Skip to content
CNMS ICSE - 7 Std
Share
Explore

Answer key

Chapter: 03. Elements Compounds And Mixtures

⁠

Section A: Multiple Choice Questions (MCQs)

(​c​) Bronze (Explanation: Bronze is an alloy of copper and tin, whereas brass is of copper and zinc.)
(a) 1 : 8 (Explanation: The constituent elements of water, hydrogen and oxygen, are present in a fixed ratio of 1:8 by mass.)
(d) Lead (Explanation: The symbol of Lead is Pb, derived from its Latin name Plumbum.)
(b) Gravitation (Explanation: Gravitation is used when one component is heavier than water [sand settles] and the other is lighter [sawdust floats].)
(​c​) 95% alcohol and 5% water (Explanation: Ethanol and water cannot be separated completely by fractional distillation, leaving a final mixture of 95% alcohol and 5% water called rectified spirit.)
⁠

Section B: Fill in the Blanks

hydrolysis
noble gases (or inert gases)
alloy
Whatmann
deposition
⁠

Section C: True or False Questions

False (Explanation: Generally, energy is neither released nor absorbed during the physical formation of a mixture.)
True (Explanation: Non-metals are generally soft, non-lustrous, brittle, and bad conductors.)
False (Explanation: The constituents of a compound can only be separated by chemical methods, not physical methods.)
True (Explanation: Brass is a homogeneous mixture of copper and zinc.)
True (Explanation: Chromatography has a vital role in testing sports athletes for performance-enhancing drugs.)
⁠

Section D: Explanations of Terms

Metalloids: Elements that exhibit some physical and chemical properties of metals and some properties of non-metals (e.g., boron, silicon, arsenic).
Distillate: The pure liquid obtained in the receiving flask by condensing the vapour formed during the process of distillation.
Emulsion: A mixture of two or more liquid components that are completely immiscible (i.e., they cannot be mixed completely, such as oil and water).
⁠

Section E: Complete the Series

Kalium (Explanation: Symbol of Sodium is from Natrium, and Potassium is from Kalium.)
Homogeneous mixture (Explanation: Sand in water is heterogeneous, whereas salt dissolves completely to form a homogeneous solution.)
Sublimation (Explanation: Iron filings are separated magnetically, while ammonium chloride is a sublimable solid separated from non-sublimable salt by sublimation.)
⁠

Section F: Differentiate Between Concepts

Pure Substance vs. Impure Substance (Mixture):
Composition: Pure substances have a definite and uniform composition throughout. Mixtures have a variable composition, with components present in any proportion.
Melting/Boiling Points: Pure substances have fixed melting and boiling points. Mixtures do not have fixed melting and boiling points (they depend on the proportions of the components).
Miscible Liquids vs. Immiscible Liquids:
Solubility/Layers formed: Miscible liquids dissolve completely in each other to form a single uniform layer (e.g., alcohol and water). Immiscible liquids do not dissolve in each other and separate into distinct layers (e.g., oil and water).
Separation Method: Miscible liquids are separated by fractional distillation (if boiling points differ by less than 25°C). Immiscible liquids are separated using a separating funnel.
⁠

Section G: Short Answer Questions

Pure iron is soft in its pure form and stretches easily when heated. Steel is an alloy of iron with a small amount of carbon, which makes it hard, strong, and highly resistant to rusting.
Two characteristics of a compound are:
It is made up of two or more elements chemically combined in a fixed proportion by mass.
Its physical and chemical properties are entirely different from the properties of its constituent elements.
Its constituents can be separated only by chemical methods. (Any two)
Mustard oil is lighter (less dense) than water, and since the two liquids are immiscible, the oil rises to the top to form a separate layer.
The principle of centrifugation is based on separating tiny insoluble solid particles from a liquid by rotating the mixture at a very high speed. Under high-speed rotation, the heavier solid particles (higher density) are forced to the bottom, while the lighter particles (lower density) float to the top.
⁠

Section H: Diagram-Based Questions

Sublimation (and deposition).
The cotton plug is used to block the nozzle of the funnel to prevent the sublimed vapour of ammonium chloride from escaping into the surrounding air.
Sodium chloride (common salt) remains behind in the china dish as it is non-sublimable.
⁠

Section I: Observation-Based & Everyday Use Questions

(a) The chemical name of the dark grey substance is iron sulphide. (b) No, it will not be attracted to a magnet. This is because iron sulphide is a compound, and the properties of a compound are entirely different from its constituent elements (iron, which is magnetic, and sulphur, which is non-magnetic).
Step-by-step separation:
Pour the mixture of water and kerosene oil into a separating funnel and let it stand undisturbed.
The mixture will separate into two distinct layers: kerosene oil on top (lighter) and water at the bottom (heavier).
Place a beaker under the funnel and slowly open the stopcock to drain out the lower layer of water.
Close the stopcock tightly as soon as the upper kerosene oil layer reaches the opening.
Place another beaker under the funnel, open the stopcock, and collect the separated kerosene oil.
The method is called Centrifugation. Milk is placed in a centrifuge machine and rotated at a very high speed. Because cream is lighter than the rest of the milk components, it floats to the top during rotation and can be easily skimmed off or separated.
⁠

Section J: Long Answer Questions

Separation of Sand, Sawdust, and Salt:
Step 1: Adding Water: Add water to the mixture and stir well. Common salt is soluble and dissolves in water to form a salt solution. Sand and sawdust are insoluble.
Step 2: Gravitation (Separating Sawdust): Since sawdust is lighter than water, it floats on the surface of the liquid. Sand, being heavier, settles at the bottom of the container. Collect the floating sawdust from the top using a spoon and leave it to dry.
Step 3: Filtration (Separating Sand): Pour the remaining mixture of sand and salt solution through a filter paper. The insoluble sand remains on the filter paper as residue, while the clear salt solution passes through as the filtrate.
Step 4: Evaporation (Separating Salt): Heat the salt solution filtrate in a china dish. The water evaporates into the air as vapour, leaving behind pure solid common salt in the dish.
Paper Chromatography:
Definition: Chromatography is the process of separating the different dissolved components of a mixture by their adsorption on an appropriate material (stationary phase) using a moving solvent (mobile phase).
Experimental Process:
Take a thin strip of Whatmann filter paper and draw a pencil reference line about 3 cm above one end.
Put a small spot of the marker pen ink at the center of the pencil line and let it dry.
Suspend the paper strip vertically in a chromatographic jar containing water (the level of water must be below the pencil line).
As water rises up the paper strip by capillary action, it carries the dissolved ink pigments along with it.
Since different color pigments in the ink have different solubilities in water, they interact differently with the paper. The pigment that is more soluble rises faster and reaches a greater height, separating the ink into distinct colored spots on the chromatogram.
Gallery
 
Want to print your doc?
This is not the way.
Try clicking the ··· in the right corner or using a keyboard shortcut (
CtrlP
) instead.