MAK LABS — Std 7 General Science Chapter 14 Elements, Compounds and Mixtures

1Who are my companions?

Group ‘A’	Group ‘B’
1. Stainless steel	(e) Alloy
2. Silver	(f) Metal
3. Bhajani mixture for milling	(c) Mixture
4. Salt	(b) Compound
5. Coal	(a) Non-metal
6. Hydrogen	(d) Element

2Write the names of elements from the following symbols: Zn, Cd, Xe, Br, Ti, Cu, Fe, Si, Ir, Pt.

Zn – Zinc
Cd – Cadmium
Xe – Xenon
Br – Bromine
Ti – Titanium
Cu – Copper
Fe – Iron
Si – Silicon
Ir – Iridium
Pt – Platinum

3What are the molecular formulae of the following compounds? Hydrochloric acid, sulphuric acid, sodium chloride, glucose, methane.

Hydrochloric acid – HCl
Sulphuric acid – H₂SO₄
Sodium chloride – NaCl
Glucose – C₆H₁₂O₆
Methane – CH₄

4Give scientific reasons.

(a) Buttermilk is churned to get butter.

Buttermilk contains tiny particles of fat. They do not separate on their own. When it is churned, the fat particles join together. They form lumps of butter. Butter is lighter, so it floats on top. Thus, churning separates butter.

(b) In chromatography, the ingredients of a mixture rise up to a limited height when water rises up to the upper end of the paper.

Each component of a mixture has different solubility in water. Each also sticks to the paper differently. So each component moves up only to a certain height. Water alone rises to the top. In this way, the components get separated.

(c) A wet cloth is wrapped around a water storage container in summer.

In summer, the water in the wet cloth evaporates quickly. For evaporation, it takes heat from the container. So the water inside the container becomes cool.

5Explain the difference.

(a) Metals and non-metals

Metals	Non-metals
They are shiny.	They are not shiny.
They can be beaten into sheets and drawn into wires.	They are brittle.
They are good conductors of heat and electricity.	They are bad conductors (except graphite).
They make a ringing sound.	They do not make a ringing sound.
Examples: iron, copper.	Examples: carbon, sulphur.

(b) Mixtures and compounds

Mixtures	Compounds
Formed by simply mixing substances.	Formed by chemical combination of elements.
No new substance is formed.	A new substance is formed.
Proportion of components is not fixed.	Proportion of elements is fixed.
Components can be separated easily.	Elements cannot be separated easily.
Examples: air, soil.	Examples: water, salt.

(c) Atoms and molecules

Atoms	Molecules
The smallest particle of an element.	The smallest particle of a substance that can exist independently.
Usually cannot exist independently.	Can exist independently.
Example: oxygen atom (O).	Example: oxygen molecule (O₂), water molecule (H₂O).

(d) Separation by distillation and by separating funnel

Distillation	Separating funnel
Used to separate a solid dissolved in a liquid.	Used to separate liquids which do not mix.
Based on evaporation and condensation.	Based on difference in density.
Heating is needed.	Heating is not needed.
Example: pure water from salt water.	Example: kerosene and water.

6Write answers to the following questions in your own words.

(a) How are the components of mixtures separated by simple methods?

Picking: Stones are picked out from rice or dal by hand.
Winnowing: Husk is separated from grain by wind.
Sieving: Bran is separated from flour using a sieve.
Filtering: Tea leaves are separated from tea using a strainer.
Settling: Mud settles at the bottom of muddy water.
Using a magnet: Iron filings are separated from rava.

(b) Which elements (metals and non-metals), compounds and mixtures do we use in our day-to-day life?

Metals: Iron, aluminium, copper, gold, silver.
Non-metals: Oxygen, carbon, nitrogen, chlorine, sulphur.
Compounds: Water, salt, sugar, baking soda, washing soda.
Mixtures: Air, milk, soil, sherbet, brass, steel.

(c) In everyday life, where and for what purpose do we use centrifugation?

In dairies, to separate cream from milk.
In washing machines, to remove water from wet clothes.
In laboratories, to separate parts of blood and to test urine.
To take out honey from honeycombs.

(d) Where are the methods of separation by distillation and by separating funnel used? Why?

Distillation is used to make pure (distilled) water for laboratories, medicines and car batteries. It removes all dissolved impurities.

Separating funnel is used to separate liquids which do not mix, like kerosene and water. Such liquids form two separate layers. The lower layer can be easily drained out.

(e) Which precaution will you take while using the methods of distillation and separation by separating funnel?

Distillation:

Fill the flask only half.
Fit the corks tightly.
Keep cold water flowing through the condenser.
Do not heat the flask until it becomes dry.

Separating funnel:

Close the stopcock before pouring the mixture.
Let the mixture stand until two clear layers form.
Open the stopcock slowly.
Close it as soon as the lower layer drains out.

★Project: Visit a jaggery or a sugar factory. Obtain information about the methods that are used to separate the components of the mixture while making jaggery or sugar. Present it in the class.

Sugarcane is crushed to get juice. The fibre (bagasse) is separated.
The juice is filtered to remove dirt.
The juice is heated. Impurities rise as scum and are removed.
The juice is boiled. Water evaporates and thick syrup is left.
For jaggery, the syrup is poured into moulds and cooled.
For sugar, crystals are formed. They are separated by centrifugation.

Methods used: filtering, settling, evaporation, crystallization and centrifugation.

14. Elements, Compounds and Mixtures

Exercises