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CHEMISTRY [3 Mark Question Answer]

Elements, Compounds and Mixtures · ICSE STD 8 (ICSE - English Medium). 4 questions.

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[3 Mark Question Answer]

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Question 13 Marks
Compare the properties of iron $\ce{[II]}$ sulphide with iron $-$ sulphur mixture, considering iron $\ce{[II]}$ sulphide as a compound particles of iron sulphur mixed together as an example of a mixture.
Answer
Iron and sulphur mixture :
The constituents i.e. iron and sulphur are seen separately.
Iron attracts and clings to magnet.
On adding dil. $\ce{HCl}$ to the mixture $H_2$ gas evolves.
Iron $\ce{[II]}$ sulphide is a compound
It is black solid.
On bringing a magnet near it we cannot separate iron.
On adding dil. $\ce{HCl}$ to it $\ce{H_2S}$ gas evolves
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Question 23 Marks
Define the term 'compound'. In the compound carbon dioxide $-$ the elements carbon $\&$ oxygen are combined in a fixed ratio. Explain.
Answer
A compound is a pure substance made up of two or more different elements $[$atoms$]$ combined chemically in a fixed proportion.
$(a)$ Contains $-$ two or more kinds of atoms, e.g. compound $-$ carbon dioxide $\left[\ce{CO_2}\right]$.
$(b)$ Can be broken down $-$ into two or more simpler substances $-$ by chemical means.
$(c)$ Properties of compounds $-$ differ from those of their elements, e.g. $\ce{CO_2}$ contains elements $-$ Carbon
$\ce{[C]}$ is combustible and Oxygen $\ce{[O]}$ is supporter of combustion - but carbon dioxide is non$-$combustible and non$-$supporter of combustion.
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Question 33 Marks
With reference to elements $–$ define the term ‘molecule’. Give two examples each of a monoatomic, diatomic $\&$ polyatomic molecule.
Answer
Molecule : Atoms of the same element or different elements combine to form a molecule. It is the smallest particle of a substance $-$ which can normally exist independently and can retain, the physical and chemical properties of the substance.
Examples
Monoatomic molecule : Metals $- \ce{Na , Mg}$ etc.
Diatomic molecule : $\ce{H_2, O_2, N_2}$.
Polyatomic molecule : $\ce{O_3, P_4}$.
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Question 43 Marks
Define the terms elements, compounds $\&$ mixtures with a view to show their basic difference.
Answer
Element is a pure substance :
$(i)$ It is the basic unit of matter and cannot be broken down into two or more simpler substances by any means.
$(ii)$ It is mainly classified into metals, non$-$metals, metalloids and noble gases.
Compound is a pure substance :
$(i)$ It is formed by combination of two or more elements.
$(ii)$ The elements are combined together in a fixed ratio.
$(iii)$ It can be broken down into its elements by chemical means.
Mixture is an impure substance :
$(i)$ It is formed by combination of two or more elements, compounds or both.
$(ii)$ The substances are mechanically mixed together in any ratio.
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[3 Mark Question Answer] - CHEMISTRY STD 8 Questions - Vidyadip