Question
In the following unbalanced equation, X represents an element.
Which oxide reacts with water as shown?
\( ………. + H_2O \rightarrow X(OH)_2\)
A. \(Na_2O\)
B. $MgO$
C. \(NO_2\)
D. \(SO_3\)
▶️Answer/Explanation
Markscheme: B
The balanced equation suggests the formation of a hydroxide (\(\rm OH_2\)) when an oxide reacts with water. This is indicative of a basic oxide, as basic oxides react with water to form hydroxides.
Among the given options:
A. \(\rm Na_2O\) – This is a basic oxide (sodium oxide), and it reacts with water to form sodium hydroxide (\(\rm NaOH\)).
B. \(\rm MgO\) – This is a basic oxide (magnesium oxide), and it reacts with water to form magnesium hydroxide (\(\rm Mg(OH)_2\)).
C. \(\rm NO_2\) – This is not an oxide; it’s a nitrogen dioxide molecule.
D. \(\rm SO_3\) – This is an acidic oxide (sulfur trioxide) and reacts with water to form sulfuric acid (\(\rm H_2SO_4\)).
So, the oxide that reacts as shown is \(\rm MgO\), and the correct answer is B.
Question
What is the sum of the coefficients when the equation is balanced with the lowest whole number ratio?
__Na2S2O3(aq) + __HCl(aq) → __S(s) + __SO2(g) + __NaCl(aq) + __H2O(l)
A. 6
B. 7
C. 8
D. 9
▶️Answer/Explanation
C
Explanation: Firstly, count the number of Na atoms in reactants and products side. There are 2 Na atoms in the reactants side, then there must be 2 Na atoms in the products side also. Hence, place the coefficient 2 in front of NaCl to balance Na.
Na2S2O3(aq) + HCl(aq) → S(s) + SO2(g) + 2NaCl(aq) + H2O(l)
Similarly for Sulfur atoms, the number of S atoms on both sides is equal i.e. 2.
For Oxygen atoms, there are 3 O atoms on both sides.
There is only 1 H atom in the reactants, but there are two H atoms in products. Hence, place the coefficient 2 in front of HCl. After doing this, Cl atoms also get balanced on both sides.
So, the equation becomes :
1Na2S2O3(aq) + 2HCl(aq) → 1S(s) + 1SO2(g) + 2NaCl(aq) + 1H2O(l)
Now, Sum of the coefficients: 1 + 2 + 1 + 1 + 2 + 1 i.e 8.
Hence, correct answer is C.
Question
Which is a homogeneous mixture?
A. Oil and water
B. Sand and water
C. Ethanol and water
D. Chalk and sand
▶️Answer/Explanation
C
Option A : Oil and water do not mix. They forms two phases as there is a boundary of separation between oil and water, it is a heterogeneous mixture.
Option B: Sand doesn’t dissolve in water, it settles down in it, hence it is a heterogeneous mixture.
Option C: Ethanol combines with water to form a single phase only and there is no boundary of separation between ethanol and water. Hence the mixture is homogeneous in nature.
Option D: Chalk doesn’t mix with sand. it forms a heterogeneous mixture.
Hence correct answer is C.
Question
Which statements about mixtures are correct?
A. I and II only
B. I and III only
C. II and III only
D. I, II and III
▶️Answer/Explanation
Markscheme
B
Explanation : In a mixture, the components are substances that are physically combined but not chemically bonded. As a result, the components of a mixture retain their individual properties. This means that each component maintains its own characteristic properties, such as its chemical composition, physical state, and reactivity.
The components could be in solid, liquid or gas phase and they could be elements or compounds.
Question
Some sodium chloride is dissolved in water. Which term describes the role of sodium chloride in this process?
A. Solute
B. Solvent
C. Solution
D. Saturated
▶️Answer/Explanation
Markscheme
A
Solute: The solute is the substance that is being dissolved in a solution. It is the component that is present in a smaller quantity or is being added to the solvent. The solute can be a solid, liquid, or gas. When a solute is added to a solvent, it disperses or dissolves to form a homogeneous mixture.
Solvent: The solvent is the substance that dissolves the solute to form a solution. It is the component that is present in a larger quantity or is used as the dissolving medium. The solvent is typically a liquid, but it can also be a gas or a solid.
Sodium chloride is the solute here and water is the solvent.