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Nature of Matter: Elements, Compounds, and Mixtures - What Are the Types of Pure Substances?

Grade 8CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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A pure substance consists of a single type of particles (atoms or molecules) and has a fixed chemical composition throughout.

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Elements are the most basic form of pure substances that cannot be broken down into simpler substances by chemical means. They are categorized into Metals, Non-metals, and Metalloids.

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Metals (e.g., FeFe, AuAu, CuCu) are typically malleable, ductile, and good conductors of heat and electricity.

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Non-metals (e.g., HH, OO, CC) are usually poor conductors and are not malleable or ductile.

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Metalloids (e.g., SiSi, GeGe) exhibit properties intermediate between metals and non-metals.

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Compounds are substances composed of two or more elements chemically combined in a fixed proportion by mass. For example, pure Water (H2OH_2O) always contains Hydrogen and Oxygen.

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The properties of a compound are entirely different from the properties of its constituent elements. For instance, H2H_2 is a gas and O2O_2 supports combustion, but H2OH_2O is a liquid used to extinguish fires.

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In a compound, the constituents can only be separated by chemical or electrochemical reactions, not by physical methods.

📐Formulae

Mass Ratio of Water (H2O)=1:8 (Hydrogen : Oxygen)\text{Mass Ratio of Water } (H_2O) = 1:8 \text{ (Hydrogen : Oxygen)}

Mass Ratio of Ammonia (NH3)=14:3 (Nitrogen : Hydrogen)\text{Mass Ratio of Ammonia } (NH_3) = 14:3 \text{ (Nitrogen : Hydrogen)}

Mass Ratio of Carbon Dioxide (CO2)=3:8 (Carbon : Oxygen)\text{Mass Ratio of Carbon Dioxide } (CO_2) = 3:8 \text{ (Carbon : Oxygen)}

Chemical Formula Concept: AxBy\text{Chemical Formula Concept: } A_x B_y

💡Examples

Problem 1:

In a sample of Water (H2OH_2O), the ratio of the mass of Hydrogen to the mass of Oxygen is always 1:81:8. If we have 9g9g of Hydrogen, how many grams of Oxygen are required to form Water?

Solution:

9×8=72g9 \times 8 = 72g

Explanation:

Since the ratio of H:OH:O is 1:81:8, for every 1g1g of Hydrogen, we need 8g8g of Oxygen. Therefore, for 9g9g of Hydrogen, the amount of Oxygen required is calculated as: 8×972\begin{array}{r} 8 \\ \times 9 \\ \hline 72 \end{array}

Problem 2:

Identify which of the following are pure substances: (a) Iron, (b) Air, (c) Water, (d) Mercury.

Solution:

Pure substances: (a) Iron, (c) Water, (d) Mercury.

Explanation:

Iron (FeFe) and Mercury (HgHg) are elements. Water (H2OH_2O) is a compound. Air is a mixture of various gases like N2N_2, O2O_2, and CO2CO_2, so it is not a pure substance.

Problem 3:

Calculate the total mass of a compound formed if 12g12g of Carbon reacts completely with 32g32g of Oxygen to form Carbon Dioxide (CO2CO_2).

Solution:

12+32=44g12 + 32 = 44g

Explanation:

According to the Law of Conservation of Mass, the mass of the product in a chemical reaction is equal to the sum of the masses of the reactants: 12+3244\begin{array}{r} 12 \\ + 32 \\ \hline 44 \end{array} So, 44g44g of CO2CO_2 is formed.