Chemical Reaction and Equations Class 10th Important Questions and Answers

 1. What causes rancidity? Name an antioxidant which prevents rancidity.

Answer: 

When fats and oils are left open, they get oxidized in the presence of air, their smell and taste change. This means the oxidation is the main cause for rancidity in fats and oil. The substances which prevent oxidation of the food items are called antioxidants.
Example: Nitrogen is an inert gas and does not oxidize the food, thus is used as antioxidants by chips manufactures.

2. Identify the substance oxidized and reduced in the chemical reaction: 2Mg + O2 → 2MgO.

Answer:

O2 is an oxidizing agent and Mg is reducing agent.
2Mg + O2 → 2MgO
In the given reaction, O2 is reduced by losing oxygen atoms. Thus, O2 is an oxidizing agent as it undergoes reduction.
Mg is oxidized by gaining oxygen atoms. Thus, Mg is a reducing agent as it undergoes oxidation.

3.What happens chemically when quicklime is added to water?

Answer:

When quicklime (calcium oxide- Cao) is added to water, it reacts vigorously with water to produce slaked lime (calcium hydroxide) and releases a large amount of heat.

The reaction that takes place is:

The above reaction is an example of a combination reaction in which calcium oxide and water combine together to form a single product, calcium hydroxide.

4. Why respiration and decomposition processes are considered to be an exothermic process?

 Answer:

Respiration and decomposition processes are exothermic process because:

• We all get energy from the food we eat.

• During digestion, food is broken down into simpler substances.

• For example, Carbohydrates (rice, potatoes, bread) are broken down to form glucose.

• This glucose combines with oxygen and provides energy to the whole body.

5. A student has found black coating on his silver coins and green coating on his copper coins. Which chemical phenomenon is responsible for this? Write the chemical name of these coatings.

Answer: 

The chemical phenomenon which is responsible is corrosion.

When a metal is attacked by substances around it such as moisture, acids, etc. it is said to corrode, and this process is called corrosion.

After corrosion, the color of:

Silver changes to → black

Copper changes to → green

Hence, the chemical names of coatings are silver coating and copper coating.

6.Define oxidizing and reducing agents by giving suitable examples.

Answer:

 Oxidizing agent: An oxidizing agent is an element that gains electrons. Since the oxidizing agent means to gain electrons; it is said to have been reduced.
The element which undergoes reduction (gets reduced) is called oxidizing agent.
For example: 2Mg + O2 → 2MgO
In the given reaction, O2 is reduced by losing oxygen atoms. Thus, O2 is an oxidizing agent as it undergoes reduction.

Reducing agent: 

A reducing agent is an element that loses electrons. The reducing agent means to lose electrons; it is said to have been oxidized.
The element which undergoes oxidation (gets oxidized) is called a reducing agent.
For example:  4NH3 + 5O2→ 4NO + 6H2O
In the given reaction, nitrogen is oxidized to NO by gaining oxygen atoms. Thus, NH3 is reducing agent as it undergoes oxidation.

7. What happens when:

(i) Lead nitrate is heated

(ii) crystals of FeSO4 are heated?

Answer: 

(i) When lead nitrate [Pb (NO3)2] is heated, it forms lead oxide. The emission of brown fumes is also observed. These brown fumes are of nitrogen dioxide (NO2).
The reaction that takes place is:
Pb (NO3)2 + heat → 2PbO + 4NO2 + O2
The above reaction is an example of decomposition reaction in which a single reactant breaks down to give simpler products.

(ii) When green crystals of ferrous sulfate [FeSO4] are heated, the crystals lose water and the color of the crystals changes. It then decomposes to ferric oxide [Fe2O3], sulfur dioxide, and sulfur dioxide.
The reaction that takes place is:
FeSO4 + heat → Fe2O3 + SO2 + SO3
The above reaction is also an example of a decomposition reaction.

8.White colored AgCl turns gray when kept in sunlight.

Answer:

 White silver chloride turns gray in sunlight due to the decomposition of silver chloride into silver and chlorine by light.
The reaction that takes place is:
2AgCl + sunlight → 2Ag + Cl2
The above reaction is an endothermic reaction.

9. What is observed when:
(i) Potassium iodide is added to aqueous lead nitrate.
(ii) Identify the type of reaction.
(iii) Give a balanced chemical equation for this.

Answer:

 (i) When potassium iodide is added to aqueous lead nitrate, it forms potassium nitrate and a precipitate of lead iodide which is yellow in color.
The reaction that takes place is:
Pb (NO3)2 + 2KI → 2PbI + 2KNO3

Note: The insoluble substance formed is known as precipitate.  Precipitate settles down at the bottom of the test tube.

(ii) The type of reaction is double displacement reaction or precipitation reaction.  Any reaction that produces a precipitate can be called a precipitation reaction.

(iii) The balanced chemical equation is:
Pb (NO3)2 + 2KI → 2PbI + 2KNO3

10.What is meant by rancidity? How can it be prevented?

Answer: 

When fats and oils are oxidized in the presence of air, they become rancid and their smell and taste change. This phenomenon is called rancidity. Rancidity can be prevented using antioxidants:

Using antioxidants- substances which prevent oxidation are called antioxidants. They are added to foods containing fats and oil.
For example: Nitrogen gas is filled in the packets of chips to prevent them from oxidation.

11.What happens when:

(a) Copper powder is warmed in a China dish.

(b) Hydrogen gas is disregarded as hot copper (II) oxide?

Answer:

(a) When copper powder is warmed within the sight of air (oxygen), copper responds with oxygen to shape copper oxide. The copper oxide shape is dark in variety. The dark tone is shaped because of the oxidation of copper.

The synthetic condition of the response that happens is given underneath:

2Cu + O2→ 2CuO

(b) When hydrogen gas is disregarded as hot copper oxide, the dark covering on a superficial level becomes brown as the converse response happens and copper is obtained.

The response that happens is:

CuO + H2 → Cu + H2O

12.Distinguish the sort of response:

Fe + CuSO4(aq) → FeSO4(aq) + Cu(s)

Answer:

The kind of response is an uprooting response. In the given response the iron metal being more receptive than copper dislodges it and structures another compound that is FeSO4.

13. Give reason why:

(a) Magnesium ribbon should be cleaned before burning in the air.

(b) Iron should be protected from moist air.

Answer:

 (a) Magnesium ribbon should be cleaned before burning at air because:

• Magnesium is a very reactive metal.

• When it is stored, it forms a layer of magnesium oxide (MgO) by reacting with oxygen.

• This layer of MgO stops the further reaction of magnesium with oxygen.

• Thus, magnesium ribbon should be cleaned to before burning to air to remove this layer.

(b) Iron should be protected from the moist air because:

• When iron (a metal) is attacked by moist air (moisture), it is said to corrode.

• This process is called corrosion or rusting of iron.

• Rusting of iron causes damage to car bodies, bridges, iron railings to all objects made of iron.

• Every year a big amount of money is spent to replace damaged iron.

• Hence, we should protect iron from moist air.