CBSE Class 10 Chemistry Chemical Reactions And Equations Worksheet Set 09

This is Printable Worksheet Set 09 for CBSE Class 10 Chemistry Chapter 1, Chemical Reactions and Equations, with 24 questions and answers. It begins with true or false statements on oxidation, reduction and reaction types and follows with very short answer questions on writing and balancing equations with state symbols, displacement reactions and signs of a chemical reaction.

True or False

Question 1. State whether the following statement is true or false: The number of atoms of each element is conserved in any chemical reaction.
Answer:
True.
Atoms are neither created nor destroyed in a reaction; they only rearrange.
In simple words: Atoms are only rearranged, never lost.

Teacher's Note:
1. This is why equations must be balanced.
2. It follows from the law of conservation of mass.

Question 2. State whether the following statement is true or false: Oxidation is the loss of electrons from a substance.
Answer:
True.
In the electronic view, a substance that loses electrons is oxidised, for example Na → Na⁺ + e⁻.
In simple words: Losing electrons means oxidation.

Teacher's Note:
1. Oxidation can also be described as gain of oxygen or loss of hydrogen.
2. Use the same idea for all three descriptions.

Question 3. State whether the following statement is true or false: Reduction is the gain of electrons by a substance.
Answer:
True.
A substance that gains electrons is reduced, for example Cu²⁺ + 2e⁻ → Cu.
In simple words: Gaining electrons means reduction.

Teacher's Note:
1. Oxidation and reduction occur together as a redox reaction.
2. Reduction is also removal of oxygen or addition of hydrogen.

Question 4. State whether the following statement is true or false: A complete chemical equation represents the reactants, products and their physical states symbolically.
Answer:
True.
State symbols (s), (l), (g) and (aq) are written after the formulae, so the equation gives the physical states too.
In simple words: State symbols make the equation more informative.

Teacher's Note:
1. (aq) means dissolved in water.
2. Conditions such as heat or catalyst are written above the arrow.

Question 5. State whether the following statement is true or false: A magnesium ribbon burns with a dazzling flame in air (oxygen) and changes into a white substance, magnesium oxide.
Answer:
True.
2Mg + O₂ → 2MgO. Magnesium burns with a dazzling white flame and leaves white magnesium oxide ash.
In simple words: Mg burns brightly and leaves white ash.

Teacher's Note:
1. Clean the ribbon with sandpaper before burning it.
2. Never look directly at the flame.

Question 6. State whether the following statement is true or false: Rusting is a double decomposition reaction.
Answer:
False.
Rusting is the slow oxidation of iron by oxygen and moisture to form hydrated iron(III) oxide. It is not a double displacement (double decomposition) reaction.
In simple words: Rusting is oxidation, not a swap of ions.

Teacher's Note:
1. Double displacement involves exchange of ions between two compounds.
2. Rust is Fe₂O₃·xH₂O.

Question 7. State whether the following statement is true or false: The reaction between nitrogen and hydrogen to give ammonia is an example of a combination reaction.
Answer:
True.
N₂ + 3H₂ → 2NH₃. Two reactants combine to give a single product.
In simple words: Two elements join to give one compound.

Teacher's Note:
1. This reaction is used in the Haber process.
2. It needs a catalyst and high pressure.

Question 8. State whether the following statement is true or false: Action of heat on ferrous sulphate is an example of a decomposition reaction.
Answer:
True.
2FeSO₄ → Fe₂O₃ + SO₂ + SO₃. One compound breaks into three simpler substances.
In simple words: One compound splits into several.

Teacher's Note:
1. It is a thermal decomposition reaction.
2. Gases with the smell of burning sulphur are given out.

Question 9. State whether the following statement is true or false: The formation of Cu and H₂O in the reaction of copper oxide with hydrogen is an example of a redox reaction.
Answer:
True.
CuO + H₂ → Cu + H₂O. CuO loses oxygen (reduced) and H₂ gains oxygen (oxidised), so both happen together.
In simple words: One substance is oxidised while the other is reduced.

Teacher's Note:
1. Redox means reduction and oxidation together.
2. Hydrogen is the reducing agent here.

Very Short Answer Questions

Question 10. Write a balanced chemical equation when magnesium metal reacts with aqueous hydrochloric acid to produce a solution of magnesium chloride and hydrogen gas.
Answer:
Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
Check: Mg 1 = 1, H 2 = 2, Cl 2 = 2.
In simple words: Magnesium pushes out hydrogen from the acid.

Teacher's Note:
1. Write the formulae first and then balance.
2. Hydrogen gas burns with a pop sound.

Question 11. Can a combination reaction be a redox reaction? Explain with an example.
Answer:
Yes, a combination reaction can also be a redox reaction.
Example: 2Mg + O₂ → 2MgO. Here magnesium gains oxygen (oxidised) and oxygen is reduced to oxide ions, so oxidation and reduction occur together.
In simple words: Joining with oxygen is both combination and oxidation.

Teacher's Note:
1. Not all combination reactions are redox, for example CaO + H₂O → Ca(OH)₂.
2. Check whether any substance gains or loses oxygen or electrons.

Question 12. What are the different types of chemical reactions?
Answer:
The main types of chemical reactions are:
1. Combination reactions
2. Decomposition reactions
3. Displacement reactions
4. Double displacement reactions
5. Oxidation–reduction (redox) reactions
In simple words: There are five main types.

Teacher's Note:
1. Give one example of each type for full marks.
2. Precipitation and neutralisation are double displacement reactions.

Question 13. Define a displacement reaction and give one example.
Answer:
A reaction in which a more reactive element displaces a less reactive element from its compound is called a displacement reaction.
Example: Zn + CuSO₄ → ZnSO₄ + Cu.
In simple words: A strong metal pushes a weaker metal out of its salt.

Teacher's Note:
1. Use the reactivity series to decide whether the reaction occurs.
2. Cu cannot displace Zn from ZnSO₄.

Question 14. What happens when sodium reacts with water?
Answer:
Sodium is more reactive than hydrogen, so it displaces hydrogen from water.
2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g). The reaction is violent and exothermic, and the hydrogen gas may catch fire.
In simple words: Sodium reacts violently, giving NaOH and hydrogen.

Teacher's Note:
1. Sodium is stored in kerosene.
2. The solution formed turns red litmus blue.

Question 15. Write the chemical equation and name the reaction when a solution of sodium chloride is mixed with a solution of silver nitrate and a white precipitate of silver chloride is formed.
Answer:
NaCl(aq) + AgNO₃(aq) → AgCl(s)↓ + NaNO₃(aq)
The two compounds exchange ions and an insoluble solid forms, so it is a double displacement (precipitation) reaction.
In simple words: Two solutions mix and a white solid appears.

Teacher's Note:
1. The white precipitate is silver chloride.
2. It turns grey in sunlight.

Question 16. Potassium chlorate (KClO₃) on heating forms potassium chloride and oxygen. Write a balanced equation for this reaction.
Answer:
Skeleton: KClO₃ → KCl + O₂
Balance oxygen: 2KClO₃ → 2KCl + 3O₂
2KClO₃ →^heat 2KCl + 3O₂. K 2 = 2, Cl 2 = 2, O 6 = 6.
In simple words: Put 2 before KClO₃ and KCl and 3 before O₂.

Teacher's Note:
1. Balance oxygen last when it is in several places.
2. It is a thermal decomposition reaction.

Question 17. Give an example of a chemical reaction characterised by a change in temperature.
Answer:
When quicklime reacts with water, a large amount of heat is released and the temperature rises.
CaO(s) + H₂O(l) → Ca(OH)₂(aq) + heat
In simple words: Quicklime with water becomes hot.

Teacher's Note:
1. Temperature change is one sign of a chemical reaction.
2. Other signs are change in colour, state, and evolution of gas.

Question 18. To balance a chemical equation, can we change the formulae of either reactants or products? Give a reason.
Answer:
No. The formula of a compound shows its true composition.
Changing a formula would change the substance itself. Only the numbers in front (coefficients) can be changed.
In simple words: Change only the coefficients, never the formulae.

Teacher's Note:
1. Subscripts are fixed for a compound.
2. Adjust coefficients one element at a time.

Question 19. Write balanced chemical equations with state symbols for the following reactions: (a) Aqueous solutions of barium chloride and sodium sulphate react to give insoluble barium sulphate and a solution of sodium chloride. (b) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.
Answer:
(a) BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s)↓ + 2NaCl(aq)
(b) NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
In simple words: Use (aq) for dissolved, (s) for solid and (l) for liquid.

Teacher's Note:
1. Show the precipitate with (s) and a downward arrow.
2. Part (b) is a neutralisation reaction.

Question 20. State one basic difference between a physical change and a chemical change.
Answer:
In a physical change no new substance is formed, but in a chemical change one or more new substances with new properties are formed.
In simple words: Chemical change makes new substances.

Teacher's Note:
1. Melting of ice is physical; rusting of iron is chemical.
2. Chemical changes are usually irreversible.

Question 21. What is meant by a chemical reaction?
Answer:
A chemical reaction is a process in which one or more substances (reactants) are changed into new substances (products) with different properties.
In it, old bonds in the reactants break and new bonds form in the products.
In simple words: Old substances change into new ones.

Teacher's Note:
1. Signs: change in colour, state, temperature or gas evolution.
2. Example: burning of magnesium.

Question 22. In the equations AgNO₃(aq) + NaCl(aq) → AgCl(s) ↓ + NaNO₃(aq) and FeS + H₂SO₄ → FeSO₄ + H₂S ↑, what do the two different arrows (↓ and ↑) indicate?
Answer:
The downward arrow (↓) shows that the product is formed as a precipitate (an insoluble solid).
The upward arrow (↑) shows that the product is a gas that escapes.
In simple words: Down arrow means a solid settles; up arrow means a gas goes out.

Teacher's Note:
1. Arrows next to the products make the equation more informative.
2. (s) and (g) state symbols convey the same idea.

Question 23. When limestone is heated it decomposes into quicklime and carbon dioxide gas. Write the chemical equation for this reaction.
Answer:
CaCO₃(s) →^heat CaO(s) + CO₂(g)
Limestone is CaCO₃ and quicklime is CaO.
In simple words: Heating limestone gives quicklime and CO₂.

Teacher's Note:
1. It is a thermal decomposition reaction.
2. The reaction is endothermic.

Question 24. What happens when a magnesium ribbon burns in air?
Answer:
Magnesium burns with a dazzling white flame and combines with oxygen of the air to form white magnesium oxide powder.
2Mg(s) + O₂(g) → 2MgO(s)
In simple words: Mg burns brightly and leaves white ash.

Teacher's Note:
1. The reaction is exothermic and a combination reaction.
2. The ash is MgO.