition reaction (B) combination reaction (C) displacement reaction (D) double displacement reaction
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Why must a skeletal chemical equation be balanced? Relate your answer to the number of atoms before and after a reaction.
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25 more probable questions in this chapter, every one with a full answer.
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