CBSE Class 10 Science Chemical Reactions and Equations VBQs Set 06

This VBQs Set 06 for CBSE Class 10 Science Chapter 1, Chemical Reactions and Equations, has 13 real-life and competency questions with stepwise answers. The questions link rancidity, corrosion, photography, respiration and burning magnesium to everyday situations and the careful use of food and metals.

Very Short Answer Questions

Question 1. Food items can be preserved for longer time when kept in a refrigerator. What is the reason behind this fact?
Answer:
At a lower temperature, oxidation of the food is slowed down, so the food keeps for a longer time.
In simple words: Cold slows down the reaction that spoils food.

Teacher's Note:
1. Mention oxidation.
2. Temperature affects reaction speed.

Question 2. Why do we store silver chloride in dark coloured bottles? Explain in brief.
Answer:
In sunlight silver chloride decomposes to silver and chlorine: 2AgCl →^sunlight 2Ag + Cl₂. Dark bottles keep light away, so it stays unchanged.
In simple words: Light would decompose it.

Teacher's Note:
1. It is a photochemical reaction.
2. Silver bromide is also light sensitive.

Short Answer Questions

Question 3. A silver article generally turns black when kept in the open for a few days. The article when rubbed with toothpaste again starts shining. (A) Why do silver articles turn black when kept in the open for a few days? Name the phenomenon involved. (B) Name the black substance formed and give its chemical formula.
Answer:
(a) Silver reacts with sulphur compounds (such as H₂S) in the air to form a black coating. The phenomenon is corrosion.
(b) The black substance is silver sulphide, Ag₂S.
Toothpaste rubs off the coating and the shine returns.
In simple words: Air slowly corrodes silver into black silver sulphide.

Teacher's Note:
1. Only the surface is affected.
2. Rubbing removes the layer.

Question 4. A student has been collecting silver coins and copper coins. One day she observed a black coating on silver coins. Which chemical phenomenon is responsible for these coatings? Write the chemical name of black and green coatings.
Answer:
The phenomenon is corrosion.
Silver gets a black coating of silver sulphide (Ag₂S).
Copper gets a green coating of basic copper carbonate.
In simple words: Air and moisture slowly change the metal surfaces.

Teacher's Note:
1. Name the coloured coatings.
2. Both are metal compounds.

Question 5. Assertion (A): Quick lime is used for whitewashing walls.
Reason (R): The solution of slaked lime reacts slowly with the carbon dioxide in air to form a thin layer of calcium carbonate on the walls after two to three days which gives a shiny finish to the walls.
(a) Both (A) and (R) are true, and (R) is the correct explanation of (A). (b) Both (A) and (R) are true, and (R) is not the correct explanation of (A). (c) (A) is true but (R) is false. (d) (A) is false but (R) is true.
Answer:
Option (a). Both (A) and (R) are true and (R) explains (A).
Quicklime reacts with water to give slaked lime: CaO + H₂O → Ca(OH)₂.
Slaked lime slowly reacts with carbon dioxide of the air: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. The thin layer of calcium carbonate gives a shiny finish.
In simple words: Slaked lime turns slowly into shiny calcium carbonate.

Teacher's Note:
1. Write both equations.
2. The change takes a few days.

Question 6. Tell whether respiration is exothermic reaction or endothermic reaction. Why?
Answer:
During respiration, glucose combines with oxygen in the cells to give carbon dioxide and water, with release of energy: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy
Since energy is released, it is exothermic.
In simple words: Our body uses the heat energy released from food.

Teacher's Note:
1. Glucose is the fuel.
2. Heat is released.

Question 7. Name two salts that are used in black and white photography. Give reactions when they are exposed to light.
Answer:
The salts are silver chloride and silver bromide.
2AgCl →^sunlight 2Ag + Cl₂
2AgBr →^sunlight 2Ag + Br₂
In simple words: Light decomposes silver salts into silver.

Teacher's Note:
1. Both are photochemical decompositions.
2. The silver formed gives the dark image.

Question 8. While Abhi was about to burn magnesium ribbon in the chemistry laboratory, the teacher asked him to clean the ribbon with the sand paper before burning it. What could be the reason for the above instruction by the teacher? After burning the magnesium ribbon, Abhi obtained a white coloured residue. Name this residue. What type of chemical reaction has occurred? Write a balanced chemical equation to explain the reaction.
Answer:
The ribbon is coated with magnesium oxide, which would stop it from burning. Cleaning removes it.
The white residue is magnesium oxide.
2Mg + O₂ → 2MgO. It is a combination reaction.
In simple words: Clean magnesium burns with a dazzling white flame to give white MgO.

Teacher's Note:
1. Never look at the flame directly.
2. It is also oxidation.

Question 9. Write two examples of every day life, where redox reactions are taking place.
Answer:
(i) Tarnishing of silver jewellery: silver is oxidised by H₂S of the air to black silver sulphide.
(ii) Rusting of iron: iron is oxidised to iron oxide.
In simple words: Rusting and tarnishing are daily redox changes.

Teacher's Note:
1. Name what is oxidised.
2. Both are corrosion.

Question 10. Give reasons for the following: (i) Keeping food in air tight containers help in preventing rancidity. (ii) Moist air and acidic gases are not good for some metals. (iii) Chips manufacturers usually flush bags of chips with nitrogen gas.
Answer:
(i) It does not allow air (oxygen) to reach the food, preventing oxidation.
(ii) Moist air corrodes iron, while acidic gases corrode metals such as copper and silver.
(iii) Nitrogen is inert and prevents oxidation of the chips.
In simple words: Keep oxygen and moisture away from food and metals.

Teacher's Note:
1. Give one reason each.
2. Rancidity is oxidation of fats and oils.

Long Answer Questions

Question 11. What happens when food materials containing fats and oils are left for a long time? List two observable changes and suggest three ways by which this phenomenon can be prevented.
Answer:
They get oxidised and become rancid.
Observable changes: the smell becomes unpleasant and the taste changes (colour also changes).
Prevention: (i) add antioxidants, (ii) keep food in airtight containers, (iii) store in a refrigerator (flushing with nitrogen is also used).
In simple words: Fats and oils go bad when oxygen reaches them.

Teacher's Note:
1. Name the process: rancidity.
2. Give three methods.

Case Based Questions

Question 12. While burning fireworks, we experience fire with multiple colours. One of the ways to achieve this is by mixing different metal powders while manufacturing the crackers. For instance, copper burns with a bright green colour and magnesium with a bright white colour. Let us dig deeper into this simple but enjoyable experiment with burning magnesium which can be easily performed in a lab or even at home.
(A) Identify the type of reaction that involves burning of a strip of magnesium ribbon. Also write a balanced chemical equation with the type of state symbols.
(B) While burning a strip of magnesium ribbon, what is the colour (i) of flame? (ii) of the ash formed?
(C) (i) How will you find out the nature of the ash formed? (ii) Give an example of a combination reaction in which reaction takes place (1) between two elements. (2) between two compounds.
Answer:
(A) Combination reaction: 2Mg(s) + O₂(g) → 2MgO(s)
(B) (i) Dazzling white flame (ii) white ash
(C) Dissolve the ash in water and test with litmus: red litmus turns blue, so it is basic (MgO + H₂O → Mg(OH)₂).
(1) C(s) + O₂(g) → CO₂(g) (2) NH₃(g) + HCl(g) → NH₄Cl(s)
In simple words: Burning magnesium gives white basic magnesium oxide.

Teacher's Note:
1. Litmus tells the nature.
2. Give examples with states.

Question 13. Arjun decided to investigate the effect of water and air on some metallic objects under identical experimental conditions. He kept a pair of silver earrings, an iron nail, a copper vessel and an aluminium utensil outside for few days. He observed that a pair of silver earrings turns black, iron nail gets a brown flaky coating, the copper vessel gets a coating of green layer and the aluminium utensil loses its lustre and its surface becomes dull.
(A) List and state the phenomena involved in the given case.
(B) Which substances are formed on the surface of silver and copper and why?
(C) How can the surface of copper vessel be cleaned?
Answer:
(A) Corrosion in all cases; for iron it is called rusting.
(B) Silver forms black silver sulphide with sulphur compounds in air. Copper forms green basic copper carbonate with moist carbon dioxide of air.
(C) Rub it with lemon or tamarind (acidic) which dissolves the green layer, and then wash it.
In simple words: All the metals are slowly corroded by air and moisture.

Teacher's Note:
1. Aluminium gets a dull oxide layer.
2. Use a mild acid to clean copper.