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Chemistry - Experimental Design

Grade 9IGCSE

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

🔑Concepts

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Experimental variables: The Independent Variable is the factor changed by the investigator (e.g., temperature). The Dependent Variable is the factor being measured (e.g., volume of gas produced). Controlled Variables are factors kept constant to ensure a fair test.

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Apparatus selection: Use a Burette (0.1 cm30.1 \text{ cm}^3 precision) or Pipette (fixed volume) for accurate liquid measurements. Use a Gas Syringe to measure the volume of a gas produced in a reaction.

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Gas collection methods: Displacement of water is used for gases insoluble in water (e.g., H2H_2, O2O_2). Downward delivery is used for gases denser than air (e.g., CO2CO_2, Cl2Cl_2). Upward delivery is used for gases less dense than air (e.g., NH3NH_3, H2H_2).

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Purity and Chromatography: A pure substance has a sharp, fixed melting and boiling point. In paper chromatography, the RfR_f value is a ratio used to identify substances.

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Methods of separation: Filtration removes insoluble solids from liquids. Crystallization obtains a solute from a solution. Simple Distillation separates a solvent from a solution. Fractional Distillation separates liquids with different boiling points.

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Safety and Reliability: Repeat experiments at least three times to identify anomalies and calculate a mean. Use safety goggles and lab coats to minimize risk.

📐Formulae

Rf=distance moved by the substancedistance moved by the solvent frontR_f = \frac{\text{distance moved by the substance}}{\text{distance moved by the solvent front}}

Rate of Reaction=Change in Amount of Reactant or ProductTime Taken\text{Rate of Reaction} = \frac{\text{Change in Amount of Reactant or Product}}{\text{Time Taken}}

Percentage Purity=Mass of Pure SubstanceTotal Mass of Sample×100%\text{Percentage Purity} = \frac{\text{Mass of Pure Substance}}{\text{Total Mass of Sample}} \times 100\%

Average Rate=ΔVΔt or ΔmΔt\text{Average Rate} = \frac{\Delta V}{\Delta t} \text{ or } \frac{\Delta m}{\Delta t}

💡Examples

Problem 1:

A student investigates the rate of reaction between magnesium ribbon and dilute hydrochloric acid. 50 cm350 \text{ cm}^3 of 1.0 mol/dm31.0 \text{ mol/dm}^3 HClHCl is used. Suggest the independent, dependent, and three controlled variables.

Solution:

Independent variable: Surface area of Magnesium or Concentration of HClHCl. Dependent variable: Volume of H2H_2 gas produced per unit time. Controlled variables: Temperature of the acid, Volume of the acid, Mass of Magnesium used.

Explanation:

To make the experiment a fair test, only one factor should be changed at a time (independent), while the result is measured (dependent) and all other factors are kept constant (controlled).

Problem 2:

In a chromatography experiment, the solvent front travels 10.0 cm10.0 \text{ cm} from the baseline. A red dye spot travels 7.5 cm7.5 \text{ cm} from the baseline. Calculate the RfR_f value of the red dye.

Solution:

Rf=7.5 cm10.0 cm=0.75R_f = \frac{7.5 \text{ cm}}{10.0 \text{ cm}} = 0.75

Explanation:

The RfR_f value is a dimensionless ratio. It is calculated by dividing the distance traveled by the component by the distance traveled by the solvent front. It must always be less than 11.

Problem 3:

A student wants to separate a mixture of ethanol (boiling point 78∘C78 ^\circ C) and water (boiling point 100∘C100 ^\circ C). Describe the best method and the necessary apparatus.

Solution:

Method: Fractional Distillation. Apparatus: Round-bottom flask, Fractionating column, Thermometer, Liebig condenser, Receiver flask.

Explanation:

Fractional distillation is used because the liquids are miscible and have different boiling points. The fractionating column allows for better separation by providing a surface for repeated evaporation and condensation.