Class 10 Science Chapter 1 Important Questions

Whenever a chemical change occurs, we can say that a chemical reaction has taken place. For example, food is cooked, milk is left at room temperature during summers, an iron tawa/pan/nail is left exposed to humid atmosphere, grapes get fermented, food is cooked, food gets digested in our body, respiration etc. Chemical change involves change in state, change in colour, evolution of a gas, change in temperature.

The description of a chemical reaction in a shorter form is called chemical equation The simplest way to do this is to write it in the form of a word-equation.
For example:
Magnesium + Oxygen → Magnesium oxide
(Reactants) (Product)
Magnesium and oxygen are reacting with each other so it is called reactants and magnesium oxide, formed during the reaction, as a product. 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.

A chemical reaction written in the form of symbols and formulae is called skeletal chemical equation. The above word-equation can be written as a balanced chemical equation:
2Mg + O₂ → 2MgO
In this equation there are two oxygen atom on the LHS side while there is only one oxygen atom on the RHS side.

A chemical reaction in which total mass of the elements present in the products of a chemical reaction has to be equal to the total mass of the elements present in the reactants. In other words, the number of atoms of each element remains the same, before and after a chemical reaction. Hence, we need to balance a skeletal chemical equation.
For example: Zn + H₂SO₄ → ZnSO₄ + H₂

The reaction in which two or more element or compound combine together to form a single compound are called Combination reaction.
Example:
(i) CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat
(Quick lime) (Slaked lime)

(ii) C(s) + O₂(g) → CO₂(g)

(iii) 2H₂(g) + O₂(g) → 2H₂O(l)

It is a type of reaction in which a single compound broke down to give two or more simpler substance.
Example:
2FeSO₄(s) → Fe₂O₃(s) + SO₂(g) + SO₃(g)
(Ferrous sulphate) (Ferric oxide)
There are three different type of Chemical reaction are:
i). Thermal Decomposition: When the decomposition take place due to heat then this type of reaction is known as thermal decomposition.
Example: Ca(OH)₂(aq) → CaO(s) + H₂O(l)

ii). Electrical Decomposition: When the decomposition take place due to electricity then this type of reaction is known as electrical decomposition.
Example: 2H₂O→2H₂+O₂

iii). Photolytic Decomposition: When the decomposition take place in the presence of sunlight then this type of reaction is known as photolytic decomposition.
Example: 2AgCl(s) → 2Ag(s) + Cl₂

Those reactions in which a more active element displaces a less active element from its compound are called displacement reaction.

Example: Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)

Those reactions in which two ionic compound in the solution react by exchange of their ions to form new compound are called double displacement reaction.
Example:
Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)
(Sodium (Barium (Barium (Sodium
Sulphate) chloride) sulphate) chloride)

Those reactions in which aqua solution of two compound on mixing react to form an insoluble compound which separate out as a solid or such an insoluble compound is forms when a gas passed through the solution of compound are called precipitation reaction.
Example:
CuSO₄(aq) + H₂ → Cu + H₂SO₄

It is a type of reaction in which a acid react with a base to form salt and water this is known as Neutralization reaction
Example:
NaOH + HCl → NaCl + H₂O

The gain of oxygen or loose of hydrogen is known as Oxidation reaction.
The gain of hydrogen or loosing of oxygen is known as Reduction reaction.

Corrosion: When a metal is attacked by substance around it such as moisture, acids, etc., it is said to be corrode and this process is known as corrosion. For example, iron metal are shiny when new but with the passage of time they get coated with reddish brown powder called rust. Silver articles became black and copper articles became green on the surface due to corrosion. Corrosion cause damage to car body, bridges, iron railing, ships, etc.
Rancidity: When fats and oils are oxidized (or kept in open for some days), their smell and taste became changes. We can say that they have become rancid. This phenomenon is called rancidity. Antioxidants are adding to food containing oils and fats and oils to prevent rancidity. The antioxidant substance that are added to food are preferentially oxidized and thus they prevent the oxidation of food. Keeping food in air tight containers helps to slow down oxidation. For example: Chips manufacturer flush bags of chips with nitrogen to prevent chips from getting oxidized.

In a decomposition reaction, a single reactant decomposes to give 2 or more products. Decomposition reactions require energy in the form of heat, light or electricity
Types of decomposition reactions:
a). Decomposition reactions which require heat are known as thermolytic decomposition reactions
b). Decomposition reactions which require light are known as photolytic decomposition reactions
c). Decomposition reactions which require electricity are known as electrolytic decomposition

Chemical equation: The symbolic representation of a chemical reaction is called a chemical equation.

Exothermic and endothermic reactions: If heat is evolved during a reaction, then such a reaction is known as exothermic reaction. If heat is absorbed from the surroundings, then such a reaction is known as endothermic reaction
Combination reaction: Combination reaction is a reaction in which two or more substances combine to give a single product.
Decomposition reaction: In a decomposition reaction, a single reactant decomposes to give 2 or more products. Decomposition reactions require energy in the form of heat, light or electricity
8. Displacement reaction: A reaction in which a more active element displaces less active element from its salt solution.

The one basic difference is that In a Physical change no new substance is formed but in a Chemical change a new substance is formed.

The reaction in which a chemical change occurs it is known as chemical reaction. A chemical reaction can be observed with the help of any of the following observations:
a) Evolution of a gas
b) Change in temperature
c) Formation of a precipitate
d) Change in colour
e) Change of state

2FeSO₄ → Fe₂O₃ + SO₂ + SO₃

It is because the properties of compound (H₂O) are different from the properties of its constituting elements which is H₂ and O₂.

In endothermic reaction, heat is absorbed but in the exothermic reaction, heat is evolved.

Flushing bags of chips with nitrogen cuts off oxygen and protect the food from rancidity.

Co + 2H₂ → CH₃OH

WE get a black coating of CuO, copper oxide.

It is a combination reaction.

Fermentation of fruit juice is a chemical change.

It is a chemical change. The hydrocarbon of wax burns to produce CO₂ and H₂O

Quick lime react with water to form slaked lime and produces lot of heat and hissing sound.

CaO + H₂O → Ca(OH)₂ + heat + hissing sound

It is based on the law of conservation of mass which said that mass can neither be created nor be destroy in a chemical reaction.

While balancing a chemical equation, we follow the law of conservation of mass which states mass can neither be created nor be destroy in a chemical reaction.

Oxidation id addition of oxygen or removal of hydrogen. Reduction is addition of hydrogen or removal of oxygen.

Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)

The balanced chemical equation is

2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O

(i) 2AgBr → 2Ag + Br₂

(ii) 2Na + 2H₂O → 2NaOH + H₂

(i) The colour of ferrous sulphate is pale green. The colour changes to reddish brown on heating due to formation of Iron (III) oxide.

(ii) 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃

In a displacement reaction, more reactive can displace the less reactive metal from its salt solution e.g.
Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag

In double displacement reactions ,two compound exchange their ions to form two new compounds e.g.
NaOH + HCL → NaCl + H₂O

(i)
Pb(NO₃)₂ + 2Kl → 2Pbl₂ + 2KNO₃

(ii)
CaO + H₂O → Ca(OH)₂ + heat.

Thermal decomposition reaction:
Ca(OH)₂(aq) → CaO(s) + H₂O(l)

Photochemical decomposition:
2AgCl(s) → 2Ag(s) + Cl₂

(a) ‘X’ is calcium oxide (CaO)
CaO + H₂O → Ca(OH)₂ + Heat

(b) It is because iron displaces copper from CuSO₄ to form FeSo4 Which is pale green
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)

A reaction in which two elements or compounds combine to form a single compound is called combination reaction.
CaO + H₂O → Ca(OH)₂ + Heat

It is also an exothermic reaction because heat is evolved.

White precipitate of barium sulphate is formed. If both reactants are in solid state, then the reaction will not take place between them.
BaCl₂(aq) + Na₂SO₄ → BaSO₄ + 2NaCl

It is double displacement as well as precipitation reaction.

The reaction in which oxidation (loss of electrons) and reduction (gain of electrons) take place simultaneously are called redox reaction.

2Mg + O₂ → 2MgO

Magnesium is getting oxidised because it is losing electrons to form Mg+2 and oxygen is gaining electrons to form O-2 therefore it is getting reduced.

Any two of these observations will suggest chemical reaction has taken place
(i) Change in state
(ii) Change in colour
(iii) Evolution of gas
(iv) Change in temperature

e.g. lead nitrate is white crystalline solid, on heating gives yellowish brown solid (lead monoxide), brown gas is evolved and a colourless gas is evolved . It shows chemical reaction has taken place.

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

(i) In the first step, oxidation is taking place.
In second step, redox reaction take place.

(ii) Metal in the powder form is copper
2Cu + O₂→ 2CuO
CuO + H₂ → Cu + H₂O

The reaction in which oxygen or electronegative element is added, Hydrogen or electropositive element is removed or loss of electrons takes place is called oxidation reaction.

Fe+2 → Fe+3 + e- (loss of electron )
C + O₂ → CO₂ + Heat

Oxidation reaction are mostly exothermic in nature because heat is evolved in this process.