Category: Questions

Answer:

The dissociation of HCl or HNO₃ to form hydrogen ions always occurs in the presence of water. Hydrogen ions (H⁺) combine with H₂O to form hydronium ions (H₃O⁺).

The reaction is as follows:

HCl (aq) → H3O+ (aq) + Cl− (aq)

Although aqueous solutions of glucose and alcohol contain hydrogen, these cannot dissociate in water to form hydrogen ions. Hence, they do not show acidic character.

Answer:

The reaction involves the evolution of carbon dioxide gas (which extinguishes a burning candle) when calcium carbonate reacts with hydrochloric acid.

The balanced chemical equation for the reaction is:

CaCO₃ (s) + 2HCl (aq) → CaCl₂ (aq) + H₂O (l) + CO₂ (g)

Answer:

Hydrogen gas is usually liberated when an acid reacts with a metal.

Take a few pieces of zinc granules and add 5 ml of dilute H₂SO₄. Shake it and pass the gas produced into a soap solution. The bubbles of the soap solution are formed. These soap bubbles contain hydrogen gas.

We can test the evolved hydrogen gas by its burning with a pop sound when a candle is brought near the soap bubbles.

Answer:

If the colour of red litmus paper gets changed to blue, then it is a base, and if there is no colour change, then it is either acidic or neutral. Thus, the basic solution can be easily identified.

Let us mark the three test tubes as A, B, and C. A drop of the solution in A is put on the red litmus paper. The same is repeated with solutions B and C. If either of them changes colour to blue, then it is basic. Therefore, out of the three, one is eliminated.

Out of the remaining two, any one can be acidic or neutral. Now a drop of the basic solution is mixed with a drop of each of the remaining two solutions separately, and then the nature of the drops of the mixtures is checked. If the colour of the red litmus turns blue, then the second solution is neutral, and if there is no change in colour, then the second solution is acidic. This is because acidic and basic solutions neutralize each other. Hence, we can distinguish between the three types of solutions.

(a) Corrosion:

Corrosion is defined as a process where materials, usually metals, deteriorate as a result of a chemical reaction with air, moisture, chemicals, etc.

Example:

4Fe (s) + 3O2 (g) + 6H2O (l) → 4Fe(OH)3 (s)

This is the formation of rust, which is hydrated iron oxide.

(b) Rancidity:

The process of oxidation of fats and oils that can be easily noticed by the change in taste and smell is known as rancidity.

Example:

The taste and smell of butter change when kept for a long time.

Rancidity can be avoided by:

• Storing food in airtight containers
• Storing food in refrigerators
• Adding antioxidants
• Storing food in an environment of nitrogen

Answer:

Nitrogen is an inert gas and does not easily react with these substances. On the other hand, oxygen reacts with food substances and makes them rancid. Thus, bags used in packing food items are flushed with nitrogen gas to remove oxygen inside the pack. When oxygen is not present inside the pack, rancidity of oil and fat-containing food items is avoided.

Answer:

Iron articles are painted because it prevents them from rusting. When painted, the contact of iron articles with moisture and air is cut off. Hence, rusting is prevented as their presence is essential for rusting to take place.

Answer:

‘X’ is copper (Cu) and the black-coloured compound formed is copper oxide (CuO). The equation of the reaction involved on heating copper is given below:

2Cu (s) + O2 (g) → 2CuO (s)

(a) Oxidation:

Oxidation is the gain of oxygen. This process occurs when a substance combines with oxygen to form an oxide.

Example 1:

2 H2 (g) + O2 (g) → 2 H2O (l)

In this equation, hydrogen (H₂) is oxidized to form water (H₂O) by gaining oxygen.

Example 2:

2 Cu (s) + O2 (g) → 2 CuO (s)

In this equation, copper (Cu) is oxidized to form copper oxide (CuO) by gaining oxygen.

(b) Reduction:

Reduction is the loss of oxygen. It occurs when a substance loses oxygen to form a simpler compound.

Example 1:

CO2 (g) + H2 (g) → CO (g) + H2O (g)

In this equation, carbon dioxide (CO₂) is reduced to carbon monoxide (CO) by losing oxygen.

Example 2:

CuO (s) + H2 (g) → Cu (s) + H2O (g)

In this equation, copper oxide (CuO) is reduced to copper (Cu) by losing oxygen.

Answer:

A reaction in which an insoluble solid (called precipitate) is formed is called a precipitation reaction.

Example 1:

CaCl2 (aq) + Na2CO3 (aq) → CaCO3 (s) + 2 NaCl (aq)

In this reaction, calcium carbonate (CaCO₃) is obtained as a precipitate. Hence, it is a precipitation reaction.

Example 2:

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

In this reaction, barium sulphate (BaSO₄) is obtained as a precipitate.