2. Experimental Techniques Flashcards
What can time be measured with?
Time can be measured using a stopwatch or stop-clock which are usually accurate to one or two decimal places
What unit is used for time?
The units of time normally used are seconds or minutes although other units may be used for extremely slow reactions (e.g. rusting)
What can temperature be measured with?
- Temperature is measured with a thermometer which can normally give readings to the nearest degree Celsius
- Digital thermometers are available which are more precise than traditional thermometers
How do traditional thermometers work?
Traditional thermometers rely upon the uniform expansion and contraction of a liquid substance with temperature;
Is a traditional or a digital thermometer more accurate?
Traditional thermometers tend to be more accurate than digital thermometers which often need to be re-calibrated
What unit is used for temperature?
degrees Celsius (ºC)
What can mass be measured with?
Mass is measured using a digital balance which normally give readings to two decimal places. These must be set to zero before use
What unit is used for mass?
The standard unit of mass is kilograms (kg) but in chemistry grams (g) are most often used
What can the volume of a liquid be measured with?
The volume of a liquid can be determined using several types of apparatus, depending on the level of accuracy needed
What apparatus can be used for approximate volumes?
For approximate volumes where accuracy isn’t an important factor, measuring cylinders are used. These are graduated (have a scale so can be used to measure)
What is the most accurate way of measuring a fixed volume of a liquid?
Using a pipette
What is the most accurate way of measuring a variable volume of a liquid?
Burette
How can a the volume of a gas be measured?
The volume of a gas sometimes needs to be measured and is done by collecting it in a graduated measuring apparatus
What apparatus is used in measuring the volume of a gas?
A gas syringe
What determines whether the graduated cylinder is inverted or can be used upright?
- A graduated cylinder inverted in water may also be used, provided the gas isn’t water-soluble
- If the gas happens to be heavier than air and is coloured, the cylinder can be used upright
When is paper chromatography used?
This technique is used to separate substances that have different solubilities in a given solvent (e.g. different coloured inks that have been mixed to make black ink)
How is the paper set up?
paper chromatography
A pencil line is drawn on chromatography paper and spots of the sample are placed on it.
Why is pencil, not pen used to draw a line? (paper chromatography)
Pencil is used for this as ink would run into the chromatogram along with the samples
What is the paper lowered into?
paper chromatography
The paper is then lowered into the solvent container
What is important to remember when lowering the paper into the solvent?
(paper chromatography)
making sure that the pencil line sits above the level of the solvent so the samples don’t wash into the solvent container
How does the solvent travel up the paper?
paper chromatography
The solvent travels up the paper by capillary action, taking some of the coloured substances with it
What causes the different substances to travel at different rates and what does this from?
(paper chromatography)
Different substances have different solubilities so will travel at different rates, causing the substances to spread apart.
What correlation is there between the solubility of a substance and the distance it travels?
(paper chromatography)
Those substances with higher solubility will travel further than the others
What does paper chromatography show?
This will show the different components of the ink / dye
If substances are the same, will they produce the same chromatograms?
YES
If two or more substances are the same, they will produce identical chromatograms
What will happen to a substance during paper chromatography if it is a mixture?
If the substance is a mixture, it will separate on the paper to show all the different components as separate spots
How can you tell the difference between a pure and impure substance using paper chromatography?
An impure substance will show up with more than one spot, a pure substance should only show up with one spot
What is the difference between the boiling point of pure substances and mixtures?
- Pure substances melt and boil at specific and sharp temperatures
- Mixtures have a range of melting and boiling points as they consist of different substances that melt or boil at different temperatures
How can an unknown pure substance be identified?
An unknown pure substance can be identified by experimentally determining its m.p and b.p and comparing to data tables
Why do mixtures melt over a range of temperatures?
Mixtures melt over a range of temperatures as they contain two or more substances
What does a pure substance consist of?
A pure substance consists of only one substance and contains nothing else.
Where and why is it important to have pure substances?
To have a pure substance for food and drugs is very important as impurities could be dangerous even in small amounts
What is routinely assessed during the production of food and drugs?
Melting and boiling point analysis is routinely used to assess the purity of food and drugs
What are Retention Factor (Rf) values used to identify?
These values are used to identify the components of mixtures
Is the Rf value of a particular compound always the same?
Yes
What does calculating Rf values allow scientists to do?
Calculating the Rf value allows chemists to identify unknown substances because it can be compared with Rf values of known substances under the same conditions
How do you calculate the Rf value?
Retention factor = distance moved by compound ÷ distance moved by solvent
Does the Rf value have units?
NO
- The Rf value is a ratio and therefore has no units
When are locating agents used and what do they do?
paper chromatography
Locating agents are substances which react with the sample and produce a coloured product which is then visible
What substances are hard to see using paper chromatography?
What is used to solve this problem?
invisible samples such as proteins
A locating agent
When is the locating agent applied to the chromatogram?
The chromatogram is treated with the agent after the chromatography run has been carried out, making the sample runs visible to the naked eye
What does the choice of the method of separation depend on?
the nature of the substances being separated
What is one similarity between all separation methods?
All methods rely on there being a difference of some sort, usually in a physical property such as b.p., between the substances being separated.
To separate mixtures of solids, what differences can be used to carry this process out?
Differences in density, magnetic properties, sublimation and solubility can be used
If using a difference in solubility to separate a mixture of solids, what must be taken into account?
For a difference in solubility, a suitable solvent must be chosen to ensure the desired substance only dissolves in it and not other substances or impurities
What are immiscible liquids?
two or more liquids which do not mix together
How can immiscible liquids be separated?
Immiscible liquids can be separated using a separating funnel or by decanting (pouring carefully)
When is filtration used?
Used to separate an undissolved solid from a mixture of the solid and a liquid / solution ( e.g. sand from a mixture of sand and water
What is placed in the funnel?
(filtration)
1.
Filter paper is placed in a filter funnel above another beaker
- What is poured into the funnel?
filtration
Mixture of insoluble solid and liquid is poured into the filter funnel
- How does the filter paper allow filtration to be carried out?
Filter paper will only allow small liquid particles to pass through as the filtrate
- What is left behind in the filter paper and funnel?
filtration
Solid particles are too large to pass through the filter paper so will stay behind as a residue
When is crystallisation used?
Used to separate a dissolved solid from a solution, when the solid is much more soluble in hot solvent than in cold (e.g. copper sulphate from a solution of copper (II) sulphate in water)
- What is the first step of crystallisation?
The solution is heated, allowing the solvent to evaporate to leave a saturated solution behind
- How can you test the solution to see if it’s saturated?
crystallisation
Test if the solution is saturated by dipping a clean, dry, cold glass rod into the solution. If the solution is saturated, crystals will form on the glass rod
- What is done with the saturated solution?
crystallisation
The saturated solution is allowed to cool slowly and solids will come out of the solution as the solubility decreases, and crystals will grow
- How are the crystals collected?
crystallisation
Crystals are collected by filtering the solution
- What is done with the collected crystals?
crystallisation
They are then washed with cold, distilled water to remove impurities and allowed to dry
When is simple distillation used?
Used to separate a liquid and soluble solid from a solution (e.g. water from a solution of saltwater) or a pure liquid from a mixture of liquids
- What is the first step in simple distillation?
The solution is heated and pure water evaporates producing a vapour which rises through the neck of the round-bottomed flask
- What happens to the vapour?
fractional distillation
The vapour passes through the condenser, where it cools and condenses, turning into pure liquid H2O which is collected in a beaker
- What happens after all the water is evaporated from the solution?
(crystallisation)
After all the water is evaporated from the solution, only the solid solute will be left behind
When is fractional distillation used?
Used to separate two or more liquids that are miscible (can mix) with one another (e.g. ethanol and water from a mixture of the two)
- To what point is the solution heated?
F distillation
The solution is heated to the temperature of the substance with the lowest boiling point
- Which substance will rise first and what will happen to it?
(F distillation)
This substance will rise and evaporate first, and vapours will pass through a condenser, where they cool and condense, turning into a liquid that will be collected in a beaker
- Once this substance is fully evaporated, what is left behind?
(F distillation)
All of the substance is evaporated and collected, leaving behind the other components(s) of the mixture
