All questions of Chemical Reactions and Equations for Class 10 Exam

Physical and Chemical Changes

Physical changes are reversible changes that do not involve a change in the chemical composition of a substance. Examples of physical changes include changes in state (solid, liquid, gas), changes in shape or size, and changes in color or texture. In contrast, chemical changes involve a change in the chemical composition of a substance, resulting in the formation of a new substance(s) with different properties.

Explanation

Of the given options, option 'D' - combustion of liquefied petroleum gas (LPG) is not a physical change. Combustion is a chemical change that involves the reaction of a fuel (LPG in this case) with oxygen in the air, producing heat and light energy. The products of the combustion of LPG are carbon dioxide and water vapor, which are different from the original fuel.

On the other hand, boiling of water to give water vapor, melting of ice to give water, and dissolution of salt in water are all physical changes. In these processes, no new substances are formed, and the original substance(s) retain their chemical composition.

Conclusion

In summary, physical changes are characterized by the absence of a chemical reaction, and the original substance(s) retain their chemical composition. Option 'D' is the correct answer as it represents combustion, which is a chemical change that involves the formation of new substances with different properties.

Answer C
its absolutely correct

Heating Ferrous Sulphate Crystals

Ferrous sulphate crystals have the chemical formula FeSO4·7H2O. When these crystals are strongly heated, they undergo a decomposition reaction, breaking down into different compounds.

Decomposition Reaction

The decomposition reaction of ferrous sulphate crystals is as follows:

FeSO4·7H2O(s) → Fe2O3(s) + SO3(g) + H2O(g)

According to this reaction, the ferrous sulphate crystals break down into solid iron(III) oxide (Fe2O3), gaseous sulphur trioxide (SO3), and water vapor (H2O).

Evolved Gas/Vapour

The question asks us to identify the gas/vapour that is not evolved during the heating process. From the above reaction, we can see that three different compounds are produced when ferrous sulphate crystals are strongly heated. These are:

- Iron(III) oxide (Fe2O3)
- Sulphur trioxide (SO3)
- Water vapor (H2O)

Out of these three compounds, only one is not a gas/vapour. That compound is iron(III) oxide (Fe2O3).

Therefore, the correct answer to the question is option 'C', which is O2. This is not one of the compounds produced during the heating of ferrous sulphate crystals, and hence it is not the gas/vapour that is not evolved.

Chemical and Physical Changes

Chemical changes involve the formation of new substances with different chemical properties, while physical changes involve changes in the physical state or appearance of a substance without changing its chemical composition.

Cooking of Food

Cooking of food involves chemical changes, as the heat and other ingredients cause chemical reactions that alter the taste, texture, and nutritional value of the food.

Evaporation of Water

Evaporation of water is a physical change, as it involves the change of water from a liquid to a gas without any change in its chemical composition.

Burning of Candle Wax

Burning of candle wax is a chemical change, as the heat and oxygen cause the wax to react and form new compounds such as carbon dioxide and water.

Digestion of Food in our Body

Digestion of food in our body involves chemical changes, as the enzymes in our digestive system break down the food into simpler substances that can be used by our body for energy and growth.

Conclusion

Therefore, the correct answer is option B - Evaporation of water, as it is a physical change and does not involve any chemical reactions or the formation of new substances.

Answer:

Introduction:
When storing a fresh sample of oil for a long time, it is important to prevent its degradation or oxidation. This can be achieved by using an appropriate gas to create an inert atmosphere that minimizes contact with oxygen, moisture, and other reactive substances. Among the options given, nitrogen or helium are the most suitable gases for this purpose.

Explanation:
Nitrogen:
- Nitrogen is an inert gas, meaning it is chemically unreactive and does not readily react with other substances.
- It is abundantly available in the atmosphere and is commonly used for various industrial and laboratory applications.
- When used for storing oil samples, nitrogen replaces the oxygen in the storage container, thus creating an oxygen-free environment.
- This prevents oxidation and slows down the degradation of the oil, helping to maintain its freshness and quality over a long period of time.

Helium:
- Helium is another inert gas that is chemically stable and does not react with other substances.
- It is lighter than air and has low solubility in most liquids, including oil.
- When used as a storage gas, helium creates a protective blanket over the oil sample, preventing oxygen and moisture from coming into contact with the oil.
- This helps to maintain the oil's freshness and prevents degradation or oxidation.

Comparison:
Both nitrogen and helium are suitable for storing oil samples for a long time. However, there are a few key differences between the two gases:

- Availability: Nitrogen is more readily available and less expensive compared to helium, which is relatively rare and more expensive.

- Density: Helium is lighter than air and has low solubility in oil, making it more effective in creating a protective blanket over the oil sample.

- Cost: Nitrogen is generally more cost-effective and practical for most applications. Helium is typically reserved for specialized or high-value samples due to its higher cost.

Conclusion:
In conclusion, nitrogen or helium can be used for the storage of fresh samples of oil for a long time. Both gases create an inert atmosphere that prevents oxidation and degradation of the oil. Nitrogen is more commonly used due to its availability and cost-effectiveness, while helium is preferred for specialized or high-value samples.

Understanding Electrolysis of Water
Electrolysis of water involves splitting water (H2O) into its constituent gases, hydrogen and oxygen, using an electric current. This process can be represented by the chemical equation:
2 H2O(l) → 2 H2(g) + O2(g)
Mole Ratio of Gases
In the reaction:
- 2 moles of water produce:
- 2 moles of hydrogen gas (H2)
- 1 mole of oxygen gas (O2)
From this equation, we can derive the mole ratio of hydrogen to oxygen.
Mole Ratio Calculation
- Hydrogen (H2) produced: 2 moles
- Oxygen (O2) produced: 1 mole
Thus, the mole ratio of hydrogen to oxygen is:
- 2 moles H2 : 1 mole O2
This simplifies to:
- 2 : 1
Conclusion
Therefore, during the electrolysis of water, the correct mole ratio of hydrogen to oxygen gases liberated is indeed 2 : 1. This means for every 2 moles of hydrogen produced, 1 mole of oxygen is released, confirming that option 'B' is correct.
Understanding this ratio is crucial in various applications, including fuel cell technology and industrial processes involving hydrogen production.

Endothermic Processes

Dilution of sulphuric acid
- Dilution of sulphuric acid is an exothermic process as it releases heat during the reaction.

Sublimation of dry ice
- Sublimation of dry ice is an endothermic process as it requires heat input to change the solid directly into a gas without passing through the liquid phase.

Condensation of water vapours
- Condensation of water vapours is an exothermic process as it releases heat when water vapours change into liquid water.

Evaporation of water
- Evaporation of water is an endothermic process as it requires heat input to change liquid water into water vapour.
Therefore, the endothermic processes in the given options are (ii) Sublimation of dry ice and (iv) Evaporation of water. So, the correct answer is option B - (ii) and (iv).

The reaction given is not complete. Please provide the complete reaction so that I can determine whether it is a double displacement reaction or not.

Understanding the Reaction
In the reaction:
PbS(s) + 4H2O2(aq) → PbSO4(s) + 4H2O,
we need to identify which substance is reduced.
Reduction Process
- Definition of Reduction: Reduction is a chemical reaction that involves the gain of electrons or a decrease in oxidation state by a molecule, atom, or ion.
Analyzing Hydrogen Peroxide
- Hydrogen Peroxide (H2O2): In this reaction, H2O2 acts as an oxidizing agent. It is known to undergo reduction during the reaction.
- Oxidation State Change: The oxidation state of oxygen in H2O2 is -1, and in water (H2O), it is -2. This change indicates that H2O2 is gaining electrons and being reduced.
Analyzing Lead Sulfide
- Lead Sulfide (PbS): In this reaction, PbS is converted to PbSO4.
- Oxidation State Change: Lead (Pb) in PbS has an oxidation state of +2 in PbSO4, but the lead does not gain electrons. Instead, it is oxidized.
Conclusion
- Final Identification: Since H2O2 is reduced (gaining electrons) and PbS is oxidized (losing electrons), the correct answer is indeed b) Hydrogen peroxide.
Thus, understanding the changes in oxidation states helps clarify the reduction process in this reaction.

Explanation:

The reaction between a metal and an acid usually results in the evolution of hydrogen gas. However, there are certain conditions under which this reaction may not occur. In this case, we are given four different reactions and we have to identify which one does not result in the evolution of H2 gas.

Zinc and hydrochloric acid:
When zinc reacts with hydrochloric acid, it undergoes a displacement reaction and produces zinc chloride and hydrogen gas. The balanced chemical equation for this reaction is:

Zn + 2HCl → ZnCl2 + H2

Iron and sulphuric acid:
When iron reacts with sulfuric acid, it also undergoes a displacement reaction and produces iron(II) sulfate and hydrogen gas. The balanced chemical equation for this reaction is:

Fe + H2SO4 → FeSO4 + H2

Magnesium and very dilute nitric acid:
When magnesium reacts with nitric acid, it undergoes a redox reaction and produces magnesium nitrate, nitrogen dioxide gas, and water. The balanced chemical equation for this reaction is:

3Mg + 8HNO3 → 3Mg(NO3)2 + 2NO + 4H2O

Aluminium and nitric acid:
When aluminium reacts with nitric acid, it also undergoes a redox reaction and produces aluminium nitrate, nitrogen dioxide gas, and water. The balanced chemical equation for this reaction is:

2Al + 6HNO3 → 2Al(NO3)3 + 3H2O + 3NO2

From the above equations, we can see that all the reactions except the one involving aluminium and nitric acid result in the evolution of hydrogen gas. Therefore, the correct answer is option 'D' - Aluminium and nitric acid.

Explanation:

Iron sulphide and dilute sulphuric acid:
- When iron sulphide reacts with dilute sulphuric acid, it forms hydrogen sulfide gas and iron(II) sulfate.
- Since hydrogen sulfide gas is a gas and not a solid, no precipitate is formed in this reaction.
- Therefore, this combination will not form a precipitate.

Lead acetate and potassium iodide:
- When lead acetate reacts with potassium iodide, it forms lead iodide which is insoluble in water and hence, a yellow precipitate of lead iodide is formed.

Lead nitrate and sulphuric acid:
- When lead nitrate reacts with sulphuric acid, it forms lead sulfate which is insoluble in water and hence, a white precipitate of lead sulfate is formed.

Potassium bromide and silver nitrate:
- When potassium bromide reacts with silver nitrate, it forms silver bromide which is insoluble in water and hence, a cream precipitate of silver bromide is formed.
Therefore, out of the given options, only the combination of iron sulphide and dilute sulphuric acid will not form a precipitate.

**Answer:**

**Introduction:**
When a metal reacts with an acid, it displaces hydrogen from the acid, resulting in the formation of a salt and the evolution of hydrogen gas. This type of reaction is known as a displacement reaction.

**Explanation:**
Among the given options, the metal that will react with dilute hydrochloric acid to give out hydrogen gas is Pb (lead). The reaction between lead and hydrochloric acid can be represented by the following equation:

Pb + 2HCl → PbCl2 + H2

**Reasoning:**
To determine which metal will react with hydrochloric acid, we need to consider the reactivity series of metals. The reactivity series is a list of metals in order of their reactivity, with the most reactive metal at the top and the least reactive metal at the bottom.

The reactivity series of metals is as follows:
Potassium (K) > Sodium (Na) > Calcium (Ca) > Magnesium (Mg) > Aluminum (Al) > Zinc (Zn) > Iron (Fe) > Lead (Pb) > Hydrogen (H) > Copper (Cu) > Silver (Ag) > Gold (Au)

According to the reactivity series, metals higher in the series are more reactive than metals lower in the series. This means that metals higher in the series can displace metals lower in the series from their compounds.

In the given options, lead (Pb) is below hydrogen (H) in the reactivity series. This means that lead can displace hydrogen from hydrochloric acid to form lead chloride (PbCl2) and release hydrogen gas (H2).

Therefore, the correct answer is option B - Pb.

Green coating on copper in rainy season is due to the formation of CuCO3.Cu(OH)2.

Explanation:
Copper is a metal that is highly reactive with oxygen and water in the atmosphere. When copper comes in contact with air and moisture, a series of chemical reactions occur, resulting in the formation of a green coating on the surface of the copper. This green coating is known as copper carbonate hydroxide or copper(II) carbonate dihydroxide, with the chemical formula CuCO3.Cu(OH)2.

The formation of the green coating on copper can be explained in the following steps:

1. Oxidation: When copper is exposed to air, it undergoes oxidation. The copper atoms lose electrons and form copper(II) ions (Cu2+). This reaction can be represented as follows:
2Cu(s) + O2(g) → 2Cu2+(aq)

2. Reaction with carbon dioxide: The copper(II) ions in the presence of carbon dioxide in the air react to form copper(II) carbonate (CuCO3). This reaction can be represented as follows:
Cu2+(aq) + CO2(g) + H2O(l) → CuCO3(s) + H2O(l)

3. Reaction with water: The copper(II) carbonate reacts with water to form copper(II) hydroxide (Cu(OH)2) and carbon dioxide. This reaction can be represented as follows:
CuCO3(s) + H2O(l) → Cu(OH)2(s) + CO2(g)

4. Formation of green coating: The copper(II) hydroxide further reacts with copper(II) carbonate to form copper carbonate hydroxide (CuCO3.Cu(OH)2), which is the green-colored compound that coats the surface of the copper. This reaction can be represented as follows:
2Cu(OH)2(s) + CuCO3(s) → CuCO3.Cu(OH)2(s) + H2O(l)

The green coating on the copper surface provides a protective layer that helps to slow down the further corrosion of copper. It acts as a barrier between the copper and the surrounding environment, preventing direct contact with moisture and oxygen. However, over time, this coating can also undergo further reactions and degrade, exposing the underlying copper to corrosion again.

In conclusion, the green coating on copper in the rainy season is due to the formation of copper carbonate hydroxide (CuCO3.Cu(OH)2), which is a result of the reactions between copper, oxygen, carbon dioxide, and water in the atmosphere.

Rise of temperature on mixing with water: Sodium chloride

When certain substances are mixed with water, they can either absorb heat energy from the surroundings (endothermic reaction) or release heat energy into the surroundings (exothermic reaction). In this case, we are looking for the substance that will result in a rise of temperature when mixed with water.

Explanation:

- Sodium chloride (NaCl): When sodium chloride is mixed with water, it undergoes a highly exothermic reaction. This means that it releases heat energy into the surroundings, causing a rise in temperature. The dissolution of sodium chloride in water is an exothermic process because it involves the formation of new bonds between the sodium and chloride ions and the water molecules. The heat released during this process raises the temperature of the solution.

- Sodium hydroxide (NaOH): When sodium hydroxide is mixed with water, it also undergoes an exothermic reaction. However, the heat released is not as significant as in the case of sodium chloride. The dissolution of sodium hydroxide in water is also exothermic, but the reaction is less vigorous compared to sodium chloride.

- Potassium nitrate (KNO3): When potassium nitrate is mixed with water, it undergoes an endothermic reaction. This means that it absorbs heat energy from the surroundings, causing a decrease in temperature. The dissolution of potassium nitrate in water requires energy to break the bonds between the potassium and nitrate ions, resulting in a cooling effect.

- CuSO4·5H2O: Copper sulfate pentahydrate (CuSO4·5H2O) is a hydrated compound, which means it contains water molecules within its crystal structure. When it is mixed with water, the water molecules already present in the crystal structure are released, but this process does not result in a significant rise in temperature.

Based on the above explanations, the correct answer is option 'A' - Sodium chloride.

Explanation:

When an iron nail is dipped in a blue copper sulphate solution, a chemical reaction occurs between iron and copper sulphate. This reaction results in the displacement of copper ions by iron ions.

Colour Change:
The blue colour of the copper sulphate solution changes to a greenish color when the iron nail is dipped into it.

Reason for Colour Change:
The blue color of the copper sulphate solution is due to the presence of copper ions (Cu2+) in the solution. When the iron nail is dipped into the solution, iron ions (Fe2+) are formed by the displacement of copper ions. The greenish color is due to the formation of iron sulphate (FeSO4) in the solution.

Chemical Reaction:
The chemical reaction between iron and copper sulphate can be represented as follows:

Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)

In this reaction, iron (Fe) displaces copper (Cu) from copper sulphate (CuSO4) to form iron sulphate (FeSO4) and copper metal (Cu).

Conclusion:
When an iron nail is dipped in a blue copper sulphate solution, the blue color of the solution changes to a greenish color due to the formation of iron sulphate. This change in color is an indication of the chemical reaction between iron and copper sulphate.