States of matter and mixture

Question 1

Solids

Answer 1

A solid has a fixed shape because of the strong forces between its particles.
Solids cannot be compressed because their particles are already very close together and cannot flow.
The particles in a solid are arranged in an ordered pattern.
The particles in a solid move around a fixed point.
Solids are the densest state of matter.
The particles are packed tightly together.

Question 2

Liquids

Answer 2

Liquids cannot be compressed because their particles are already very close together.
In a liquid, the particles are in contact with one another, but they can still move. This allows a liquid to flow and take the shape of its container.
Particles in a liquid are arranged in a disordered pattern.
Liquids are less dense than solids but denser than gases.
The particles in liquids can move around each other.

Question 3

Gases

Answer 3

Gases can be compressed because their particles are very far apart.
When water evaporates to become steam (gas), its volume increases by 1000x.
The particles in a gas are free to move in any direction. Because of this, a gas can flow, has no fixed shape and completely fills its container.
The particles in a gas move randomly and are not organised in any way.
Gases are the least dense state of matter.
The particles are free to move with negligible forces between particles.

Question 4

Interconversions

Answer 4

The amount of energy needed to change state from solid to liquid and from liquid to gas depends on the strength of the forces between the particles.
The stronger the forces of attraction, the more energy that is needed to overcome them for a state change to occur.
Therefore, the stronger the forces between the particles the higher the melting point and boiling point of the substance.
When matter changes from one state to another due to changes in temperature or pressure, the change is called an interconversion of state.
It is a physical change involving changes in the forces between the particles of the substances, the particles themselves remain the same, as do the chemical properties of the substance.
Physical changes are relatively easy to reverse as no new substance is formed during interconversions of state.

Question 5

Melting

Answer 5

Melting is when a solid changes into a liquid.
The process requires heat energy which transforms into kinetic energy, allowing the particles to move.
It occurs at a specific temperature known as the melting point which is unique to each pure solid .

Question 6

Boiling

Answer 6

Boiling is when a liquid changes into a gas.
This requires heat which causes bubbles of gas to form below the surface of a liquid, allowing for liquid particles to escape from the surface and from within the liquid.
It occurs at a specific temperature known as the boiling point which is unique to each pure liquid .

Question 7

Freezing

Answer 7

Freezing is when a liquid changes into a solid.
This is the reverse of melting and occurs at exactly the same temperature as melting, hence the melting point and freezing point of a pure substance are the same.
Water for example freezes and melts at 0 ºC.
It requires a significant decrease in temperature (or loss of thermal energy) and occurs at a specific temperature which is unique for each pure substance.

Question 8

Evaporation

Answer 8

When a liquid changes into a gas.
Evaporation occurs only at the surface of liquids where high energy particles can escape from the liquids surface at low temperatures, below the boiling point of the liquid.
The larger the surface area and the warmer the liquid/surface, the more quickly a liquid can evaporate.
Evaporation occurs over a range of temperatures, but heating will speed up the process as particles need energy to escape from the surface.

Question 9

Condensation

Answer 9

When a gas changes into a liquid, usually on cooling.
When a gas is cooled its particles lose energy and when they bump into each other, they lack energy to bounce away again, instead grouping together to form a liquid.

Question 10

Sublimation

Answer 10

When a solid changes directly into a gas.
This happens to only a few solids, such as iodine or solid carbon dioxide.
The reverse reaction also happens and is called desublimation or deposition.

Question 11

Pure substance

Answer 11

In chemistry a pure substance may consist of a single element or compound which contains no other substances.
For example a beaker of a sample of pure water contains only H2O molecules and nothing else.
If salt were added to the beaker then a mixture is produced.

Question 12

Mixture

Answer 12

A mixture consists of two or more elements or compounds that are physically mixed together, they are not chemically combined.
The chemical properties of the substances in a mixture remain unchanged.
Substances in mixtures can be separated by physical means.

Question 13

Distinguishing purity

Answer 13

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 tend to lower the melting point and broaden the melting point range.
Melting and boiling points data can therefore be used to distinguish pure substances from mixtures.
Melting point analysis is routinely used to assess the purity of drugs.
This is done using a melting point apparatus which allows you to slowly heat up a small amount of the sample, making it easier to observe the exact melting point.
This is then compared to data tables.
The closer the measured value is to the actual melting or boiling point then the purer the sample is.

Question 14

Simple distillation

Answer 14

This is used to separate a liquid and soluble solid from a solution (e.g., water from a solution of salt water) or a pure liquid from a mixture of liquids.
The solution is heated, and pure water evaporates producing a vapour which rises through the neck of the round bottomed flask.
The vapour passes through the condenser, where it cools and condenses, turning into the pure liquid that is collected in a beaker.
After all the water is evaporated from the solution, only the solid solute will be left behind.

Question 15

Fractional distillation

Answer 15

This is used to separate two or more liquids that are miscible with one another (e.g., ethanol and water from a mixture of the two).
The solution is heated to the temperature of the substance with the lowest boiling point.
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.
All of the substance is evaporated and collected, leaving behind the other components(s) of the mixture.

Question 16

Filtration

Answer 16

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).
Centrifugation can also be used for this mixture.
A piece of filter paper is placed in a filter funnel above a beaker.
A mixture of insoluble solid and liquid is poured into the filter funnel.
The filter paper will only allow small liquid particles to pass through as filtrate.
Solid particles are too large to pass through the filter paper so will stay behind as a residue.

Question 17

Crystallisation

Answer 17

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).
The solution is heated, allowing the solvent to evaporate, leaving a saturated solution behind.
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.
The saturated solution is allowed to cool slowly.
Crystals begin to grow as solids will come out of solution due to decreasing solubility.
The crystals are collected by filtering the solution, they are washed with cold distilled water to remove impurities and are then allowed to dry.

Question 18

Paper Chromatography

Answer 18

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).
A pencil line is drawn on chromatography paper and spots of the sample are placed on it. Pencil is used for this as ink would run into the chromatogram along with the samples.
The paper is then lowered into the solvent container, making sure that the pencil line sits above the level of the solvent, so the samples don’t wash into the solvent container.
The solvent travels up the paper by capillary action, taking some of the coloured substances with it.
Different substances have different solubilities so will travel at different rates, causing the substances to spread apart.
Those substances with higher solubility will travel further than the others.

Question 19

Identifying mixtures

Answer 19

Pure substances will produce only one spot on the chromatogram.
If two or more substances are the same, they will produce identical chromatograms.
If the substance is a mixture, it will separate on the paper to show all the different components as separate spots.
An impure substance therefore will produce a chromatogram with more than one spot.

Question 20

Rf values

Answer 20

These values are used to identify the components of mixtures
Rf stands for retention factor.
The Rf value of a particular compound is always the same but it is dependent, however, on the solvent used.
If the solvent is changed then the value changes.
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.
These values are known as reference values.
Rf = distance travelled by substance ÷ distance travelled by solvent
The Rf value will always lie between 0 and 1; the closer it is to 1, the more soluble is that component in the solvent

Question 21

Potable water

Answer 21

Ground water from aquifers is relatively clean but surface water (from rivers & lakes) and waste water need significant treatment in order to be fit for human consumption.
Untreated water contains soluble and insoluble impurities
Insoluble impurities include soil, pieces of plants and other organic matter and soluble impurities include calcium, metallic compounds and inorganic pollutants.
Unclean water also contains microbes which can cause illness.
Potable water means water that is clean enough for human consumption.

Question 22

Purifying water

Answer 22

Filtration - This process removes large insoluble particles by passing the water through layers of sand and gravel filters that trap larger particles.
Wire mesh filters are sometimes used, depending on the level of impurities in the water.
Sedimentation - Large insoluble particles sink to the bottom of a tank of water that has been left still for some time.
Iron sulphate or aluminium sulphate is sometimes added to help the fine particles clump together.
Chlorination - This process is used to kill bacteria and microorganisms which are too small to be trapped by the filters.
Cholera and typhoid are examples of bacterial diseases which can arise by the consumption of untreated water.

Question 23

Making sea water potable

Answer 23

This process is done in some areas of the world where very hot and dry climates prevail and where a lack of water.
Sea water contains mainly salts and can therefore be distilled to separate the water and the salts.
The salt remains in the liquid while the steam is cooled and condensed to make potable water.
The process is extremely expensive as a lot of energy is required to heat the large volumes of water to 100 ºC.
The wastewater produced is also extremely toxic due to the very high concentration of salts and must be disposed of correctly.

Question 24

Water in chemical analysis

Answer 24

Most chemical investigations involve the use of water at some stage of the process.
Normally deionised water is used, which is water that has had metallic ions such as calcium or copper removed.
Deionisation uses specifically designed ion-exchange resins that remove ions by exchange with hydrogen and hydroxide ions in water, which then recombine to form water molecules.
Deionised water is used as the ions could react with the substances under analysis and would give the experiment a false result.