Topic 2 : Experimental techniques

Question 1

How do you choose between a burette, pipette, graduated pipette, and measuring cylinder?

Answer 1

Choose based on accuracy vs control vs speed:

Volumetric pipette: one fixed, very accurate volume

Burette: accurate and controlled delivery (unknown volume)

Graduated pipette: variable volume, moderate accuracy

Measuring cylinder: fast but inaccurate

Question 2

Why is a burette used in titrations?

Answer 2

Because the required volume is unknown and must be added slowly and precisely until an endpoint is reached.

Question 3

When is a volumetric pipette used?

Answer 3

When an exact, known volume (e.g. 25.0 cm³) must be transferred with maximum accuracy.

Question 4

Why use a graduated pipette instead of a volumetric pipette?

Answer 4

When different volumes are needed with reasonable accuracy, not just one fixed volume.

Question 5

Why is a measuring cylinder unsuitable for precise chemistry?

Answer 5

Large reading uncertainty → significant percentage error in volume.

Question 6

What is a thistle funnel used for?

Answer 6

Safely adding liquids into a reaction vessel, often through a bung, without splashing or gas loss.

Question 7

What problem does a tap funnel solve?

Answer 7

Separates immiscible liquids based on density differences, with controlled release.

Question 8

Why is a crucible used instead of glassware?

Answer 8

It withstands very high temperatures and is chemically inert, allowing strong heating without reacting.

Question 9

Why is a mortar and pestle used in experiments?

Answer 9

To grind solids into a fine powder, increasing surface area so reactions, dissolving, or heating occur more efficiently and uniformly.

Question 10

What is the difference between a trough and a beaker?

Answer 10

Trough: holds water for collecting gases (usually by water displacement)

Beaker: general-purpose container for holding, mixing, or heating liquids

A trough is wide and shallow, allowing gas jars to be inverted and filled without air entering.

Question 11

When do you use a gas jar instead of a gas syringe, and why?

Answer 11

Gas jar: used to collect gases (often by water displacement or downward/upward delivery) when exact volume is not critical

Gas syringe: used to measure the volume of gas accurately as it is produced

Question 12

Why is a gas jar suitable for collecting gases?

Answer 12

It allows gases to be collected based on density or water displacement, without needing precise volume measurements.

Question 13

What is the difference between a concentrated and a dilute solution?

Answer 13

Concentrated: large amount of solute per unit volume of solution

Dilute: small amount of solute per unit volume of solution

Question 14

What is a saturated solution?

Answer 14

A solution that contains the maximum amount of solute that can dissolve at a given temperature.

Question 15

What is an unsaturated solution?

Answer 15

A solution that can still dissolve more solute at that temperature.

Question 16

What does solubility mean?

Answer 16

The maximum mass of solute that can dissolve in 100 g of solvent at a specified temperature.

Question 17

What factors increase the solubility of most solids in liquids?

Answer 17

Higher temperature (usually)

Suitable solvent (“like dissolves like”)

Question 18

What increases the rate of dissolving ? and how ?

Answer 18

Stirring : Removes the saturated layer of solution around the solid. I e, the part that already dissolved.

Crushing : Increases surface area. Therefore : More solute particles are exposed to the solvent at the same time
More collisions between solute and solvent particles per second

Question 19

Why does the solubility of gases in liquids usually decrease when temperature increases?

Answer 19

Heating gives gas particles more kinetic energy, making it easier for them to escape the liquid, so fewer remain dissolved.

Question 20

How does evaporation separate a dissolved solid from a liquid?

Answer 20

The solution is boiled till dryness to evaporate the solvent, the powder solid (solute) is left behind

Question 21

How does simple distillation separate a solvent from a solution?

Answer 21

The solution is heated to the solvent’s boiling point, and so it turns to vapor, rises into the condenser, then it is condensed back to a pure liquid and is collected, leaving the solid behind.

Question 22

How does crystallization separate a solid from a solution?

Answer 22

The solution is heated till crystallization point, the hot saturated solution is cooled so the solid forms crystals as solubility decreases. then the solid

Question 23

Why do crystals form as the hot saturated solution cools down ?

Answer 23

At high temperature, more solute can dissolve. The solution is saturated at that temperature, not at room temperature. As temperature falls, so does solubility. The solution now contains more solute than it can hold. Attractions between solute particles become energetically favorable again, excess solute particles come together to form an ordered lattice and crystals grow.

Question 24

What is the difference between evaporation and crystallization ?

Answer 24

Evaporation removes solvent; crystallization removes excess solute.

Question 25

Why filtering is necessary after crystallization ?

Answer 25

After crystallization, you have: Solid crystals Remaining solution (mother liquor) You filter to: Separate solid from liquid Stop crystals from redissolving Collect the purified solid

Question 26

What is decantation ? when do we use it ?

Answer 26

Carefully pouring off a liquid from a solid that has settled at the bottom. When to use it : Solid particles are large High precision is not required You just want a quick separation

Question 27

What is filtration ? when do we use it ?

Answer 27

Passing a mixture through filter paper to trap insoluble solids. When to use it : Solid is insoluble You want a clean separation Particle size is sufficient to be trapped

Question 28

What is centrifugation ? when do we use it ?

Answer 28

Spinning a mixture at high speed to force solids to the bottom. When to use it : Particles are very fine Decantation would take too long Filtration is difficult or ineffective

Question 29

How do we separate liquids do not mix easily, i,e : immiscible ? and how would that work ?

Answer 29

Using a separating funnel (tap funnel). Liquids form separate layers due to different densities The denser liquid settles at the bottom Tap allows controlled release of the lower layer

Question 30

How do we separate two miscible liquids that are uniformally mixed ?

Answer 30

Simple distillation if there are large differences in their boiling points. Fractional distillation if they have similar boiling points.

Question 31

How does fractional distillation work ?

Answer 31

Both liquids start to evaporate, Vapor contains more of the lower-boiling liquid, but still some higher-boiling one. As the vapor rises, temperature decreases vapor meets cooler glass beads. The higher boiling point liquid condenses first, while the lower boiling point liquid remains gaseous. This is because the higher boiling point liquid requires more heat to stay gaseous, which it loses to the glass beads. Some of the low boiling point vapor is re-condensed, but then it evaporates again and the same process happens. The low boiling point vapor rises into the condenser, where it liquefies and is collected.

Question 32

How do we separate components of air ?

Answer 32

Through fractional distillation. Air is liquefied by applying high pressure and low temperature, then allowed to warm up. The components are separated according to their boiling point as : Nitrogen boils first at -196 C, Argon boils second at -186 C, and oxygen boils third at -183 C.

Question 33

How do we separate solid/solid mixture ?

Answer 33

By magnetic if an element with magnetic properties is involved like iron, cobalt and nickel. Or by solvent extraction.

Question 34

How is solvent extraction of a solid/solid mixture done.

Answer 34

The mixture is crushed to increase rate of dissolving, then a solvent which only one of the solids is soluble in is added, then it is stirred with gentle heating to increase rate of dissolving further more. After, we filter the solution, and heat the acquired filtrate till crystallization point, and leave to cool to get crystals, or alternatively evaporate the filtrate till dryness to obtain a powder.

Question 35

What solvent is appropriate for separating sugar and salt ? and why ? and what is a precaution we have to take when separating them with said solvent ?

Answer 35

Ethanol. Sugar dissolves in ethanol, but salt does not. Ethanol is flammable, so the process should be done using a water bath.

Question 36

What is the retention factor formula ? what value range should it be in ?

Answer 36

Distance moved by substance/distance moved by solvent. Must be smaller than <1.