Answer: (1) ammonia
Explanation: Ammonia [tex]NH_3[/tex] is a compound which is formed by the elements nitrogen and hydrogen in a fixed ratio by mass. It can be separated into its constituent elements by chemical means.
[tex]2NH_3\rightarrow N_2+3H_2[/tex]
Phosphorous (P), Oxygen (O) and Silicon (Si) are all elements and thus cannot be decomposed further.
[tex]\boxed{\left( 1 \right){\text{ ammonia}}}[/tex] is the substance that can be decomposed by chemical means.
Further Explanation:
The pure form of matter is defined as substance but the mixture is termed as the combination of atoms and molecules.
Classification of substances:
(a). Element
It is a type of pure substance and is the simplest form that cannot be broken down by any chemical means. Copper, iron, and aluminium are some of the examples of elements.
(b) Compound
It is composed of two or more different elements that are held together by chemical bonds. These can be decomposed into their respective constituents. Compounds have no similarity in properties with those of their constituent particles. NaCl, [tex]{\text{C}}{{\text{O}}_{\text{2}}}[/tex] and [tex]{\text{N}}{{\text{H}}_3}[/tex] are examples of compounds.
(1) Ammonia contains one nitrogen and three hydrogen atoms so it is a compound. Since compounds can be decomposed chemically, ammonia is also decomposed by chemical means.
(2) Oxygen (O) is an element so it is the simplest form in which it can exist. Therefore it cannot be decomposed by chemical means.
(3) Phosphorus (P) is an element so it is the simplest form in which it can exist. Therefore it cannot be decomposed by chemical means.
(4) Silicon (Si) is an element so it is the simplest form in which it can exist. Therefore it cannot be decomposed by chemical means.
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Answer details:
Grade: High School
Subject: Chemistry
Chapter: Elements, compounds, and mixtures
Keywords: substance, ammonia, oxygen, phosphorus, silicon, element, compound, chemical means, decomposed, broken down, simplest form.