RBSE Class 10 Science Important Questions Chapter 1 Chemical Reactions and Equations

Rajasthan Board RBSE Class 10 Science Important Questions Chapter 1 Chemical Reactions and Equations Important Questions and Answers.

RBSE Class 10 Science Chapter 1 Important Questions Chemical Reactions and Equations

Objective Type Questions:

Question 1.
When potassium metal reacts with water then it is:
(a) oxidised
(b) reduced
(c) remains unaffected
(d) hydrolysed
Answer:
oxidised

Question 2.
Which of the following compound is used in black and white photography?
(a) AgCl
(b) AgBr
(c) Agl
(d) All of the above
Answer:
AgBr

Question 3.
Which among the following does not corrode?
(a) Iron
(b) Calcium
(c) Copper
(d) Silver
Answer:
Calcium

Question 4.
Burning of magnisium in air is:
(a) reduction
(b) vaporisation
(c) oxidation
(d) sublimation
Answer:
oxidation

Question 5.
When solution of NaCl is mixed in the solution of AgNO₃ then white precipitate of which compound is formed?
(a) NaNO₃
(b) AgCl
(c) HCl
(d) AgBr
Answer:
AgCl

Question 6.
Which gas is evolved by the reaction of Zn with dil. H₂SO₄?
(a) O₂
(b) H₂
(c) SO₂
(d) None of the above
Answer:
H₂

Question 7.
Which of the following in exothermic reaction?
(a) Dissolution of CaO in water
(b) Conversion of CaCO₃ into CaO
(c) Burning of natural gas
(d) All of the above
Answer:
Conversion of CaCO₃ into CaO

Question 8.
In ZnO + C → Zn + CO
(a) Oxidation of ZnO is taking place.
(b) C is oxidised.
(c) ZnO is reduced.
(d) All of the above.
Answer:
C is oxidised.

Very Short Answer Type Questions:

Question 1.
What is a ‘skeletal’ chemical equation?
Answer:
Those chemical equation in which the number of atoms of each element on both the sides of the arrow are not same, are called ‘skeletal’ chemical equation.

Question 2.
Write the dissociation reaction of ferrous sulphate.
Answer:

Question 3.
Write the following chemical equation in balanced form:

Answer:

Question 4.
When quicklime is mixed in water then chemically what happens?
Answer:
Slaked lime is formed by mixing of quicklime in water.
CaO(s) + H₂O(l) → Ca(OH)₂(aq)

Question 5.
What is the basis of balancing of a chemical equation?
Answer:
The chemical equation is balanced on the basis of law of conservation of mass.

Question 6.
To stop the oxidation which gas is flushed during the packing of chips?
Answer:
Nitrogen gas.

Question 7.
What is the role of a catalyst in a chemical reaction?
Answer:
Catalyst may increase or decrease the rate of a chemical reaction but it self remains unchanged.

Question 8.
What is the chemical reaction?
Answer:
Conversion of one or more substances into another substance having different composition and property is called chemical reaction.

Question 9.
In a chemical equation, how the physical states of reactants and products are represented ?
Answer:
In a chemical equation, the gaseous, liquid, aqueous and solid states of reactants and products are represented by the notations (g), (l), (aq) and (s) respectively.

Question 10.
Write an example of precipitation reaction.
Answer:

Question 11.
When calcium oxide is dissolved in water, what change occurs in the heat?
Answer:
Heat is emitted when calcium oxide (CaO) is dissolved in water.

Question 12.
Which gas is evolved when a active metal reacts with dilute sulphuric acid?
Answer:
Hydrogen gas is evolved.
Example – Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)↑

Question 13.
Write the reaction which is used in black and white photography?
Answer:

Question 14.
Which type of reaction is combustion of coal?
Answer:
Combination reaction and oxidation:
C(s) + O₂(g) → CO₂(g)

Question 15.
Which type of reaction is ?
Pb(s) + CuCl₂ (aq) → PbCl ₂ (aq) + Cu(s)
Answer:
Displacement reaction.

Question 16.
What is redox reaction?
Answer:
The reaction in which a reactant is oxidised and other one is reduced is called redox (oxidation reduction) reaction.

Question 17.
What is formed when magnesium ribbon is bum in air?
Answer:
White magnesium oxide.

Question 18.
When magnesium ribbon is bum in air, magnesium is oxidised or reduced?
2Mg(s) + O₂(g) → 2MgO(s)
Answer:
Magnesium is oxidised.

Question 19.
Balance the following equation:
CH₄(g)+O₂(g) → CO₂(g)+H₂O(l)
Answer:
CH₄(g) + 2O₂(g) → CO₂ (g) + 2H₂O(g)

Question 20.
Write the balanced equation for the reaction of N₂ and H₂.
Answer:
N₂(g) + 3H₂(g) → 2NH₃(g)

Question 21.
Write the balanced chemical equation for the reaction:
Sodium sulphite + Water + Chlorine → Sodium sulphate + Hydrochloric acid
Answer:
Na₂SO₃(aq) + H₂O(l) + Cl₂(g) → Na₂SO₄(aq) + 2HCl(g)

Question 22.
What happens when light green crystals of ferrous sulphate are heated?
Answer:
Ferrous sulphate crystals becomes white due to removal of water.

Question 23.
Write the names of three metals which are more reactive than copper.
Answer:
Iron (Fe), Zinc (Zn) and Magnesium (Mg).

Question 24.
In the reaction H₂S + Br₂ → 2HBr + S, which substance is reduced?
Answer:
Br₂ (Bromine)

Question 25.
What is the chemical formulae of marble?
Answer:.
CaCO₃ (Calcium carbonate)

Question 26.
Write the reactivity order of Zn, Pb and Cu.
Answer:
Zn → Pb → Cu

Question 27.
What causes new substances to form in a chemical reaction ?
Answer:
Chemical reactions involve the breaking and making of bonds between atoms to produce new substances.

Question 28.
Name any three metals that corrode.
Answer:
(i) Iron
(ii) Copper
(iii) Silver.

Short Answer Type Questions:

Question 1.
What is combination reaction ? Write the combination reaction for quicklime and water.
Answer:
Combination reaction: A reaction in which a single product is formed from two or more reactants is known as a combination reaction.
Example:  Calcium oxide reacts vigorously with water to produce slaked lime (calcium hydroxide) releasing a large amount of heat.

A solution of slaked lime Ca(OH)₂ is used for whitewashing.

Question 2.
What is redox reaction ? In reaction, ZnO + C Zn + CO, which substance is oxidised and which is reduced?
Answer:
Redox reaction: Reactions in which one reactant gets oxidised while the other gets reduced during a reaction, such reactions are called redox reaction. In the reaction, given in the question Zn is reduced and C is oxidised.

Question 3.
In the reaction CuO + H₂ → Cu + H₂O, which substance gets oxidised and which gets reduced? Give one more example of such type of reaction.
Answer:
In the reaction CuO + H₂ → Cu + H₂O, H₂ gets oxidised and CuO gets reduced because H₂ gets oxygen and CuO losses the oxygen. The other example of such type of reaction is
MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂

Question 4.
(a) What is displacement reaction?
(b) Identify A in the following reactions:
(i) Zn + CuSO₂ → A + Cu
(ii) Na₂SO₄+ BaCl₂ → A + 2NaCl
Answer:
(a) When an element displaces another element from its compound. This process is called displacement reaction.

Here A in (i) is ZnSO₄ and A in (ii) is BaSO₄.

Question 5.
Identify X, Y and Z in the following equations:

Answer:
X = CuCO₃, Y = H₂S, Z = SO₂

Question 6.
What happens when:
(i) CuSO₄(aq) + Fe(s) →
(ii) Zn(s) + H₂SO₄(l) →
(iii) 4Na(s) + O₂(g) →
Answer:
(i) CuSO₄(aq) + Fe(s) → FeSO₄(aq) + Cu(s)
(ii) Zn(s) + H₂SO₄(l) → ZnSO₄(s) + H₂(g)
(iii) 4Na(s) + O₂(g) → 2Na₂O(s)

Question 7.
On which observations it is determined that a chemical reaction has taken place? Explain.
Answer:
The following observations helps us to determine that a chemical reaction has taken place:
(i) Change in physical state of substance.
(ii) Change in colour of substance.
(iii) Evolution of a gas.
(iv) Change in temperature of reaction.

Question 8.
What is chemical equation? Explain.
Answer:
In any chemical reaction the presentation of all reactants and products and their physical state in the form of symbols present, is called chemical equation. The description of a chemical reaction in a sentence form is quite long, so it can be written in a shorter form. The simplest way to do this, is to write it in the form of word equation.
Example:  Magnesium oxide is formed by the reaction of magnesium with oxygen. The word equation for this reaction would be.

In this reaction, magnesium and oxygen are those substances that undergo chemical change and are called reactants. The new substance, magnesium oxide, formed during the reaction is called product.

Question 9.
How the chemical equation is written? Explain with example.
Answer:
Chemical equation can be written by two types:
(1) Word equation: In a word equation, formation of reactants from products are shown by an arrow between them. The reactants are written on the left hand side (LHS) with a plus sign (+) between them. Similarly, products are written on the right hand side (RHS) with a plus sign (+) between them. The arrowhead points towards the products, and shows the direction of the reaction.

(2) Equation in the form of formulae:
Chemical equations become more concise and useful when we use chemical formulae instead of words. A chemical equation represents a chemical reaction.
Example: 2Mg(s) + O₂(g) → 2MgO(s)

Question 10.
What is oxidation? Explain with example.
Answer:
Oxidation: The chemical reaction in which a substance gains oxygen or losses hydrogen is called oxidation.
Example:
(i) 2Mg + O₂ → 2MgO
(ii) H₂S + Cl₂ → 2HCl + S
In the example (i) O₂is gained by Mg, while in example (ii) H₂S is losing H₂.

Question 11.
What results are obtained when copper powder is heated in a dish?
Answer:
When copper powder is heated, black layer of copper (II) oxide is formed on the surface of it. This black substance (copper oxide) is formed by the combination of oxygen with copper.

Question 12.
What will happen if hydrogen gas is passed over the heated CuO (copper oxide) ?
Answer:
If hydrogen gas is passed over the heated CuO (copper oxide) then its colour turns brown and copper is obtained.

Question 13.
Write the chemical equation of the following reactions:
(i) Iron with water vapours
(ii) Sodium with water
(iii) Zinc with sulphuric acid.
Answer:
(i) 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g)
(ii) 2Na(s) + 2H₂O(Z) → 2NaOH(aq) + H₂(g)
(iii) Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)

Question 14.
What happens when (Give only chemical equation)
(1) Carbon dioxide gas is passed through lime water.
(2) Zinc metal reacts with sodium hydroxide.
(3) Chlorine reacts with slaked lime.
Answer:

Question 15.
(a) What will you observe when the solution of sodium sulphate is mixed in a test tube containing Barium chloride solution?
(b) Mention the type of above reaction.
(c) Write a balanced chemical equation to represent the above reaction.
Answer:
(a) White precipitate of Barium sulphate is formed on mixing solution of sodium sulphate in a test tube containing Barium chloride solution.
(b) Above reaction is called ‘precipitation’.

Question 16.
(i) What happens when dilute hydrochloric acid reacts with Bleaching Powder?
(ii) Write the balanced chemical equation of copper oxide with dilute sulphuric acid.
(iii) What happens when a small piece of sodium is droped in ethanol. (Write only the equation of reaction)
(iv) Which of the following metal reacts with hot water but not with cold water?
Na, Mg and Fe
Answer:
(i) Chlorine gas is formed by the reaction of dilute hydrochloric acid with bleaching powder.

(ii) Copper sulphate is formed by the reaction of copper oxide with dilute sulphuric acid

(iv) Among Na, Mg and Fe, only Mg metal reacts with hot water not with cold water.

Question 17.
Why a shinny finish to the walls is obtained after two to three days of whitewashing ?
Answer:
A solution of slaked lime [Ca (OH)₂(aq)] is used for whitewashing walls. Calcium hydroxide reacts slowly with the carbon dioxide in the air to form a thin layer of calcium carbonate on the walls. Calcium carbonate is formed after two to three days of whitewashing and gives a shinny finish to the walls.

Question 18.
What is thermal decomposition? Give example.
Answer:
When a decomposition reaction is carried out by heating, it is called thermal decomposition.
Example:

Question 19.
Balance the following reactions and mention the type of each reaction also.
(i) CH₄ + Cl₂ → CH₃Cl + HCl
(ii) Na + Cl₂ → NaCl
(iii) H₂SO₄ + NaOH → Na₂SO₄ + H₂O
(iv) Ba(OH)₂ + HBr → BaBr₂ + H₂O
(v) BaCl₂ + Al₂ (SO₄)₃ → AlCl₃ + BaSO₄
Answer:

Question 20.
What type of chemical reactions are photosynthesis and respiration ? Give their balanced chemical equations.
Answer:
(i) Respiration:  Respiration is an ‘exothermic’ reaction.

(ii) Photosynthesis: It is an endothermic reaction.

Question 21.
What measures are used to prevent the rancidity of fat and oil containing food materials ?
Answer:
To prevent the rancidity of fat and oil containing food materials, the following measures are used:
(i) Usually substances which prevent oxidation (antioxidants) are added to food containing fats and oil.
(ii) Keeping food in airtight containers helps to slow down oxidation.
(iii) By removing oxygen from food bags, they are filled with less active gases like nitrogen gas so that oxidation cannot occur.

Essay Type Questions:

Question 1.
How the chemical equation is balanced ? Explain the different steps of it with, examples.
Answer:
Balancing of chemical equation:
When the number of all the elements before and after the chemical reaction are same, means number of atoms of reactants and products are same, then this chemical equation is called balanced chemical equation.
A balanced chemical equation can be presented as
Zn + H₂SO₄ →  ZnSO₄ + H₂
In this equation the number of atoms of reactants are equal to the number of atoms of products so this equation is balanced. Mainly seven steps are involved to balance an equation.
Balancing of following equation occurs in the steps given below:
Fe + H₂O →  Fe₃O₄ + H₂
Step 1.
To balance a chemical equation, first draw the boxes around each formula.

Step 2.
List the number of atoms of different elements present in the unbalanced equation.

Elements	Number of atoms in reactants (LHS)	Number of atoms in products (RHS)
Fe	1	3
H	2	2
O	1	4

Step 3.
Now for conveniency by select the element in reactant or product containing maximum number of atoms. On the basis of this we select the oxygen.
To balance the oxygen atoms:

Atoms of Oxygen	In reactants	In Products
(i) Initial (ii) To balance	1 (in H2O) 1 x 4	4 (in Fe3O4)4

To equalise the number of atoms, we cannot alter the formulae of the compounds or elements involved in the reactions. So to balance oxygen atoms put coefficient ‘4’ as 4H₂O. NOW the partly balanced equation becomes:

Step 4.
Now to balance the hydrogen atoms make the number of molecules of hydrogen as four on the RHS.

Atoms of Hydrogen	In reactants	In products
(i)  Initial (ii) To balance	8 (in 4H2O) 8	2 (in H2) 2 x 4

The Equation would be

Step 5.
To balance the Fe we take three atoms of Fe on the LHS.

Step 6.
In the end to check the balanced equation we count atoms of each element on both sides of the equation. The numbers of atoms of elements on both sides of this equation are equal.

So this is balanced equation. This method of balancing chemical equations is called hit- and-trial method as we make trials to balance the equation by using the smallest whole number coefficient.

Step 7.
Writing symbols of physical states: In this step the physical states of the reactants and products are mentioned along with their chemical formulae. The gaseous, liquid, aqueous and solid states of reactants and products are represented by the notations (g), (l), (aq) and (s) respectively.
The balanced equation becomes:  3Fe(s) + 4H₂O(g) → Fe₂O₄(s) + 4H₂(g)
The symbol (g) is used with H₂O to indicate that in this reaction water is used in the form of steam.

Question 2.
Classify the chemical reactions in different types. Explain the combination and decomposition reactions with examples.
Answer:
Chemical reaction: In a chemical reaction new substances are formed by breaking and formation of bonds between atoms. Chemical reactions are mainly of following types:
(1) Combination reaction
(2) Decomposition reaction
(3) Displacement reaction
(4) Double displacement reaction
(5) Oxidation and reduction
(6) Precipitation reaction

(1) Combination reaction: A reaction in which a new substance (single product) is formed by the combination of two or more substances (reactants) is known as combination reaction.
Example: (i) Calcium oxide (quick – lime) reacts vigorously with water to produce slaked lime (calcium hydroxide) releasing a large amount of heat.

(ii) Burning of coal
C(s) + O₂ → CO₂

(2) Decomposition reaction:  The reaction in which a single substance (reactant) decomposes to give two or more substances (Product), is called Decomposition reaction.
Example:
(i) On heating lead nitrate, lead oxide, NO₂ and O₂ are formed. In this reaction brown fumes are emitted due to NO₂.

(ii) On heating calcium carbonate (CaCO₃), CaO and CO₂ gas is formed.

(iii) When white silver chloride is placed in sunlight it turns in gray because silver chloride is decomposed in silver and chlorine

Question 3
When mixing solutions of lead (II) nitrate and potassium iodide;
(i) What colour precipitate is formed?
(ii) Name the precipitated compound.
(iii) Write a balance chemical equation for this reaction.
(iv) What type of reaction is this?
Answer:
(i) On mixing solutions of lead (II) nitrate and potassium iodide, yellow colour precipitate is formed.
(ii) The precipitated compound is ‘lead iodide’.
(iii) Balanced chemical equation:

(iv) This is a double displacement reaction.

Question 4.
Write the method of electrolysis of water. Write the required chemical reaction and draw the labelled diagram.
Answer:
Method of Electrolysis of Water:
(i) First of all take a plastic mug. Drill two holes at its base and fit rubber stoppers in these holes. Now insert carbon electrodes in these rubber stoppers and connect with 6 volt battery.

(ii) Then fill the mug with water so that the electrodes are immersed. Add a few drops of dilute sulphuric acid to the water also.

(iii) After this take two calibrated test tubes filled with water and invert them over the two carbon electrodes. Switch on the current and leave undisturbed for some time.

(iv) We observe the formation of bubbles at both the electrodes. These bubbles displace water in the calibrated test tubes.

(v) Once the test tubes are filled with the respective gases, remove them carefully and test these gases one by one by bringing a burning candle close to the mouth of the test tubes.
Conclusion Water is decomposed into hydrogen and oxygen gas and these gases are filled in both test tubes.

Question 5.
(a) Write the method of formation of slaked lime. Write the related chemical reaction and draw the labelled diagram.
(b) What is double displacement reaction ? Explain with example.
Answer:
(a) Method of formation of slaked lime:
(i) Small amount of calcium oxide (quicklime) is taken in a beaker and slowly water is added in it.

(ii) Now when we touch the beaker then it is found that the temperature of beaker is changed and it becomes hot


(b) Reactions in which there is exchange of ions between the reactants are called double displacement reactions.
Example:  When solutions of sodium sulphate and Barium chloride are mixed then the following reaction takes place:

In this reaction, the sulphate ion of sodium sulphate is displaced with the chloride ion whereas the chloride ion of barium chloride is displaced with the sulphate ion. As a result of this barium sulphate and sodium chloride are obtained as products.