Answer key

Prepared by: learnloophq@gmail.com

Last edited 25 days ago by Learn LoopHQ.

Chapter: 03. Elements And Compounds

⁠

Part A: Multiple Choice Questions

c) Sugar

c) Atom

c) Mercury

b) Carbon (as diamond)

b) Metalloids

c) To prevent the filament from reacting

c) 3

c) Elements chemically combined in a fixed proportion

Part B: Fill in the Blanks

mixture

fixed

ductile

Nitrogen

symbol

molecule

tetraatomic

CO_2

Part C: True/False Questions

False

True

False

True

False

Part D: Explanations of Terms

Malleability: It is the property of a material, typically a metal, that allows it to be hammered or pressed into thin sheets or foils without breaking.

Atomicity: It is the number of atoms present in one molecule of an element. For example, the atomicity of oxygen in O_2 is 2.

Noble Gases: These are gaseous elements that are chemically unreactive, meaning they do not easily combine with other elements. Examples include helium and neon.

Part E: Identifying and Differentiate Concepts

Homogeneous substances have a uniform composition throughout, meaning their properties are consistent in every part (e.g., pure sugar). Heterogeneous substances do not have a uniform composition; their properties vary from one part to another (e.g., soil, which contains different particles).

Elements are pure substances made of only one kind of atom and cannot be broken down into simpler substances by any physical or chemical method. Compounds are pure substances formed when two or more different elements chemically combine in a fixed proportion. Also, the properties of a compound are entirely different from its constituent elements, which is not the case for elements.

Part F: Short Answer Questions

Water is considered a pure substance because it is made up of only water molecules and has a fixed chemical composition (hydrogen and oxygen in a 1:8 ratio by mass). If sugar is mixed in water, its boiling point changes, indicating that it now contains an impurity.

Gold is suitable for making jewellery for two main reasons: it is lustrous (shiny) which makes ornaments attractive, and it is highly ductile and malleable, allowing it to be drawn into fine wires and hammered into intricate shapes.

Modern symbols for elements are derived in two ways from their English names:

Using the first letter, capitalized (e.g., O for Oxygen, C for Carbon).

Using the first letter (capitalized) and another significant letter (lowercase) (e.g., Ca for Calcium, Cl for Chlorine).

Two ways in which the properties of a compound differ from its constituent elements are:

Physical state: Hydrogen and oxygen are gases, but their compound, water, is a liquid under normal conditions.

Chemical reactivity: Hydrogen is flammable and oxygen supports burning, but their compound, water, is used to put out fires.

A chemical formula is important because it provides concise information about a substance. It tells us which elements are present in the compound, the exact number of atoms of each kind in one molecule, and their ratio. It also allows for the calculation of the mass of one molecule of the compound.

Part G: Diagram-Based Question

The substance filled inside this instrument is Mercury.

This substance is a metal.

Mercury is chosen for thermometers because it is a liquid metal that expands and contracts uniformly with temperature changes, making it ideal for accurate temperature measurement. It also has a relatively high boiling point and low freezing point, making it suitable for a wide range of temperatures.

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