7th Std Science 14 Elements, Compounds and Mixtures l Question and Answer

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Science Studies -  7th Standard

14. Elements, Compounds and Mixtures

Question And Answer

1. Who are my companions ? 

2. Write the names of elements from the following symbols : Zn,

Cd, Xe, Br, Ti, Cu, Fe, Si, Ir, Pt.

Ans: Zn- Zink

Cd- Cadmium

Xe- Xenon

Br- Bromine

Ti- Titanium

Cu- Copper/Cuprus

Fe- Iron/ Ferrum

Si- Silicon

Ir- Iridium

Pt- Platinum

3. What are the molecular formulae of the following compounds ?

Ans: Hydrochloric acid- HCl

Sulphuric acid- H2SO4

Sodium chloride- NaCl

Glucose- C6H12O6

Methane- CH4

3. Give scientific reasons.

(a) Buttermilk is churned to get butter.

Ans: Churning is one of the method of separation. The

particles of butter are all spread out in the buttermilk. By

stirring they aggregate due to centrifugal force and form one

solid lump. To separate the butter, the buttermilk is

therefore churned.

(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.

Ans: In chromatography method two mutually opposite

properties of substances are used. These are:

1) The solubility of the substance in the solvent that moves

up.

2) The ability of the substance to stick to the stationary filter

paper. These properties are different for different substances.

Since water is a solvent, it rises rapidly across the

chromatography filter paper. But the solutes will not rise

with that speed. Therefore the ingredients of a mixture riseup to a limited height when water rises up to the upper end

of the paper. 

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

summer.

Ans: A wet cloth is wrapped around water containers in

summers to keep the container and the water inside it cools.

The water present in the cloth evapourates and along with

that water the heat also evapourates thus keeping the

container and water cool.

5. Explain the difference.

(a) Metals and non-metals

Ans:

 (b) Mixtures and compounds

Ans:

(c) Atoms and molecules

Ans:

(d) Separation by distillation and by separating funnel

Ans:

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

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

methods ?

Ans: There are various methods to separate the mixtures.

Some of these are used in our daily life such as straining,

shifting, picking, sorting, winnowing, combing with a magnet

and sublimation.

1) Straining- To separate insoluble substances from the

solution, we use the method of straining.

2) Shifting- Shifting is done to separate solids of various sizes.

The larger particles and the smaller particles can be

separated by shifting.

3) Picking and Sorting- By this method a substances that are

needed or do not needed are separated out.

4) Winnowing- The substances which are lighter in weight

and those that are unwanted are removed from the mixture.

The grains in the farm are particularly cleaned by the

process of winnowing.

5) Combing with a magnet- If there is adulteration of

semolina with iron, the magnet is combed through it for removing these iron particles. The mixture that contains iron

particles is specially combed with a magnet.

(b) Which elements (metals and non-metals), compounds and

mixtures do we use in our day-to-day life ?

Ans: Metals- Iron, Gold, Silver, Copper

Non-metals- Carbon, Oxygen, Hydrogen, Nitrogen

Compounds- Sugar, Salt, Oil

Mixtures- Soap, Alloy like steel, Brass, 22 Carat Gold

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

centrifugation?

Ans: Use of centrifuge machine is never done in our common

house-hold work. By using method of centrifugation the

particles of smaller size are separated due to centrifugal

force. Based on this principle, we use stirrer for making

buttermilk from yogurt. The stirrer helps to separate

particles of butter from the suspension of the buttermilk.

Due to centrifugal force, the particles of butter aggregate and

are thus easily removed.

(d) Where are the methods of separation by distillation and by

centrifugation ?

Ans: 1) Distillation- Distillation is the separation method

used to separate impurities or dissolved salts from the

solution. By this method, the solute and solvent are

separated. For removing salt from sea water, distillation

method is used. This helps in making portable water from

sea water.

2)Using separating funnel- When two liquids in a mixture

which are immiscible with each other are to be separated,

the method using separating funnel is used. This method can

be used separation of mixture of kerosene and water.

(e) Which precaution will you take while using the methods of

distillation and separation by separating funnel ?

Ans: In the method of distillation, the entire apparatus

should be properly set up. The distillation flask should be

carefully heated. The condenser tube should be properly

cooled. In the separating funnel experiment care should be

taken to well manipulate the stopcock. The malfunctioning

stopcock many a times releases the liquids inadvertently.

The separation of lower phase liquid should be done with

precision by opening and closing and stopcock.