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Ch.14: Elements, Compounds and Mixtures

Elements, Compounds and Mixtures | Class 7 | Science | Chapter 14 | Maharashtra State Board

Measurement of Physical Quantitites | Class 7 | Science | Chapter 6 | Maharashtra State Board

Explore the fascinating world of chemistry with our education channel! Dive into lessons on the particulate nature of matter, properties of substances, and the distinctions between elements, compounds, and mixtures. Uncover the secrets of atoms and compounds, and understand combustion processes. Learn essential methods such as distillation, separating funnel techniques, centrifugation, and chromatography. Elevate your knowledge and ignite your curiosity with our engaging content, designed to make chemistry concepts come to life. Join us on a journey of discovery and deepen your understanding of the fundamental building blocks of the universe.


Questions & Answers

1. Distinguish between the following.

(a) Physical change and chemical change.

Column 'A'

Answer

Column 'B'

1. Stainless steel

Alloy

Non-metal

2. Silver

Metal

Compound

3. Bhajani mixture for milling

Mixture

Mixture

4. Salt

Compound

Element

5. Coal

Non-metal

Alloy

6. Hydrogen

Element

Metal


2. Write the names of elements from the following symbols.

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

Ans.

Symbol

Element

Zn

Zinc

Cd

Cadmium

Xe

Xenon

Br

Bromine

Ti

Titanium

Cu

Copper

Fe

Iron

Si

Silicon

Ir

Iridium

Pt

Platinum

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

(Hydrochloric acid, Sulphuric acid, Sodium chloride, Glucose, Methane)

Ans.

· Hydrochloric acid → HCl

· Sulphuric acid → H2SO4

· Sodium Chloride → NaCl

· Glucose → C6H12O6

· Methane → CH4

· Water → H2O

· Carbon dioxide → CO2

· Sucrose (sugar) → C12H22O11


4. Give scientific reasons:

(a) Buttermilk is churned to get butter.

Ans. Buttermilk consists of two main components—water and fat/cream. Churning helps in the separation of these two components. The fat/cream collects on the top as a result of churning, while the remaining part is left behind as liquid.


(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, the ingredients of a mixture rise up a limited height on paper due to capillary action. The paper's fibers act as a capillary, drawing the liquid (water and the mixture) upward. As the solvent (water) moves, it carries the components of the mixture at different rates based on their affinity for the paper and solvent, resulting in separated bands or spots. Capillary action and differential affinity drive this process.


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

Ans. Wrapping a wet cloth around a water storage container in summer helps cool the water through evaporation. As the cloth evaporates, it absorbs heat from the surroundings, including the water container, causing a cooling effect. This is because the energy required for evaporation is taken from the surrounding heat, lowering the temperature of the water inside the container.


5. Explain the difference.

(a) Metals and Non-metals

Ans.


Metals

Non-metals

1.

Metals are elements that are typically hard, shiny, fusible, malleable, and ductile.

Often lack luster, are poor conductors of heat and electricity, and are typically brittle in solid form.

2.

Most metals are solids at room temperature, with the exception of mercury, which is a liquid.

Can exist in various states at room temperature, including solids (e.g., sulfur), liquids (e.g., bromine), and gases (e.g., oxygen).

3.

Generally, have high density, meaning they are heavy for their size.

Tend to have lower density compared to metals.

4.

Typically have high melting and boiling points.

Generally, have lower melting and boiling points compared to metals.

5.

Tend to form positive ions (cations) by losing electrons in chemical reactions. They are often reactive with acids, forming salts and hydrogen gas.

Tend to form negative ions (anions) by gaining electrons. They may show varied reactivity with acids but generally do not form salts and hydrogen gas as metals do.

6.

Form basic oxides, which typically react with water to form alkaline solutions.

Form acidic or neutral oxides, which may react with water to produce acidic solutions.

(b) Mixture and Compound

Ans. 


Mixture

Compound

1.

Mixtures are substances created by physically mixing two or more substances. Their composition can vary.

Compounds are substances formed by chemically combining two or more elements. They have a fixed chemical composition.

2.

Mixtures are mainly of two types: homogeneous (uniform composition) and heterogeneous (non-uniform composition).

Compounds can be covalent, metallic, or ionic. They can also be classified as organic or inorganic based on carbon presence.

3.

Mixtures are impure substances.

Compounds fall under pure substances.

4.

Mixtures can be separated using physical methods like filtration.

Compounds require chemical or electrochemical methods for separation.

5.

Mixtures retain the properties of their constituents.

Compounds have unique properties distinct from their constituent elements.

6.

Mixtures do not create new substances.

Compounds form new substances after chemical combination.

7.

Mixtures lack specific points.

Compounds have defined melting and boiling points.

8.

Examples: Oil and water, sand and water, smog (smoke + fog).

Examples: Water (H₂O), salt (NaCl), baking soda (NaHCO₃).

(c) Atoms and Molecules

Ans. 


Atoms

Molecules

1.

Atoms are the basic building blocks of matter.

Molecules are formed when two or more atoms chemically bond together.

2.

They are the smallest units of an element that retain the chemical properties of that element.

They can be made up of atoms of the same element (as in diatomic molecules like O2 or N2) or different elements (as in compounds like H2O or CO2).

3.

Composed of protons, neutrons, and electrons.

Molecules have a distinct chemical structure and properties different from those of the individual atoms that compose them.

4.

Elements are made up of only one type of atom.

The atoms in a molecule are held together by chemical bonds, such as covalent bonds.

(d) Separation by distillation and Separation by separating funnel

Ans. 


Separation by distillation

Separation by separating funnel

1.

Distillation involves heating a liquid mixture to create vapor and then cooling the vapor to obtain a liquid, separating the components based on their different boiling points.

A separating funnel is a vertical, cone-shaped container with a stopper and a small, detachable tap at the bottom. It is used to separate immiscible liquids based on their difference in density.

2.

It is commonly used to separate liquids from a solution, especially when the components have significantly different boiling points.

It is particularly useful for separating liquids that do not dissolve in each other, such as oil and water.

3.

The component with the lower boiling point will vaporize first, and the vapor is then condensed back into a liquid, yielding two or more fractions.

The liquids in the funnel form distinct layers due to their different densities. By opening the tap, the heavier liquid (lower density) can be drained out, leaving the other liquid behind.

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

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

Ans. Some simple methods to separate the components of a mixture:

Filtration: This is used for separating an insoluble solid from a liquid. For example, sand from water.

Evaporation: This method is used to separate a soluble solid from a liquid. For example, salt from saltwater.

Distillation: This is used for separating a liquid from a solution. For example, water from saltwater.

Magnetism: This method is used to separate magnetic materials from non-magnetic ones. For example, iron filings from sand.

Decantation: This is used to separate a liquid from a solid. For example, water from sand.

Centrifugation: This is used to separate heavier components from lighter ones in a heterogeneous mixture. For example, cream from milk.


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

Ans. In our day-to-day life, we use-

Elements – non-metal

Oxygen, nitrogen → present in air

Hydrogen → present in water

Silicon → in memory card

Lithium → to make batteries

Neon → in neon signs/lights

Sulphur → used in water treatment, agricultural pesticides

Elements – metals

Calcium → in milk

Silver, gold → used in jewellery

Aluminium, copper, iron → Kitchen vessels

Mercury → in thermometer

Copper → electric wires

Compounds:

Sodium chloride → table salt

Sodium carbonate → washing soda

Sodium bicarbonate → baking soda

Sodium hypochlorite → bleaching powder

Sodium hydroxide, Potassium hydroxide → in making soaps

Calcium oxide, Calcium hydroxide → in white washing the buildings.

Hydrochloric acid → in cleaning toilets

Sucrose → sugar used in cooking and baking.

Mixtures:

Bhel → mixture of puffed rice, sev, groundnuts, lemon juice

Concrete → mixture of cement, sand and rocks

Salad → mixture of onion, cucumber, tomato, lettuce, etc. Sherbet and saltwater are also mixtures.

 

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

Ans. Centrifugation is a technique that can separate different components of a mixture based on their density, size, shape, or viscosity. Some examples of where and for what purpose we use centrifugation in everyday life are:

Washing machines: Centrifugation is used to remove water from the clothes after washing them. The clothes are spun at high speed in a drum, which forces the water out through the holes.

Salad spinners: Centrifugation is used to dry the lettuce and other greens after washing them. The greens are placed in a basket, which is spun rapidly, causing the water to fly off.

Blood tests: Centrifugation is used to separate blood cells from plasma, which can be used for various medical tests. The blood sample is placed in a tube, which is spun in a centrifuge, resulting in the formation of layers of different blood components.

Dairy products: Centrifugation is used to separate cream from milk, which can be used to make butter, cheese, or other products. The milk is spun in a centrifuge, which causes the fat globules to rise to the top, forming the cream layer.


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

Ans.

Distillation:

Water Purification: Distillation is used to remove impurities from water.

Alcoholic Beverages: It’s used in the production of alcoholic beverages like whiskey, rum, and brandy.

Petroleum Products: Crude oil is refined into gasoline, diesel fuel, lubricating oil, fuel oil, paraffin wax, and petrochemicals through a process known as fractional distillation.

Perfume Production: Distillation is used to extract essential oils from plants for making perfumes.

Food Flavourings: Natural food flavourings are created using steam distillation.

Separating Funnel:

Oil and Water Separation: In industries, a separating funnel is used to separate oil and water.

Gasoline and Water Separation: It’s used to separate gasoline and water.

Hexane and Water Separation: Hexane and water can also be separated using a separating funnel.

Laboratory Uses: In laboratories, separating funnels are used for filtration, purification, solvent extraction, concentration, de-mixing, and separating liquids.


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

Ans.

Some precautions to take while using distillation and a separating funnel.

Distillation:

Set up the apparatus in a well-ventilated area.

Perform distillations of flammable solvents under inert gas.

Never leave an active distillation unattended.

Allow the assembly to cool to room temperature before disassembling.

Always keep a fire extinguisher handy.

Separating Funnel:

Firmly hold the stopper when shaking and regularly vent the funnel.

Place a beaker below the funnel to catch solutions that leak or spill.

Wear appropriate personal protective equipment.

Check for cracks or leaks in the funnel, stopper, or stopcock before using it.

Leave the liquid in the funnel undisturbed to allow it to separate.

 

E X T R A

1. Fill in the blanks and rewrite the sentences:

1. Solid, liquid, gaseous and are three states of matter.


2. Molecules in solid phase of matter are tightly packed together.


3. A liquid has a definite volume, but not a definite shape, it takes the shape of its container.


4. The temperature at which a liquid becomes a solid is called freezing point.


5. Anything that takes up space and has mass is called matter.


6. A gas has no definite shape and no definite volume.


7. The amount of space that matter occupies is called volume.


8. A solid has a definite shape and definite volume.


9. When a liquid is heated it will expand.


10. A mixture is formed by mixing different elements or compounds.


11. The substance formed by a chemical combination of two or more elements is called a compound.


12. The elements that show some properties of metals and non-metals are called metalloids.


13. Elements are generally classified into metal and non-metal.


14. The smallest particles of substances are called molecules.


15. To date, scientists have discovered 118 elements and of these 92 elements occur in nature.


16. Berzelius was the first scientist to use symbols for elements.


17. Symbol for Tungsten is and it is 'W' and it is derived from its German name Wolfram.


18. 22-carat gold is an alloy of gold.


19. Molecular formula of a compound is a short form of its name written using symbols of its constituent elements.


20. For separating insoluble particles of blood (blood cells) from its liquid part (plasma), centrifugation method is used.


2. Answer the following.

1. Saltwater is a mixture. Give scientific reason.

Ans. Saltwater is a mixture because it is made up of two or more substances that are combined physically, but not chemically. When salt, which is composed of sodium and chlorine ions, dissolves in water, the two substances form a homogeneous solution, where the salt ions are evenly distributed throughout the water. This physical combination of salt and water can be easily separated through processes such as evaporation, which would not be possible if the salt and water had formed a compound through a chemical reaction. This distinction is what classifies saltwater as a mixture rather than a compound.


2. Classify the following substances according to their properties (water, thermocol, soil, iron, coal, paper, rubber, copper, coir, plastic)

Ans. Metals : Copper, Iron

Mixtures : Soil, Thermocol

Compounds : Paper, Coir, Rubber, Water


3. What are objects made of?

Ans. Objects are made of matter, which is composed of atoms. Atoms are the smallest units of an element that retain the properties of that element. They can combine to form molecules, which make up everything around us. This includes solids, liquids, and gases - everything from the air we breathe, to the chair you’re sitting on, to the cells in our body. So, in short, all objects are made up of atoms and molecules.


4. What are these articles of everyday use made of?

Ans. Everyday use articles can be made from a wide range of materials, depending on their intended purpose, functionality, and design. Here are some common materials used for everyday items:

Plastics: Widely used for various items like bottles, containers, toys, and packaging materials. Different types of plastics offer various properties, such as flexibility, durability, and transparency.

Metals: Items like utensils, keys, and tools are often made from metals such as stainless steel, aluminium, and copper due to their strength and durability.

Glass: Commonly used for containers, windows, and drinkware. It’s a transparent and recyclable material.

Wood: Used for furniture, tools, and decorative items. Different types of wood offer varying levels of strength, durability, and aesthetic appeal.

Ceramics: Items like dishes, tiles, and pottery are made from ceramics, which are typically composed of clay and other minerals. They are known for their hardness and heat resistance.

Textiles: Everyday clothing items, linens, and towels are made from textiles, which can include natural fibers (cotton, linen) or synthetic fibers (polyester, nylon).

Rubber: Used in items like tires, footwear, and various household products. Natural rubber is derived from the sap of rubber trees, while synthetic rubber is manufactured.

Paper and Cardboard: Commonly used for packaging, printing, and writing materials. Paper is made from wood pulp or other plant fibers.

Leather: Used for clothing, accessories, and furniture. Leather is typically derived from the hides of animals like cows, pigs, and goats.

Electronics: Various metals and plastics are used in electronic devices, while silicon is a primary component in microchips.


5. Which metals do we use in day-to-day life?

Ans. Several metals are commonly used in day-to-day life. Some of the most prevalent include:

1. Aluminium: Used in beverage cans, foil, cookware, and construction materials.

2. Steel: Widely employed in construction, tools, appliances, and transportation.

3. Copper: Found in electrical wiring, plumbing, and various electronic devices.

4. Iron: Used in construction, vehicles, and a variety of household items.

5. Brass: Commonly used for musical instruments, plumbing fittings, and decorative items.

6. Stainless Steel: Used in kitchen appliances, cutlery, and construction due to its corrosion resistance.

7. Gold and Silver: Utilized in jewellery, coins, and electronics for their conductivity.

8. Lead: Historically used in pipes, batteries, and as a shielding material.

These metals play crucial roles in manufacturing, construction, and various consumer products.


6. Are metals elements?

Ans. Yes, metals are elements. Elements are pure substances composed of atoms with the same number of protons in their atomic nuclei. Metals are a category of elements characterized by their conductivity, malleability, and ductility. Common metallic elements include iron, copper, aluminium, gold, and silver.


7. Which element helps combustion?

Ans. Oxygen is the element that supports combustion. It is essential for the process of burning and is a key component in the majority of combustion reactions.


8. Does water help combustion?

Ans. No, water does not help combustion. In fact, water has a suppressing effect on combustion by absorbing heat and reducing the temperature of the surrounding material.


9. What are the mixtures used in everyday life?

Ans. Several mixtures are used in everyday life. Some common examples include:

1. Air: A mixture of gases, primarily nitrogen, oxygen, carbon dioxide, and trace elements, essential for breathing.

2. Alloys: Mixtures of metals, such as brass (copper and zinc) and bronze (copper and tin), used in various applications.

3. Food: Many foods are mixtures of different ingredients, like salads, stews, and sandwiches.

4. Soil: A mixture of minerals, organic matter, water, and air, vital for plant growth.

5. Concrete: A mixture of cement, sand, gravel, and water used in construction.

6. Medications: Many pharmaceuticals are mixtures of different chemicals designed for specific therapeutic purposes.

7. Cleaning Products: Household cleaners often contain a mixture of chemicals for effective cleaning.

8. Gasoline: A mixture of hydrocarbons used as fuel for vehicles.

These mixtures serve various purposes in our daily lives, ranging from nutrition to construction and cleaning.


10. Are all mixtures useful to us?

Ans. Not all mixtures are useful to us. Some mixtures may be harmful or have no practical application, while others serve essential purposes in daily life. The utility of a mixture depends on its composition and properties in relation to specific needs or applications.

 

11. What is a molecular formula?

Ans. A molecular formula is a chemical expression that specifies the total number of atoms of each element present in a molecule of a substance. For example, the molecular formula for water is H₂O, indicating two hydrogen atoms and one oxygen atom in each water molecule.


3. Write short notes on:

1. Atoms

Ans. Atoms are the basic building blocks of all matter and chemistry. They consist of three types of subatomic particles: protons, neutrons, and electrons. Protons and neutrons form the nucleus of the atom, which has a positive charge. Electrons orbit the nucleus in shells, which have a negative charge. The number of protons determines the identity of the element, while the number of neutrons determines the isotope of the element. Atoms can combine with other atoms to form molecules or ions by chemical bonds. Atoms are extremely small, typically around 100 picometers across. They cannot be seen by ordinary microscopes, but can be studied by quantum physics.

 

2. Compounds

Ans. Compounds are substances formed by chemically combining two or more different elements. They have distinct properties from their constituent elements.

Examples:

Water (H₂O): A compound of hydrogen and oxygen.

Salt (NaCl): Formed by sodium and chlorine.

Glucose (C₆H₁₂O₆): A sugar compound found in food.


3. Mixtures

Ans. Mixtures are combinations of two or more substances that are physically mixed without undergoing chemical reactions. Unlike compounds, mixtures retain the properties of their individual components.

Types:

Homogeneous Mixtures: Uniform throughout (e.g., saltwater).

Heterogeneous Mixtures: Different components visible (e.g., salad).


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