Science Olympiad Test: Acids, Bases and Salts- 1 - Class 10 MCQ

Acids change the colour of the litmus solution from blue to red. They have a sour, vinegar-like flavour and are sticky to the touch. Bases, on the other hand, change the colour of the red litmus solution to blue. They have a bitter taste, similar to baking soda, and are slippery to the touch.

Lemon juice and coffee are both acidic solutions. Lemon juice has a pH of 2, and coffee has a pH of 5.

When a glass rod dipped in the ammonia solution is brought near HCl gas, a dense white fume of ammonium chloride is formed.
This is because ammonia reacts with the fumes of HCl to give ammonium chloride.
NH_3​ + HCl → NH₄​Cl

When a small amount of acid is added to water, a concentrated acid turns into dilute acid. This process is called dilution of acid. A dilute acid is also called an aqueous solution in which acid releases hydronium ions (H⁺) and undergoes ionisation. 
HCl(aq) → H⁺(aq) + Cl⁻(aq)

Molar concentration of HCl acid solution = 0.02/2L = 0.01 mol L^–1
= 10^–2 mol L^–1  
pH = – log [H⁺]
pH = – log (10^–2)
pH = 2 log 10
pH = 2

 A, B, C and D have pH 3,4,6 and 8. The solution with highest acidic strength is 6.

Option (C):- (ii) only

• Electrical conductivity is the ability of an electrolyte to carry a current.
• Electrolyte is actually a medium which contains ions and because of the movement of those charged ions when placed in an electric field, there would be conduction of electricity.
• Electric current flows only in a closed circuit that would be formed here by the joining of two iron nails (which are the good conductor of electricity in the NaOH solution) with the bulb , battery and switch through the connecting wires.
• NaOH is a strong base and furnishes strong Na+ and OH- ions which would be responsible for the electrical conductivity.
• 

Citric acid is not a mineral acid but an organic acid present in citrus fruits

When washing soda (Na₂CO₃·10H₂O) is exposed to air, it undergoes efflorescence, meaning it loses some or all of its water of crystallization (H₂O) and forms a white powder.

Chemical Reaction:

Na₂CO₃·10H₂O (s) → Na₂CO₃·H₂O (s) + 9H₂O (g)↑

Key Observations:

1. Loss of water molecules (from 10H₂O to 1H₂O or zero).

2. White powdery coating appears on the surface.

3. No CO₂ release (unlike baking soda, which decomposes).

Why?

• Washing soda is a hydrated salt, and the water molecules are weakly bound.

• In dry air, it loses water to reach a more stable monohydrate (Na₂CO₃·H₂O) form.

Most metal oxides are insoluble in water but few of them, such as Na₂O and CaO dissolve in water to form alkalies and not salt and acid. Mainly group I and group II metals react with water to form hydroxides. Other metals such as iron, chromium, etc do not react with water.