Chemistry

The setting time of cement is lowered by adding

[amp_mcq option1=”oxides of aluminium” option2=”gypsum” option3=”oxides of magnesium” option4=”silica” correct=”option1″]

The correct answer is A) oxides of aluminium. The question asks what is added to lower the setting time of cement, which means making it set faster. Gypsum (option B) is added to *increase* the setting time (retard setting). While standard accelerators are typically calcium chloride or alkali aluminates, reactive forms of aluminium oxides or phases rich in alumina like Calcium Aluminate Cements (CAC) or specific additives containing activated alumina can act as accelerators, causing rapid setting. Therefore, among the given options, oxides of aluminium (in a suitable form or context as present in cement phases like C₃A) are the most plausible candidates for contributing to or accelerating the setting process compared to the other options which are either retarders (gypsum) or primary components/cause of expansion (magnesium oxide).

– Setting time refers to the time required for cement paste to lose its plasticity.
– Additives are used to control setting time: accelerators decrease setting time, retarders increase it.
– Gypsum is a common retarder, preventing flash set by reacting with C₃A.
– Oxides of aluminium are present in cement clinker primarily as tricalcium aluminate (C₃A), which hydrates rapidly and contributes to early setting. Certain forms of reactive alumina can act as accelerators.

– Calcium chloride (CaCl₂) is a very common and effective accelerator used in cement.
– Oxides of magnesium (MgO), if present in excess in portland cement, can cause delayed expansion and soundness issues, not primarily affecting initial setting time in a controlled manner.
– Silica (SiO₂) is a main component of cement (silicates C₃S and C₂S), which are responsible for strength development, but adding pure silica is not a method to control setting time.
– The phrasing “lowered by adding” implies acceleration. While gypsum is a crucial additive related to setting time, its effect is the opposite (increasing/retarding). This points towards A as the intended answer, assuming a context where an aluminium oxide based additive is used for acceleration.

Which one of the following species is not capable of showing disproportionation reaction ?

[amp_mcq option1=”ClO⁻” option2=”ClO₂⁻” option3=”ClO₃⁻” option4=”ClO₄⁻” correct=”option4″]

ClO₄⁻ (Perchlorate) is not capable of showing disproportionation reaction.

Disproportionation is a redox reaction where an element in a specific oxidation state is simultaneously oxidized to a higher oxidation state and reduced to a lower oxidation state. For this to happen, the element must be in an intermediate oxidation state. In ClO₄⁻, Chlorine is in its highest common oxidation state of +7. It can only be reduced to a lower state (e.g., +5, +3, +1, 0, -1), but it cannot be oxidized further. Therefore, it cannot undergo disproportionation.

In ClO⁻ (+1), ClO₂⁻ (+3), and ClO₃⁻ (+5), Chlorine is in intermediate oxidation states and can be both oxidized and reduced, hence capable of disproportionation.

Match List I with List II and select the correct answer using the code given below the Lists :

List I (Element)	List II (Highest Valency)
A. Sulfur	1. Five
B. Phosphorous	2. Six
C. Lead	3. Two
D. Silver	4. Four

Code :

	A	B	C	D
(a)	2	4	1	3
(b)	2	1	4	3
(c)	3	1	4	2
(d)	3	4	1	2

[amp_mcq option1=”2, 4, 1, 3″ option2=”2, 1, 4, 3″ option3=”3, 1, 4, 2″ option4=”3, 4, 1, 2″ correct=”option2″]

The correct match is A-2, B-1, C-4, D-3.

The highest common valency (or oxidation state) for the elements are:
– Sulfur (S): Can exhibit +6 (e.g., in sulfates), so highest valency is Six (2).
– Phosphorus (P): Can exhibit +5 (e.g., in phosphates), so highest valency is Five (1).
– Lead (Pb): Can exhibit +4 (e.g., PbO₂), so highest valency is Four (4).
– Silver (Ag): The most common valency/oxidation state is +1. While higher, unstable oxidation states exist, +1 is the standard. The option provides ‘Two’ (3) as the highest valency for Silver. Assuming this option set is intended, the pairings match option B for A, B, and C.

Based on standard chemistry, the highest common valency for Silver is 1. However, matching the provided options, A-2, B-1, C-4 align with common understanding, leading to option B being the most probable intended answer despite the inaccuracy regarding Silver’s listed highest valency.

Combination of one volume of nitrogen with three volumes of hydrogen produces

[amp_mcq option1=”one volume of ammonia” option2=”two volumes of ammonia” option3=”three volumes of ammonia” option4=”one and a half volumes of ammonia” correct=”option2″]

The reaction between nitrogen and hydrogen to produce ammonia is represented by the balanced chemical equation: N₂(g) + 3H₂(g) → 2NH₃(g). According to Avogadro’s law, at the same temperature and pressure, the ratio of the volumes of reacting gases and gaseous products is equal to the ratio of their stoichiometric coefficients in the balanced equation. Thus, 1 volume of nitrogen reacts with 3 volumes of hydrogen to produce 2 volumes of ammonia.

– The balanced equation for the Haber process is N₂(g) + 3H₂(g) → 2NH₃(g).
– The stoichiometric coefficients are 1 for N₂, 3 for H₂, and 2 for NH₃.
– At constant temperature and pressure, the volume ratio of reacting gases and gaseous products is equal to the mole ratio (and stoichiometric coefficient ratio).

This principle relating volumes of reacting gases to their stoichiometric coefficients is known as Gay-Lussac’s law of combining volumes.