Exemplar Questions and Solutions

Important CBSE Questions – 1 Mark Questions

1) Which type of chemical reaction does the general chemical reaction A + BC → AC + B represent?

Ans) A + BC → AC + B represents chemical displacement reaction

2) Which type of chemical reaction does the general chemical reaction AB → A + B represent?

Ans) AB → A + B represent chemical decomposition reaction

3) What is the chemical formula of rust?

The chemical formula for rust is Fe₂O₃.nH₂O

4) Which substance is added to foods high in oil and fat to prevent rancidity?

Ans) Antioxidants are added to foods high in oil and fat to prevent rancidity. Antioxidants prevent oxidation of fats and oils.

5) List any two changes that take place when oily foods get oxidized?

Ans) The two prominent changes that takes place when oily foods get oxidized are

a) Bad taste

b) Unpleasant smell

6) What are the two conditions that are necessary for the rusting of iron?

Ans) The two conditions that are necessary for the rusting of iron are

a) The presence of water which has an acidic P^H

b) Presence of oxygen

If any one of the above condition is absent then rusting would not happen.

7) Why should we not touch iron container in which CaO reacts with H₂O? Give reason.

Ans) We not touch iron container in which CaO reacts with H₂O because the iron container will become hot as the reaction is exothermic.

CaO(s) + H₂O(l) → Ca(OH)₂(aq) + heat

8) Why is aluminium called a self-protecting metal?

Ans) Aluminium is chemically a very reactive metal and it reacts with atmospheric oxygen to form a thin layer of aluminum oxide, which is sticky in nature. This oxide protects the metal from further corrosion.

9) Write a balanced chemical equation for chemical combination reaction in which a compound reacts with an element:

Ans) The formation of Sulphur trioxide from sulphur dioxide and oxygen is a combination reaction in which a compound reacts with an element

2SO₂(g) + O₂(g) → 2SO₃(g)

10) A rod of metal X is placed in an aqueous solution of lead nitrate. After sometime, it was observed that a thin layer of lead is deposited on the the rod of metal X. According to you, which is more reactive, lead or metal X and why?

Ans) The above given reaction is an example of displacement reaction. During displacement reaction, the more reactive metal displaces the less reactive metal. In the above reactive, we find that lead is displaced from the aqueous solution of lead nitrate and is deposited on metal X. This shows that metal X is more reactive than lead.

11) How will you come to know that an exothermic reaction has taken place in an aqueous solution?

We can come to know that an exothermic reaction has taken place in an aqueous solution by touching the vessel in which the reaction has taken place. If the reaction vessel becomes hot, then it is evident that an exothermic reaction has taken place while if the reaction vessel is cold, then it implies that an endothermic reaction has taken place.

12) On what chemical law, balancing of chemical equation is based?

Ans) Balancing of a chemical equation is based on the law of conservation of mass.

13) Represent decomposition of ferrous sulphate with the help of balanced chemical equation.

Ans) 2FeSO₄(s) → Fe₂O₃(s) + SO₂(g) + SO₃(g)

14) When methyl alcohol (CH₃OH) burns in air, it reacts with oxygen from air to form carbon dioxide and water. Write a balanced chemical equation for the reaction.

Ans) 2CH₃OH + 3O₂ → 2CO₂ + 4 H₂O

15) Write the chemical equation for reactions that takes place when lead nitrate and potassium iodide solutions are mixed.

Ans) The chemical equation for reactions that takes place when lead nitrate and potassium iodide solutions are mixed is-

Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃

16) Identify the compound oxidized in the following reaction.

H₂S (g) + Cl₂ → S(s) + 2HCl (g)

Ans) H₂S is oxidized to S by losing hydrogen atom. Thus, H₂S is the reducing agent.

Cl₂ gets reduced to HCl by gaining hydrogen atom. Thus, Cl₂ is an oxidizing agent.

17)Give an example of photochemical reaction:

Ans) The most common example of a photochemical reaction is photosynthesis-plants use solar energy to convert carbon dioxide and water into glucose and oxygen.

CO₂ + H₂O(l) → C₆H1₂O₆ + O₂(photosynthesis)

18) Hydrogen being a highly inflammable gas and oxygen being a supporter of combustion, yet water which is a compound made up of hydrogen and oxygen is used to extinguish fire. Why?

Ans) During a chemical combination reaction, the product formed have entirely new properties and the individual properties of the reactants is lost.

19) Why do we apply paint on iron articles?

Ans) Iron articles are painted to prevent them from rusting. Once an iron article is painted, the contact of iron articles from moisture and air is cut off thus preventing rusting.

20) Write the balanced chemical equations for the following reactions:

(a) Sodium carbonate solution on reaction with hydrochloric acid in equimolar concentration gives sodium chloride and sodium hydrogen carbonate.

(b) When cold water is added to sodium, violent exothermic reaction takes place and sodium hydroxide and hydrogen are produced.

(c) Potassium metal reacts with water to give potassium hydroxide and hydrogen is released.

Ans) (a) Na₂CO₃(aq) + HCl(aq) → NaCl(aq) + NaHCO₃(aq)

(b) 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂ + Heat

(c) 2K(s) + 2H₂O(l) → 2KOH(aq) + H₂

21) Name a few chemical reactions taking place in our everyday life.

Ans) Photosynthesis, respiration, digestion, corrosion, combustion of fuel, formation of curd, rotting of food, etc.

22) Why non-metals in general do not displace hydrogen from dilute acids?

Ans) Non-metals generally do not displace hydrogen from dilute acids because non-metals are less reactive than hydrogen as all the non-metals other than Carbon(C) are placed below Hydrogen in the Activity Series. Moreover, non- metals do not have a tendency to lose electrons because of which they do not displace hydrogen from acids.

23) What do you observe when a matchstick flame is brought near a test tube containing hydrogen gas?

Ans) When a matchstick flame is brought near a tube containing hydrogen gas, it burns with a pop sound.

24) Which type of chemical reaction does the general chemical reaction A+B → C represent?

Ans) A+B → C represents chemical combination reaction.

25) Why does lime water turn milky when CO₂ is passed through it?

Ans) Lime water is a solution of calcium hydroxide {Ca(OH)₂}. So, when CO₂ is passed through lime water, a white precipitate of calcium carbonate (CaCO₃) which is insoluble in water.

26) What is the method opted for balancing a chemical equation?

Ans) Hit and trial method is used for balancing chemical equations. In this method, we make trials to balance the equation by using the smallest whole number coefficient.

27) Which colour fumes are evolved when lead nitrate is heated? Write an equation and name the gas that is evolved:

Ans) Brown colour fumes of nitrogen dioxide(NO2) gas is evolved when lead nitrate is heated.

2Pb(NO₃)₂(s) → 2PbO(s) + 4 NO₂(g) + O₂(g)