Question-2. Why is magnesium ribbon cleaned before burning in air?

Ans- Before burning in air, the magnesium ribbon is cleaned with glued paper, so that the layer of magnesium oxide on the ribbon is removed and it can easily burn after reacting with the oxygen present in the air. Therefore magnesium ribbon is cleaned before burning in air.

Question-3. What changes occur as a result of a chemical reaction?

Ans- The following changes occur as a result of a chemical reaction—

• • change in status.

  • Change in colour.

  • change in temperature.

  • gas emissions

Question-4. What are reactants and products?

Ans-

Reactants:- The substance that takes part in a chemical reaction is called a reactant.

Product:- The new substance that is formed as a result of a chemical reaction is called a product.

Ex:- C + O₂ (reactant) → CO₂ (Product)

Question-5. What is a combination reaction called?

Ans- A chemical reaction in which two or more reactants combine to form a single product is called a combination reaction.

Ex:- C + O₂ → CO₂

Question-6. What is a decomposition reaction called?

Ans- A chemical reaction in which a single reactant breaks down to form smaller products is called a decomposition reaction.

Ex:- CaCO₃ (Limestone) → CaO + CO₂ (slaked lime)

Question-7. What is an exothermic chemical reaction called?

Ans- A chemical reaction in which heat is produced along with the formation of a product is called an exothermic chemical reaction.

Ex:- CH₄ + 2O₂ (methane gas) → CO₂ + 2H₂O + (heat)

Question-8. What is an endothermic chemical reaction called?

Ans- The chemical reaction in which energy is absorbed is called endothermic chemical reaction.

2AgCl → 2Ag + Cl₂

Question-9. What information is obtained by chemical equation?

Ans- The following information is obtained by chemical equation—

• • What substances are participating in the reactant and what products are formed as a result of the reaction.

  • How many atoms or molecules of the reactant react to form how many atoms or molecules of the product.

  • How much volume of reactant makes up what volume of product reacts.

  • How many weights of the reactant make up the reaction?

Question-10. Balance the following chemical equation—

(A). N₂ + H₂ → NH₃

⇒ N₂ + 3H₂ → 2NH₃

(B). Na + O₂ → Na₂O

⇒ 4Na + O₂ → 2Na₂O

(C). K + H₂O → KOH+ H₂

⇒ 2K + 2H₂O → 2KOH+ H₂

(D). Na + H₂O → NaOH + H₂

⇒ 2Na + 2H₂O → 2NaOH + H₂

(E). NH₃ + O₂ → N₂ + H₂O

⇒ 4NH₃ + 3O₂ → 2N₂ + 6H₂O

(F). NaCl + AgNO₃ → AgCl + NaNO₃

⇒ NaCl + AgNO₃ → AgCl + NaNO₃

(G). BaCl₂ + H₂SO₄ → BaSO₄ + HCl

⇒ BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl

(H). NaOH + H₂SO₄ → Na₂SO₄ + H₂O

⇒ 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O

(I). HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O

⇒ 2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O

(J). CH₄ + O₂ → CO₂ + H₂O

⇒ CH₄ + 2O₂ → CO₂ + 2H₂O

Question-11. Write one equation each of those decomposition reactions in which energy is given in the form of heat, light and electricity.

Ans-

• CaCO₃ → CaO + CO₂

• 2AgCl → 2Ag + Cl₂

• 2H₂O → 2H₂ + O₂

Question-12. Identify the oxidized and reduced substances in the following reaction.

(I). 4Na(S) + O₂(g) → 2Na₂O(S)

Ans-

• • Anabolic Substances:- Na

  • Reduced Substance:- O₂

(II). CuO(S) + H₂(g) → Cu(S) + H₂O(l)

Ans-

• Anabolic Substances:- H₂

• Anabolic Substances:- Cu

Question-13. Write the balanced chemical equations for the following reactions.

(I). hydrogen + chlorine → hydrogen chloride

Ans- H₂ + Cl₂ → 2HCl

(II). barium chloride + aluminum sulfate → barium sulfate + aluminum chloride

Ans- 3BaCl₂ + Al₂(SO₄)₃ → 3BaSO₄ + 2AlCl₃

(III). sodium + water → sodium hydrochloride + hydrogen

Ans- 2Na + 2H₂O → 2NaOH + H₂

Question-14. Write balanced chemical equations for the following reactions.

(I). calcium hydroxide + Carbon dioxide → calcium carbonate + water

Ans- Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

(II). zinc + silver nitrate → zinc nitrate + silver

Ans- Zn + 2AgNO₃ → Zn(NO₃)₂ + 2Ag

(III). aluminum + copper chloride → aluminum chloride + Copper

Ans- 2Al + 3CuCl₂ → 2AlCl₃ + 3Cu

(IV). barium chloride + potassium sulfate → barium sulfate + potassium chloride

Ans- BaCl₂ + K₂SO₄ → BaSO₄ + 2KCl

Question-15. Why are foods containing oils and fats affected by nitrogen?

Ans- A less active gas, such as nitrogen, is filled by removing oxygen from the food containers or packets to prevent oxidization of foods containing oils and fats. Therefore, due to filling of nitrogen gas in oily and fatty foods, the food does not get oxidized and its taste does not deteriorate for a long time. So foods containing oils and fats are affected by nitrogen.

Question-16. Why is respiration called an exothermic reaction?

Ans- Food contains carbohydrates. Glucose is formed from the breakdown of these carbohydrates. Oxygen reaches our cells through respiration and glucose combines with the oxygen present in our cells to provide us with energy. This reaction is called respiration reaction. Energy is also released in this respiration reaction. Hence respiration is also called exothermic reaction.

Question-17. What is rancidity?

Ans- Due to the process of oxidation in foods made from fats and oils, unpleasant smell and taste are produced in them, which is called pernicious odor. Bad odor can be prevented by the following methods—

• Keeping food items in the refrigerator.

• Keep in airtight containers.

Question-18. Why does the color of the solution change when an iron nail is dipped in a solution of copper sulphate?

Ans- When an iron nail is dipped in a solution of copper sulphate, the iron nail displaces copper from the solution of copper sulphate and forms iron sulphate. Due to the formation of iron sulphate, the dark blue color of copper sulphate becomes turbid and turns into light green colour. That is why when an iron nail is dipped in a solution of copper sulphate, the color of the solution changes.

Fe + CuSO₄ (Blue) → FeSO₄ + Cu (Green)

Question-19. What is a balanced chemical equation? Why is it necessary to balance a chemical equation?

Ans- A chemical equation in which the number of atoms of each element in the reactants and products is equal is called a balanced chemical equation.

According to the law of conservation of mass, the reactants and products have the same mass. Therefore it is necessary to balance the chemical equation to equalize the mass of the reactants and products.

Question-20. A solution of a substance X is used for whitening—

(I) Name the substance X and write its formula.

Ans- Name of substance X = Quick lime (CaO)

(II) Write the reaction of substance X with water.

Ans- CaO + H₂O → Ca(OH)₂ + Heat

Question-21. A brown shiny element X on heating in the presence of air turns black in color Name this element X and that black colored compound.

Ans-

Element X → Copper (Cu)

black compound → CuO (copper oxide)

2Cu + O₂ → 2CuO

Question-22. In the refining of silver, copper is displaced from a solution of silver nitrate to obtain silver. Write the reaction for this process.

Ans- 2AgNO₃ + Cu → 2Ag + Cu(No₃)₂

Question-23. What is a redox reaction called?

Ans- A chemical reaction in which one reactant is oxidized and another reactant is reduced. Therefore, the reaction in which both oxidation (oxidation) and reduction (reduction) take place simultaneously is called redox reaction.

Question-24. What is Corrosion? What are the ways to reduce it?

Ans- When a metal gets corrosive due to exposure to air, moisture, sunlight, acid etc., it is called corrosion. Following are the measures to reduce corrosion—

• (I) by painting things ।

• (II) by oil or grease।

Question-25. Why is a decomposition reaction called the opposite of a combination reaction? Write the equations for these reactions.

Ans- In a combination reaction two or more reactants combine to form a single product, whereas in a decomposition reaction a single reactant breaks down to form two or more products. Hence decomposition reaction is called opposite to combination reaction.

Ex:-

C + O₂ → CO₂ (combination reaction)

CaCO₃ → CaO + CO₂ (dissociation reaction)

Question-26. What is Oxidation or reduction?

Ans-

Oxidation:- When there is an increase (addition) of oxygen or decrease of hydrogen in a substance, it is called oxidation. Oxidation reaction is also called oxidation reaction.

Ex:- 2Cu + O₂ → 2CuO (oxidation of copper)

Reduction:- When there is an increase of hydrogen or a decrease in oxygen in a substance, it is called reduction. Reduction reaction is also called reduction reaction.

Ex:- H₂ + Cl₂ → 2HCl (reduction of Cl)

Question-27. How many types of chemical reaction are there?

Ans- Following are the types of chemical reaction—

• • combination reaction

  • decomposition reaction

  • exothermic reaction (combustion reaction)

  • endothermic reaction

  • Displacement (single displacement reaction)

  • double displacement reaction

  • neutralization reaction

  • precipitation reaction

Question-28. What is called displacement or single displacement reaction?

Ans- A chemical reaction in which one element displaces another element from its compound is called displacement or single displacement reaction.

Ans:- Zn + CuSo₄ → ZnSo₄ + Cu

Question-29. What is called double displacement reaction?

Ans- When two compounds exchange their ions (radicals) to form two new compounds, it is called double displacement reaction.

Ex:- Na₂So₄ + BaCl₂ → BaSo₄ + 2NaCl

Question-30. What is a neutralization reaction called?

Ans- A chemical reaction in which an acid and a base exchange their ions to form a salt and water. It is called neutralization reaction.

Ex-:- HCl (acid)+ NaOH (alkali) → NaCl (salt) + H₂O (water)

Question-31. What is a precipitation reaction called?

Ans- The chemical reaction in which a precipitate is formed along with the product is called a precipitation reaction.

Ex:- AgNO₃ + NaCl → AgCl + NaNO₃ (precipitate)