Page 109 - Innovator's Science-8

ii. C is a compound, and A and B have a fixed composi on.
iii. A and B are compounds, and C has a fixed composi on.
iv. A and B are elements, C is a compound, and has a fixed composi on.
Ans. iv. A and B are elements, C is a compound, and has a fixed composi on.

The ques on states that A and B cannot be broken down into simpler substances by chemical reac ons.
This means A and B are elements, because elements are pure substances made of only one kind of atom
and cannot be broken down chemically. When A and B combine chemically to form C, the result is a
compound. A compound is formed when two or more elements combine in a fixed ra o through a
chemical reac on. Therefore, A and B are elements, and C is a compound with a fixed composi on.

2. Asser on: Air is a mixture.
Reason: A mixture is formed when two or more substances are mixed, without undergoing any
chemical change.
i. Both Asser on and Reason are true and Reason is the correct explana on for Asser on.
ii. Both Asser on and Reason are true, but Reason is not the correct explana on for Asser on.
iii. Asser on is true, but Reason is false.
iv. Asser on is false, but Reason is true.
Ans. i. Both Asser on and Reason are true and Reason is the correct explana on for asser on.
Air is indeed a mixture because it is composed of various gases like nitrogen, oxygen, and carbon dioxide,
which are mixed without any chemical reac on between them. The proper es of these individual gases
are retained within the air.

3. Water, a compound, has different proper es compared to those of the elements oxygen and hydrogen
from which it is formed. Jus fy this statement.
Ans. Water has proper es which is completely different from hydrogen and oxygen. Like water is liquid in
form, whereas hydrogen (H) and oxygen (O) are gases. This is because a compound’s proper es depend
on its molecular structure.
4. In which of the following cases are all the examples correctly matched? Give reasons in support of
your answers.
i. Elements – water, nitrogen, iron, air.
ii. Uniform mixtures – minerals, seawater, bronze, air.

iii. Pure substances – carbon dioxide, iron, oxygen, sugar.
iv. Non-uniform mixtures – air, sand, brass, muddy water.
Ans. iii. Pure substances – Carbon dioxide, iron, oxygen, and sugar are all pure substances. (Correctly
matched).
A pure substance is composed of only one type of par cle. Carbon dioxide, iron, and oxygen are all
elements, meaning they are made up of only one type of atom. Sugar is a compound, but it is s ll
considered a pure substance because it consists of only one type of molecule.
5. Iron reacts with moist air to form iron oxide, and magnesium burns in oxygen to form magnesium
oxide. Classify all the substances involved in the above reac ons as elements, compounds, or
mixtures, with jus fica on.
Ans. Iron reacts with moist air to form iron oxide
Iron (Fe) + Oxygen (O ) + Water (H O) → Iron Oxide (Fe O )
2
2 3
2
Magnesium burns in oxygen to form magnesium oxide
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