Lab Techniques and Procedures

Physical and Chemical Changes

Understanding the distinction between physical and chemical changes is fundamental in chemistry. Recognizing the evidence for each type of change allows chemists to classify reactions and processes accurately.

• Physical Change: A change that alters the form or appearance of a substance but does not change its chemical identity. Examples include melting, freezing, dissolving, and breaking.

• Chemical Change: A process in which one or more substances are converted into new substances with different properties. This involves making or breaking chemical bonds.

• Evidence of Chemical Change:

  • Formation of a gas (bubbling, fizzing)

  • Formation of a precipitate (solid from solution)

  • Color change

  • Temperature change (exothermic or endothermic reaction)

  • Emission of light or odor

• Evidence of Physical Change:

  • Change in state (solid, liquid, gas)

  • Change in size or shape

  • Dissolving (without chemical reaction)

• Example: Dissolving sugar in water is a physical change; burning wood is a chemical change.

Lab Techniques and Procedures

Density Determination

Density is a fundamental property of matter, defined as mass per unit volume. It is used to identify substances and assess purity.

• Formula for Density: where is density, is mass, and is volume.

• Determining Density of a Solid:

  • Find the mass of the solid using a balance.

  • Determine the volume by water displacement: Submerge the solid in a graduated cylinder containing water and record the change in volume.

• Determining Density of a Liquid:

  • Weigh an empty container (such as a graduated cylinder).

  • Add the liquid and weigh again.

  • Mass of liquid = (mass of container + liquid) - (mass of empty container).

  • Measure the volume directly using the graduated cylinder.

• Example: If a solid has a mass of 10.0 g and displaces 4.0 mL of water, its density is .

Lab Techniques and Procedures

Percent Composition of a Hydrated Salt

Hydrated salts contain water molecules within their crystal structure. Determining the percent composition of water helps identify unknown hydrates.

• Percent Composition Formula:

• Procedure:

  • Weigh the hydrated salt before heating.

  • Heat to drive off water; weigh the anhydrous salt after heating.

  • Mass of water lost = (mass before heating) - (mass after heating).

  • Calculate percent water using the formula above.

• Identifying Unknown Hydrate: Compare the calculated percent water to known values for possible hydrates to identify the unknown.

• Example: If a 5.00 g sample loses 1.80 g upon heating, .

Lab Techniques and Procedures

Empirical Formula Determination

The empirical formula represents the simplest whole-number ratio of elements in a compound. For hydrated salts, it includes both the salt and water molecules.

• Steps to Determine Empirical Formula:

  1. Find the mass of each component (e.g., water lost, metal, chloride).

  2. Convert masses to moles using molar masses.

  3. Divide each mole value by the smallest number of moles to get the simplest ratio.

  4. Write the empirical formula using these ratios.

• Example: If a sample contains 0.050 mol Mg, 0.100 mol Cl, and 0.100 mol H2O, the empirical formula is MgCl2·2H2O.

Lab Techniques and Procedures

Standardization of Sodium Hydroxide (NaOH)

Standardization is the process of determining the exact concentration (molarity) of a solution. NaOH is commonly standardized using a primary standard such as potassium hydrogen phthalate (KHP).

• Key Calculations:

  • Moles of KHP:

  • Moles of HCl:

  • Moles of NaOH: Determined from stoichiometry of the reaction with KHP or HCl.

  • Molarity of NaOH:

• Example Reaction: KHP + NaOH → KNaP + H2O

• Example Calculation: If 0.500 g KHP (molar mass = 204.22 g/mol) is titrated with 25.00 mL NaOH, moles KHP = mol. If the reaction is 1:1, moles NaOH = 0.00245 mol, so M.

Appendix: Useful Mathematical Operations in Chemistry

• Mass by Difference: Used to find the mass of a sample by subtracting the mass of the container from the total mass (container + sample).

• Volume by Displacement: Used to find the volume of irregular solids by measuring the change in liquid volume after submersion.

• Stoichiometry: Use balanced chemical equations to relate moles of reactants and products.