9.1.3 Elements, Compounds and Mixtures
This is the third lecture from Chapter 1: ‘States of Matter and Phase Changes’ of the new book for 9th Class Chemistry (Punjab Board – PCTB). It discusses the types of matter based on chemical composition — substances (elements and compounds) and mixtures (homogeneous and heterogeneous). The lecture includes a multiple-choice quiz, short-answer questions, and detailed long-answer notes.
MCQs Based Quiz
Short Questions
Q1. How would you classify matter based on its chemical composition?
Matter, based on its chemical composition, can be classified into two broad categories: substances (pure matter) and mixtures (impure matter). Substances include elements and compounds, while mixtures include homogeneous mixtures and heterogeneous mixtures.
Q2. How would you define elements?
An element is a pure substance. It is the purest form of matter because it contains only one kind of atom.
Example: oxygen (O2), hydrogen (H2), and nitrogen (N2).
Q3. Why do chemists consider elements to be the purest form of matter?
Elements are the purest form of matter due to two main reasons:
- Elements contain the same kind of atoms.
- They cannot be broken down into simpler substances through chemical reactions.
Q4. How would you classify elements based on their chemical reactions?
Based on their chemical reactions, elements can be categorized as metals, non-metals, metalloids and noble gases.
Q6. What are the different forms in which elements can exist?
Elements can exist in the form of atoms, molecules, ions and isotopes.
Q8. What is the definition of a compound?
A compound is a substance in which atoms of two or more elements are chemically bonded together in a fixed ratio.
Example: Water (H2O), carbon dioxide (CO2) and sodium chloride (NaCl)
Q7. What are the different types of compounds based on chemical bonding?
Based on chemical bonding, compounds can be classified into four major categories: ionic compounds, molecular compounds, coordination complexes and intermetallic compounds.
Q8. How would you classify compounds based on the type of atoms they have?
Compounds, based on their atoms, can be divided into two major categories: organic compounds and inorganic compounds. Organic compounds have carbon and hydrogen as their main atoms, while inorganic compounds do not.
Q9. Why does a sample of pure water from Karachi behave just like the water from Faisalabad?
Both of these water samples behave in a similar way because the composition and properties of the substance (pure water) are uniform throughout both samples.
Q10. What is the definition of mixtures?
Mixtures are the impure form of matter because they contain more than one type of substance physically mixed in a random ratio.
Example: Lemonade, biryani and tap water
Q11. What is a homogeneous mixture?
A homogeneous mixture is a type of mixture in which the concentration of the components is uniform throughout the mixture.
Example: Lemonade, tap water, air
Q12. How would you define heterogeneous mixture?
A mixture in which the concentration of components is not uniform is called a heterogeneous mixture.
Examples: Pizza, kheer, rock
Q13. Why would someone classify rock as a heterogeneous mixture?
A rock is made of different types of materials such as granite, mica and limestone. Due to the random distribution of these materials in the rock, it is classified as a heterogeneous mixture.
Descriptive Question
Q1. Write a note on elements, compounds and mixtures.
Matter can be classified into the following two main categories:
- Substances
- Elements
- Compounds
- Mixtures
- Homogeneous Mixtures
- Heterogeneous Mixtures
Substances:
Substances are pure forms of matter because they contain only one type of units.
Examples: Iron (Fe), Oxygen (O2), Water (H2O) and Carbon Dioxide (CO2)
Properties of Substances:
- They have specific composition and properties.
- Their composition and properties remain uniform throughout all samples.
Elements:
Elements are the simplest and purest substances because they contain only one type of atom.
Examples: Iron (Fe), oxygen (O2) and hydrogen (H2)
Properties of Elements:
- Elements exist in all three states of matter. Most are solids, some are gases and only two are liquids at room temperature.
- They can exist as atoms, molecules, ions or isotopes.
- They can also be classified as metals, non-metals, metalloids and noble gases.
- They cannot be broken into anything simpler by chemical reactions.
Compounds:
A compound is a substance made of atoms of two or more elements chemically bonded in a fixed ratio.
Examples: Water (H2O), carbon dioxide (CO2) and table salt (NaCl)
Properties of Compounds:
- Compounds can be classified as molecular, ionic, intermetallic and coordination complexes.
- They are also classified into inorganic compounds and organic compounds.
Mixtures:
Mixtures are impure forms of matter made by physically mixing two or more substances in any ratio.
Examples: Tap water, milk, air and lemonade
Properties of Mixtures:
- Mixtures do not have fixed composition and properties.
- Their components can be separated easily through physical methods.
Homogeneous Mixtures:
A homogeneous mixture has a uniform composition throughout.
Examples: Air, tap water, lemonade, steel
Heterogeneous Mixtures:
A heterogeneous mixture does not have uniform composition throughout.
Examples: Rock, soil, milk, blood
Q2. What are the differences between an element and a compound?
Elements
Compounds
Elements contain only one type of atoms.
Compounds contain atoms of two or more different elements.
They are the purest form of matter.
Compounds are substances but not the purest form of matter.
They cannot be broken down into simpler substances by chemical means.
They can be chemically broken down into their constituent elements.
They can be classified into four categories: metals, non-metals, metalloids and noble gases.
They can also be classified into organic compounds and inorganic compounds.
They can exist in the form of atoms, molecules, ions and isotopes.
They are also divided into several categories: ionic compounds, molecular compounds, intermetallic compounds, and coordination complexes.
H2, O2, N2
H2O, CO2, NaCl
Q3. How would you differentiate between a substances and a mixtures?
Substances
Mixtures
Substances are the pure form of matter.
Mixtures are the impure form of matter.
Substances can be represented by a symbol or a chemical formula.
Mixtures do not have symbols or chemical formulae.
Substances have a uniform composition thought the sample.
Mixtures do not have a uniform composition throughout the sample.
Components of a substance can only be separated through a chemical reaction.
Components of a mixture can be separated through physical methods.
H2, CO2, H2O and NaCl
Lemonade, air, soil and milk
Q4. What are the differences between a homogeneous mixtures and a heterogeneous mixtures?
Homogeneous Mixture
Heterogeneous Mixture
It is a mixture with a uniform concentration.
It is a mixture with a non-uniform concentration.
Components cannot be seen with the naked eye
Components can be seen with the naked eye.
Components cannot be separated easily.
Components can be separated easily.
Air, tap water and lemonade
Soil, rock and pizza