experimental techniques ch2

Question 1

apparatus and unit of measurement for time

Answer 1

seconds (s) minutes (min)

stopwatch

Question 2

apparatus and unit of measurement for temperature

Answer 2

degrees Celsius °C

thermometer

Question 3

apparatus and unit of measurement for mass

Answer 3

grams (g)
kilograms (kg)
1000g=1kg
balance

Question 4

apparatus and unit of measurement for volume of liquid

Answer 4

cm^3 or ml (=)
dm^3 or l (=)

1000cm^3 = 1dm^3 = 1 l

measuring cylinder -> burette-> pipette
(increasing order of accuracy)

Question 5

mixture

Answer 5

two or more substances not chemically combined

Question 6

solution

Answer 6

a mixture you make by dissolving a substance in a solvent

A homogeneous mixture where the substances are fully dissolved and evenly dispersed

Question 7

solute

Answer 7

the substance you dissolve

Question 8

solvent

Answer 8

the liquid in which a solute is dissolved, to make a solution

Question 9

volatile

describe a volatile liquid

Answer 9

1) a substance that evaporates readily at room temperatures

forces between its particles are weak so volatile liquids have low boiling points

Question 10

What is paper chromatography used for

Answer 10

To find out how many substances are present in a mixture
To check on the purity of a substance; if impurities are present, they separate out
To identify the substances in a mixture, by measuring how far they travel

Question 11

How do you separation occur in paper chromatography

Answer 11

There is a stationary phase (chromatography paper) and a mobile phase (solvent)
Substances have different solubilities in the mobile phase so will travel at different rates causing separation. More soluble substances travel further.
They separate because a different solubility in the solvent, and attraction to the paper they travel over.
The most soluble a substance is in the solvent, the further it will travel. The more attracted it is to the paper, the less far will travel.

Question 12

What is an Rf value

Answer 12

It is the ratio between this distance traveled by the dissolve substance and the distance traveled by the solvent

Rf=Distance travelled by substance/distance travelled by solvent

Question 13

A paper chromatography, what affects the Rf values of a substance?

Answer 13

The solvent. Repeating the experiment with a different solvent will change the Rf value

Question 14

How many spots will be observed and a chromatogram of a pure substance

Answer 14

1

Question 15

Locating agent

Answer 15

A substance used to analyse and identify colourless substances

Question 16

What is a pure substance

Answer 16

A single element or compound not mixed with any other substance

Question 17

How can a pure substance be identified using melting or boiling points

Answer 17

Pure: Melts and boils at temperatures are unique to it, has sharp melting and boiling points

Impure: Melt/boils over a range of temperatures, not sharply.
Melts at a lower temperature than a pure substance
Boils at a higher temperature than a pure substance

Question 18

Why is the purity of substances important in every day life

Answer 18

Impurities in drugs and vaccines may cause dangerous side-effects
Impurities in food and drink may cause health problems of ingested

Question 19

Crystallisation

Answer 19

The process in which crystals form, as a saturated solution cools

Question 20

Distillation

Answer 20

A separation method that involves heating a substance for evaporation and then obtaining it through condensation. The works because different liquids have different boiling points.

Question 21

Separate an insoluble solid from a liquid

Answer 21

Filtration

The insoluble substance will not pass through the filter paper and will remain there as the residue. You can rinse it with distilled water.
The soluble substance, the filtrate, passes through the paper.

Question 22

Separating a solvent from a solution

Separating a liquid from a solution of a solid in a liquid

Answer 22

Simple distillation e.g. producing pure water, distilled water, from seawater

The solvent boils off as a gas at it’s boiling point. The gas condenses back into a liquid in the cool condenser, and drips into the beaker. The solute remains in the flask.

Question 23

Separating a liquid from a mixture of miscible liquids

Answer 23

Fractional distillation e.g. removing ethanol from a mixture of ethanol and water

Supposing A has a lower boiling point:

• Gas A passes into the condenser first, at A’ boiling point. It condenses there.
• Collect liquid A. Stop collecting when the temperature on the thermometer rises
• Keel heating. Gas B passes into the condenser, at B’s boiling point. Collect liquid B in a separate beaker.

This method works because different liquids have different boiling points. When the mixture is heated, ethanol and some water vapour rises. They rise up the fractionating column, which is cooler the higher you go. When the glass beads reach 78°C, the water vapour condenses and drips back into the flask. The ethanol vapour continues to go up and through the condenser (as it’s boiling point is lower than water: 100°C.) The ethanol condenses due to the lower temperature from the cold water, and drips into the beaker. You now have distilled ethanol.

Question 24

Separating a mixture of immiscible liquids

Answer 24

Separating funnel. E.g. separating a mixture of water and hexane
Immiscible liquids settle into two layers: the denser one at the bottom (ie hexane)
When the taps opened, the water runs to the beaker. With the bottom layer is drained, the remaining layer can be separately drained.

Question 25

Separating a mixture of similar compounds in a solution

Substances from a mixture

Answer 25

Chromatography e.g. separating the dyes present in sample of ink

The mixture is dissolved in a suitable solvent. The solutions allowed to travel across the paper.
The substances in a solution travel at different speeds and separate.
They separate because of the different solubility in the solvent and attraction to paper they travel over.
The more soluble, the higher travels
The more attracted to the paper it is the lower it travels

Question 26

Saturated solution

Answer 26

No more of the solute will dissolve in it, at that temperature

Question 27

Separating a solute which sublimes from a solid which does not sublime
Solute from a solution

Answer 27

Crystallisation
Crystallisation works because a solvent can dissolve less and less solute as it’s temperature falls
Heat the solution until it is saturated then let it cool. Filter off the salts that form. Wash them with a little distilled water but be careful since they are soluble.