ICSE Solutions
Selina Concise ICSE Solutions for Class 8 Chemistry Chapter 3 Elements, Compounds and Mixtures
Exercise 3(A)
Question 1.
Define: (a) Elements (b) Compounds
Answer:
(a) Elements: Element is a substance which cannot be broken further into simpler substances and has a definite set of properties. Elements are made up of only one kind of atoms.
(b) Compounds: Compounds are pure substances composed of two or more elements in definite proportion by mass and has properties, entirely different from those of its constituents elements.
Compound, are made up of different types of atoms combined chemically.
Question 2.
Give two examples for each of the following:
(a) Metals (b) Non-metals
(c) Metalloids (d) Inert gases
Answer:
(a) Metals: Iron, silver, gold.
(b) Non-metals: Carbon, sulphur, oxygen.
(c) Metalloids: Antimony, silicon, boron.
(d) Inert gases: Helium, argon, neon.
Question 3.
Differentiate between:
(a) Pure and impure substances
(b) Homogenous and heterogenous substances
Answer:
(a) Pure substances —
Pure substances have definite composition and definite physical and chemical properties.
They are all homogeneous i.e. their composition is uniform throughout the bulk.
Examples: Elements and compounds.
Impure substances —
Impure substances are made up of two or more pure substances mixed together in any proportion.
They may be homogeneous or hetergeneous i.e. their composition is not uniform throughout the bulk.
They are all mixtures.
Examples: air, sea water, petroleum, a solution of sugar in water are all impure substances.
(b) Homogeneous mixture — is a mixture where the components that make up the mixture are uniformly distributed throughout the mixture.
Example — air, sugar water, rain water.
Heterogeneous mixture — is a mixture, where the components of the mixture are not uniform or have localized regios with different properties.
Example—Cereal in milk, vegetable soup.
Question 4.
Write the chemical name of the following and also give their molecular formulae:
(a) Baking soda (b) Vinegar
(c) Marble (d) Sand
Answer:
(a) Sodium bicarbonate (Baking soda) — NaHCO3
(b) Acetic acid (Vinegar) — CH3COOH
(c) Calcium carbonate (Marble) — CaCO3
(d) Silicon dioxide (Sand) — SiO2
Question 5.
Name:
(a) a soft metal
(b) a metal which is brittle
(c) a non-metal which is lustrous
(d) a liquid metal
(e) a metal which is a poor conductor of electricity.
(f) a non-metal which is a good conductor of electricity.
(g) a liquid non-metal
(h) the hardest naturally occurring substance
(i) an inert gas
Answer:
(a) Gold
(b) Zinc
(c) Iodine
(d) Mercury
(e) Tungsten
(f) Graphite
(g) Bromine
(h) Diamond
(i) Neon, helium
Question 6.
How is sodium chloride different from its constituent elements ?
Answer:
The properties of sodium chloride are completely different from those of sodium and chlorine. Sodium is a soft, highly reactive metal. Chlorine is a poisonous non-metallic gas while sodium chloride is a very useful non poisonous compound which is added to our food to get minerals and also to add taste to it.
Question 7.
Why is iron sulphide a compound ?
Answer:
Iron sulphide is a compound which can be broken into the elements iron and sulphur they both have different properties. The properties of compound are entirely different from there of its constituents elements.
Exercise 3(B)
Question 1.
Classify the following substances into compounds and mixtures:
Answer:
Carbon dioxide, air, water, milk, common, salt, blood, fruit juice, iron sulphide.
Carbon dioxide — (Compound)
air — (Mixture)
water — (Compound)
milk — (Mixture)
common salt — (Compound)
blood — (Mixture)
fruit juice — (Mixture)
iron sulphide — (Compound)
Question 2.
Give one example for each of the following types of mixtures
(a) solid-solid homogenous mixture
(b) solid-liquid heterogenous mixture
(c) misicible liquids
(d) liquid-gas homogenous mixture
Answer:
(a) Solid-solid homogenous mixture — Alloys of metals e.g. brass, bronze stainless steel etc.
(b) Solid-liquid heterogenous mixture — Sand and water, mud and water, sugar and oil.
(c) Misicible liquids — water and ethanol.
(d) Liquid-gas homogenous mixture — Air
Question 3.
Suggest a suitable technique to separate the constituents of the following mixtures. Also give the reason for selecting the particular method.
(a) Salt from sea water
(b) Ammonium chloride from sand
(c) Chalk powder from water
(d) Iron from sulphur
(e) Water and alcohol
(f) Sodium chloride and potassium nitrate
(g) Calcium carbonate and sodium chloride
Answer:
(a) The technique used to separate the salt from seawater is Evaporation.
Reason – Because this method is used to separate the components of the homogeneous solid-liquid mixture. In this method, sea water is collected in a shallow bed and allowed to evaporate in the sun. When all the water is evaporated, salt is left behind. By this method, we only get solid and liquid is evaporated in its vapour form.
(b) Technique used to separate Ammonium chloride from sand is sublimation.
Because this method is used for solid mixtures in which one of the components can sublime on heating. In this method, Ammonium chloride changes into vapours on heating and salt is left behind.
(c) Technique used to separate chalk powder from water is filtration.
Reason – Because this process is used to separate the components of a heterogeneous solid-liquid mixture in which solids are lights and insoluble in liquids. Substances used as filters are sand filter paper at C. These filters allows the liquid to pass through them, but not solids.
(d) Technique to separate iron from sulpher is magnetic separation.
Because, this method is used when one of the component of mixture is Iron. Iron gets attracted towards the magnet and hence get separated.
(e) Technique used to separate water and Alcohol is Fractional Distillation.
Because in this method, the vapours of water is left behind in the original vessel as the alcohol boils at lower temperature than water. Thus these two liquids can be separated.
(f) Technique used is Fractional-crystallisation.
Because: This method is used when solubility of solid components of mixture and different in the same solvent. Here, sodium chloride and potassium nitrate. Both are soluble in water but solubility of potassium nitrate is more.
(g) Technique used is Solvent Extraction Method: Because, by this method, salts get dissolve in water while calcium carbonate being insoluble in water settles down in the container. And hence get separated about.
Question 4.
(a) Define mixture.
(b) Why is it necessary to separate the constituents of a mixture.
(c) State four differences between compounds and mixtures.
Answer:
(a) “Mixtures can be defined as. a kind of matter which is formed by mixing two or more pure substances (elements and compounds) in any proportion, such that they do not undergo any chemical change and retain their individual properties. Therefore they are impure substances.
(b) Because: The mixtures contain unwanted substances which may be harmful and may degrade the properties of mixtures. So we, need to separated them and extract useful substances.
This is necessary because
(i) It removes unwanted and harmful substances
(ii) to obtain pure and useful substances them.
Example: Sea water is rich in common salt which is an important ingredient of our food to add taste and nutrients. But sea water, cannot be directly used to get the salt.
Hence, it is necessary to separate both.
(c) Compound
A compound is formed from its constituent elements as a result of chemical reaction.
A compound is always homogeneous in nature.
In a compound the elements are present in a fixed ratio by weight.
The components of a compound can’t be separated by physical methods but can be separated by chemical methods only.
The properties of a compound are different from those of its elements.
The formation of a compound from its elements is accompanied by energy changes.
Mixture
A mixture is obtained form its (elements, compounds) components as a result of physical change.
The mixtures can be homogeneous or heterogeneous.
In a mixture the components can be present in any ratio.
The components of a mixture can be separated by physical methods only.
The properties of a mixture lie between those of-its components.
The formation of a mixture from its constituents is not accompanied by energy changes.
Question 5.
(a) What is chromatography ? For which type of mixture is it used ?
(b) What are the advantages of chromatography.
Answer:
(a) This is one of the latest techniques to separate the coloured components of a mixture when all the components are very similar in their properties. Example: Components of ink are separated by this method. Ink is a mixture of different dyes, which are separated by chromatography because some of the dyes are less soluble and some are more soluble in a solvent.
(b)
A very small quantity of the substance can be separated.
Components with very similar physical and chemical properties can be separated.
It identifies the different constitutes of a mixture.
It also helps in quantitive estimation of components of a mixture.
6. Choose the most appropriate answer from the options given below:
(a) a mixture of sand and ammonium chloride can be separated by
filtration
distillation
sublimation
crystallisation
(b) A pair of metalloids are
Na and Mg
B and Si
C and P
HeandAr
(c) Which of the following property is not shown by compounds?
They are heterogeneous.
They are homogeneous.
They have definite molecular formulae.
They have fixed melting and boiling points.
(d) A solvent of Iodine is
Water
Kerosene oil
Alcohol
Petrol
(e) Which of the gas is highly soluble in water ?
Ammonia
Nitrogen
Carbon monoxide
Oxygen
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