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The Amazing World of Solutes, Solvents, and Solutions - Why Do Objects Float or Sink in Water?

Grade 8CBSE

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

🔑Concepts

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A Solution is a homogeneous mixture of two or more substances. The component present in larger quantity is the Solvent, and the component dissolved in it is the Solute.

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Concentration of a solution is the amount of solute present in a given amount (mass or volume) of solution. It can be expressed as a mass percentage: Mass of soluteMass of solution×100\frac{\text{Mass of solute}}{\text{Mass of solution}} \times 100.

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Density is defined as mass per unit volume of a substance. It determines whether an object will sink or float. The formula is ρ=mV\rho = \frac{m}{V}.

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Buoyancy (or Upthrust) is the upward force exerted by a fluid on an object immersed in it. If the buoyant force is greater than the weight of the object, it floats.

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Archimedes' Principle states that when a body is immersed fully or partially in a fluid, it experiences an upward force that is equal to the weight of the fluid displaced by it.

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Adding a solute (like salt) to a solvent (like water) increases the density of the resulting solution. This is why it is easier to float in saltwater (like the Dead Sea) than in freshwater, as the density of saltwater is higher than the density of freshwater (ρsaltwater>ρfreshwater\rho_{saltwater} > \rho_{freshwater}).

📐Formulae

Density(ρ)=Mass(m)Volume(V)\text{Density} (\rho) = \frac{\text{Mass} (m)}{\text{Volume} (V)}

\text{Mass % of Solution} = \frac{\text{Mass of solute}}{\text{Mass of solute} + \text{Mass of solvent}} \times 100

Relative Density=Density of substanceDensity of water\text{Relative Density} = \frac{\text{Density of substance}}{\text{Density of water}}

Condition for Floating:ρobject<ρfluid\text{Condition for Floating:} \quad \rho_{\text{object}} < \rho_{\text{fluid}}

Condition for Sinking:ρobject>ρfluid\text{Condition for Sinking:} \quad \rho_{\text{object}} > \rho_{\text{fluid}}

💡Examples

Problem 1:

A solution contains 50 g50 \text{ g} of common salt in 450 g450 \text{ g} of water. Calculate the concentration in terms of mass by mass percentage of the solution.

Solution:

Mass of Solute=50 gMass of Solvent=450 gMass of Solution=500 g\begin{array}{r} \text{Mass of Solute} = 50 \text{ g} \\ \text{Mass of Solvent} = 450 \text{ g} \\ \hline \text{Mass of Solution} = 500 \text{ g} \end{array} Concentration =50500×100=10%= \frac{50}{500} \times 100 = 10\%

Explanation:

To find the mass of the solution, we add the mass of the solute and the solvent. Then, we apply the mass percentage formula.

Problem 2:

An object has a mass of 300 g300 \text{ g} and a volume of 250 cm3250 \text{ cm}^3. Given that the density of water is 1 g/cm31 \text{ g/cm}^3, determine if the object will sink or float.

Solution:

ρ=mV=300 g250 cm3=1.2 g/cm3\rho = \frac{m}{V} = \frac{300 \text{ g}}{250 \text{ cm}^3} = 1.2 \text{ g/cm}^3 Since 1.2 g/cm3>1 g/cm31.2 \text{ g/cm}^3 > 1 \text{ g/cm}^3, the object will sink.

Explanation:

We calculate the density of the object. Since the density of the object (1.2 g/cm31.2 \text{ g/cm}^3) is greater than the density of water (1 g/cm31 \text{ g/cm}^3), it will sink due to the force of gravity being stronger than the buoyant force.

Why Do Objects Float or Sink in Water? Class 8 Notes & Examples