Chapter 3 – Elements Compounds and Mixtures

Selina Concise Chemistry 2023 solutions for class 8 Chemistry. Chapter 3 – Elements Compounds and Mixtures is provided here with simple step-by-step explanations. These solutions for Elements Compounds and Mixtures are extremely popular among class 10 students. The Elements Compounds and Mixtures solution comes in handy for quickly completing your homework and preparing for exams.

Exercise 3(A)

Question 1

Define :

1. Elements
2. Compounds

Answer

1. Elements are pure substances that cannot be broken down into simpler substances by chemical reactions. They are made up of only one type of atom and have a unique set of physical and chemical properties.
2. Compounds are pure substances made up of two or more different elements that are chemically combined in a fixed proportion.

Question 2

Give two examples for each of the following

1. Metals
2. Non-metals
3. Metalloids
4. Inert gases

Answer

(a) Metals:

• Iron (Fe)
• Aluminum (Al)

(b) Non-metals:

• Oxygen (O)
• Carbon (C)

(c) Metalloids:

• Silicon (Si)
• Arsenic (As)

(d) Inert gases:

• Helium (He)
• Neon (Ne)

Question 3

Differentiate between :

1. Pure and impure substances
2. Homogeneous and heterogeneous substances

Answer

(a) Differentiate between pure and impure substances:

(b) Differentiate between homogeneous and heterogeneous substances :

Question 4

Write the chemical name of the following and also give their molecular formulae :

1. Baking soda
2. Vinegar
3. Marble
4. Sand

Answer

(a) Baking soda:

• Chemical name: Sodium bicarbonate
• Molecular formula: NaHCO₃

(b) Vinegar:

• Chemical name: Acetic acid
• Molecular formula: CH₃COOH

(c) Marble:

• Chemical name: Calcium carbonate
• Molecular formula: CaCO₃

(d) Sand:

• Chemical name: Silicon dioxide
• Molecular formula: SiO₂

Question 5

Name

1. a soft metal.
2. a metal which is brittle
3. a non-metal which is lustrous
4. a liquid metal
5. a metal which is a poor conductor of electricity.
6. a non-metal which is a good conductor of electricity.
7. a liquid non-metal.
8. the hardest naturally occurring substance.
9. an inert gas.

Answer

Question 6

How does sodium chloride differ from its constituent elements? Explain.

Answer

Sodium chloride (NaCl) is a compound made up of sodium and chlorine elements that have different physical and chemical properties from the compound. NaCl is not reactive like its constituent elements because the chemical bond between them is ionic and very strong.

Question 7

Why is iron sulphide a compound?

Answer

Iron sulfide (FeS) is a compound because it is made up of two different elements, iron and sulfur, that are chemically bonded together in a fixed ratio to form a new substance with unique properties different from those of the individual elements.

Exercise 3(B)

Question 1

Classify the following substances into compounds and mixtures:

Carbon dioxide, air, water, milk, common salt, blood, fruit juice, iron sulphide.

Answer

Question 2

Give one example for each of the following types of mixtures.

1. solid-solid homogeneous mixture
2. solid-liquid heterogeneous mixture
3. miscible liquids
4. liquid-gas homogeneous mixture

Answer

1. An alloy, such as brass, is a mixture of copper and zinc.
2. Sand and water mixture, where sand particles do not dissolve in water.
3. Ethanol and water, mix together completely and form a homogeneous mixture.
4. Air is a mixture of gases, primarily nitrogen and oxygen, and is present in a homogeneous mixture in our environment.

Question 3

Suggest a suitable technique to separate the constituents of the following mixtures. Also give the reason for selecting the particular method.

1. Salt from seawater
2. Ammonium chloride from sand
3. Chalk powder from water
4. Iron from sulphur
5. Water and alcohol
6. Sodium chloride and potassium nitrate
7. Calcium carbonate and sodium chloride

Answer

1. Salt from seawater:
   Technique: Evaporation
   Reason: Salt can be separated from seawater through the process of evaporation, as the water evaporates, the salt is left behind as a solid residue.
2. Ammonium chloride from sand:
   Technique: Sublimation
   Reason: Ammonium chloride can be separated from sand by sublimation, as it sublimes on heating, leaving behind the sand.
3. Chalk powder from water:
   Technique: Filtration
   Reason: Chalk powder can be separated from water through the process of filtration, as chalk powder is insoluble in water and can be separated using a filter.
4. Iron from sulphur:
   Technique: Magnetic Separation
   Reason: Iron can be separated from sulfur through the process of magnetic separation, as iron is magnetic and can be attracted to a magnet, whereas sulfur is not magnetic.
5. Water and alcohol:
   Technique: Distillation
   Reason: Water and alcohol can be separated through the process of distillation, as they have different boiling points. Alcohol has a lower boiling point than water, so it vaporizes first and can be condensed and collected separately.
6. Sodium chloride and potassium nitrate:
   Technique: Crystallization
   Reason: Sodium chloride and potassium nitrate can be separated through the process of crystallization, as they have different solubility properties. By heating the mixture, the less soluble substance (potassium nitrate) will form crystals first, which can be filtered out, leaving behind the more soluble substance (sodium chloride).
7. Calcium carbonate and sodium chloride:
   Technique: Filtration
   Reason: Calcium carbonate can be separated from sodium chloride through the process of filtration, as calcium carbonate is insoluble in water and can be separated using a filter, while sodium chloride remains in the filtrate.

Question 4

1. Define ‘mixture’.
2. Why is it necessary to separate the constituents of a mixture?
3. State four differences between compounds and mixtures.

Answer

1. Definition of ‘Mixture’: A mixture is a combination of two or more substances that are not chemically combined, meaning they can be separated by physical means. Mixtures retain the individual properties of their constituents and can be inhomogeneous (not uniformly distributed) or homogeneous (uniformly distributed).
2. Why it is necessary to separate the constituents of a mixture? It is necessary to separate the constituents of a mixture to obtain the individual components in a pure form or to remove unwanted substances. Separation can be achieved by physical methods such as filtration, evaporation, distillation, sublimation, etc.
3. Four differences between compounds and mixtures:

Question 5

1. What is chromatography? For which type of mixture is it used?
2. What are the advantages of chromatography?
3. Give two applications of chromatography.

Answer

(a) Chromatography is a physical method of separating the components of a mixture based on their differential partitioning between a stationary phase and a mobile phase. It is primarily used for separating complex mixtures of organic compounds, such as those found in natural products, pharmaceuticals, and biological samples.

(b) Chromatography has several advantages over other separation techniques, including:

• High separation efficiency and resolution
• Ability to handle a wide range of sample sizes and concentrations
• Can be used for both analytical and preparative purposes
• Compatible with a variety of stationary and mobile phases
• Can separate multiple components in a single run

(c) Chromatography has a wide range of applications in various fields, some of which are:

• Purification and analysis of drug compounds and intermediates in the pharmaceutical industry.
• Identification and separation of compounds found in crime scene samples in forensic science.

Question 6

Choose the most appropriate answer from the options given below:

(a) a mixture of sand and ammonium chloride can be separated by

1. filtration
2. distillation
3. sublimation
4. crystallisation

(b) A pair of metalloids are

1. Na and Mg
2. B and Si
3. C and P
4. He and Ar

(c) Which of the following property is not shown by compounds?

1. They are heterogeneous.
2. They are homogeneous.
3. They have definite molecular formulae.
4. They have fixed melting and boiling points.

(d) A solvent of Iodine is

1. Water
2. Alcohol
3. Petrol
4. Kerosene oil

(e) This gas is highly soluble in water

1. Ammonia
2. Nitrogen
3. Carbon monoxide
4. Oxygen

Answer

(a) sublimation

(b) B and Si

(c) They are heterogeneous.

(d) alcohol

(e) Ammonia