Separating Mixtures
Define the terms: - solute - solvent - solution
When a solid dissolves in a liquid - The substance that dissolves is called the solute - The liquid that dissolves is called the solvent - The liquid formed is called the solution
How do you separate sulfur and iron without using chemical separation methods?
a mixture of iron and sulphur is quite easy to separate into the two elements using a magnet. The iron sticks to the magnet and the sulphur doesn't. the elements in a compound cannot be separated by physical means. To convert iron sulphide into separate samples of iron and sulphur requires chemical reactions
what can you find out with a solubility curve?
a) Plot a solubility curve b) Use the solubility curve to find - The solubility of potassium chloride at 50°C - The maximum mass of potassium chloride that would dissolve in 50g of water at 40°C - The temperature at which crystals will first appear if you cooled a hot solution containing 51.0g of potassium chloride in 100g of water.
How can mixtures be seperated?
by physical means are things, like changing the temperature or dissolving part of the mixture in a solvent such as water; in other words, methods that don't involve any chemical reactions. or by the separation techniques: Simple distillation fractional distillation filtration chromatography crystallization
What happens when we heat a solid? what is the cycle?
1) When a solid is heated, its particles gain more energy 2) This makes the particles vibrate more, which weakens the forces that hold the solid together 3) At a certain temperature, the particles have enough energy to break free from their positions. This is called melting and the solid turns into a liquid 4) When a liquid is heated, again the particles get even more energy 5) This energy makes the particles move faster, which weakens and breaks the bonds holding the liquid together 6) At a certain temperature, the particles have enough energy to break their bonds. This is called evaporating and the liquid turns into a gas
Changing state with changed pressrue
A gas will also liquefy (turn into a liquid) if its pressure is increased enough. This is because the particles are moved close enough for bonds to form between the particles. Gas cylinders used for camping stoves and barbecues contain liquefied petroleum gas (LPG) under high pressure. As soon as the pressure is released, the liquid turns back to a gas.
Practical - investigate the solubility of a solid in water and a certain temperature:
A procedure we can use to measure the solubility of potassium nitrate in water at 40°C is as follows: 1. Weigh an evaporating basin 2. Heat a boiling tube of water to just above 40°C 3. Add potassium nitrate to the water in the boiling tube and stir rapidly until no more of it will dissolve and there is undissolved solid left over. 4. Allow the solution to cool to exactly 40°C 5. Pour of some of the solution into the evaporating basin ( it is important that you only pour off solution and no solid). You do not have to pour off all the solution 6. Weigh the evaporating basin and contents 7. Heat the evaporating basin and contents gently to evaporate off all the water 8. When it looks as if all the water has evaporated weigh evaporating basin and contents 9. Heat the evaporating basin and contents again and then re-weigh. This is to make sure that all the water has, indeed, evaporated and is called heating to constant mass. we heat the solution gently to make sure that none spits out. If some did spit out we would record a lower mass of solid and the solubility would appear to be lower than the actual value
Changing state between solid and gas: sublimation
A small number of substances can change directly from a solid to a gas, or from a gas to a solid, at normal pressure without involving any liquid in the process. The conversation of a solid into a gas is known as sublimation and the reverse process is usually called deposition An example of a substance that sublimes is carbon dioxide. At ordinary pressures, there is no such thing as liquid carbon dioxide - it turns directly from a solid to a gas at -78.5 °C, solid dioxide is known as dry ice.
Working out the physical state of a substance at a particular temperature:
A substance is a solid at temperatures below its melting point, between its melting point and its boiling point it is a liquid, and above its boiling point it is a gas. In chemistry we can decide whether a substance is a solid, a liquid or a gas at room temperature by looking at where its melting and boiling points are in relation to room temperature. A temperature line to work can be used to work out whether substances are solids, liquids or gases
Using a temperature line
A temperature line to work can be used to work out whether substances are solids, liquids or gases If we look at the temperature line, we can see heat that room temperature is above the boiling point of oxygen; this means that oxygen is a gas at room temperature. Lets look at what happens when we heat bromine from -100°C to 100°C. As -100°C is below bromine's melting point, bromine is a solid at -100°C. As it is heated to -7°C (its melting point) it becomes a liquid and it remains as a liquid until its temperature reaches the boiling point at 59°C. room temperature is between the melting point and boiling point, which means that bromine is a liquid at room temperature. Above 59°C bromine is a gas.
Dilution
Adding water to a solution in order to decrease the concentration
What is to look out for when using Rf value
An Rf value msut be between 0 and 1. If you get a number bigger than 1 you have probably divided the numbers the wrong way around. An Rf value has no units! You have to be careful using Rf values as they depend on the solvent used and on the type of paper. There was no problem in the experiment described above bevause the mixtrue and induvidual dyes were all put on the same piece of paper. However, If the mixture was put on one piece of chromatogrpahy paper and the induvidual dyes on separate piece, you can still comapre the Rf values as long as you can use the same type of paper and the same solvent.
define the term atom
Atom - the smallest piece of an element that can be recognised as that element
Define the term compound
Compound - compounds are formed when two or more elements chemically combine. The elements always combine in fixed proportions. For example, hydrogen and fluorine always combine to form hydrogen fluoride, with the formula HF. Eg: CO2- carbon dioxide, CH4 - methane.
What is crystalization
Crystallization can be used to separate a solute from a solution. For example, it could be used to separate sodium chloride from a sodium chloride solution. The solution is heated in an evaporating basin to boil off some of the water until an almost saturated solution is formed. This can be tested by dipping a glass rod into the solution and seeing if crystals form quickly on its surface when removed. The Bunsen burner is then turned off and the crystals allowed to form as more water evaporates and the solution cools. The crystals can now be removed from the mixture using a method called filtration.
What is - deposition - condensation - sublimation
Deposition: For deposition to occur, thermal energy must be removed from a gas. When the air becomes cold enough, water vapor in the air surrounding the leaf loses enough thermal energy to change into a solid Sublimation: Sublimation is the transition from the solid phase to the gas phase without passing through an intermediate liquid phase. This endothermic. It occurs when the particles of a solid absorb enough energy to completely overcome the force of attraction between them. Condensation: Condensation is the process in which molecules of a gas slow down, come together, and form a liquid.
What is diffusion
Diffusion is the spreading out of particles from where they are at a high concentration ( there are lots of them in a certain volume) to where they are at a low concentration ( there are fewer of them in a certain volume)
Diffusion in liquids
Diffusion though a liquid is very slow if the liquid is completely still. For example, If a small jar of strongly coloured solution. (such as potassium manganate(VII) solution) is placed in a gas jar of water. It can take days for the colour to dilute throughout the water. This is because the particles in a water move slower than a gas. The particles in a liquid are also much closer together than those in a gas and so there is less space for particles to move into without colliding with another one.
Define the term element
Element - Elements are substances that can't be spilt into anything simpler by chemical means. An element contain s only one type of atom. Eg: Oxygen, Pure metal like Magnesium, diamond etc.
Example of fractional distillation - ethanol and water
Ethanol and water are completely miscible (mixable) with each other. That means you can mix them together in any proportion and they will form a single liquid layer. You can separate them by taking advantage of their different boiling points: water boils at 100°C, ethanol at 78°C.
What is filtration
Filtration can be used to separate a solid from a liquid. For example, sand can be separated from water by filtration. The apparatus for filtration is: - Funnel - Filter paper - Beaker And substances collected are - Sand (residue) - Water (filtrate) the substance left in the filter paper is called the residue and the liquid that comes though is called the filtrate. Filtration can also be used to separate two rock solids from each other if only one of them is soluble in water. Rock salt consists of salt contaminated by various other earthy or rocky impurities. These impurities are insoluble in water), therefore can be filtered.
What is fractional distillation
Fractional distillation is used to separate a mixture of components/fractions/liquids such as crude oil and ethanol (alcohol) and water. Both liquids boil, but by careful heating you can control the temperature of the column so that all the water condenses in the column and trickles back into the flask. Only the ethanol remains as a vapour all the way to the top of the fractionating and condenses out into the condenser. Another example of fractionating distillation is the separation of crude oil into its fractions because they all have their different melting and boiling points.
changing from state to state
How do we change to different states of matter: · Heat must be supplied to a substance for it to melt, evaporate or boil. For example, you need to heat ice to melt it, and you need to heat water to make steam. · Heat must be removed from a substance to condense or freeze it. In other words, the substance must be cooled down. Under certain conditions, some solids turn straight into a gas when heated. This process is called sublimation. A good example is solid carbon dioxide, also called 'dry ice'. At atmospheric pressure, it turns straight into gaseous carbon dioxide. Liquid carbon dioxide can only exist under high pressure, such as in fire extinguishers. Iodine also sublimes - it turns directly from shiny purple-black crystals to a purple vapour when warmed up.
Interconnections between the three states of matter: changing between solid and liquid
If you heat a solid, the energy provided by the heat source makes the particles in the solid vibrate faster and faster. Eventually, they vibrate so fast that the forces of attraction between the particles are no longer strong enough to hold them together; the particles are then able to move around each other - the solids melts to form a liquid. The temperature at which the solid melts is called its melting point. The particles in the liquid have more kinetic energy than the particles in the solid so energy so energy has to be supplied to convert a solid into a liquid. In a liquid is cooled again, the liquid particles will move around and more slowly. Eventually, they are moving so slowly that the forces of attraction between them will hold them in a fixed position and particles pack more closely together into a solid. The liquid freezes, forming a solid. The temperature at which this occurs is called the freezing point. Although they are called different things depending which way you are going, the temperature of the melting point and that of the freezing point of a substance are exactly the same.
Arrangement of particles in liquid
In a liquid, the particles are still mostly touching, but some gap have appeared. This is why liquids are usually less dense than solids. The forces between the particles are less effective, and the particles can move around each other. The particles in a liquid are moved around randomly.
what is the difference in properties of elements and properties of compounds?
In a mixture of elements in each element keeps its own properties, but the properties of the compound are quite different. For example in a mixture of iron and sulphur, the iron is grey and the sulphur yellow. The iron reacts with dilute acids such as hydrochloric acid to produce hydrogen; the sulphur doesn't react with the acid. However, the compound iron sulphide (FeS) reacts quite differently with acids to produce poisonous hydrogen sulphide gas, which smell like bad egg A mixture of hydrogen and oxygen I a colourless gas which explodes when you put a flame to it. The compound, water, I a colourless liquid which just put out a flame.
Arrangement of particles in solid
In a solid the particles are usually arranged regularly and packed closely together. The particles are only able to vibrate about fixed positions; they can't move around. The particles have strong forces of attraction between them, which keep them together. The particles in a solid have less kinetic energy (movement) than the particles in a liquid, which have less kinetic energy than the particles in a gas.
Example of Rf value in experiment
In this experiement the Rf value = x/y Rf = 2.9/3.6 = 0.81 The Rf value of the dyes in mixture m are Blue spot: 0.9/3.6 = 0.25 Orange spot: 2.0/3.6 = 0.56 Green spot: 2.9/3.6 = 0.81 The Rf values of dyes d1 to d4 are: D1: Rf = 0.56 D2: Rf = 0.36 D3: Rf = 0.81 D4: Rf = 0.25 Because the dots in mixture m have the same Rf values as d1, d3 and d4, we can conclude that the mixture contains these dyes
Example of Paper Chromatography
In this experiment spot C has hardly moved. Either it was not very soluble in the solvent or it has a very high affinity for the paper (or both). On the other hand, spot A has moved almost as far as the solvent. It must be very soluble in the solvent and not have much affinity for the paper. The pattern you get is called a chromatogram.
What are the proportions of atoms?
In water ( a compound), every single water molecule has two hydrogen atoms combined with one oxygen atom. It never varies. In a mixture of hydrogen and oxygen gases, the two could be mixed together in any proportion. If you had some iron metal and some sulphur, you could mix them in any proportion you wanted to. In iron sulphide (FeS) a compound, the proportion of iron to sulphur always exactly the same.
What is the Rf value?
Instead of saying the spots have moved different distances we can use the Rf value to describe how far the spots move. Rf stands for retardation factor. Each time we do a chromatography experiment the solvent ( and therefore the spots) will move different distances along the paper. This means we can't just report the distance moved by a particular spot, so we have to work out a ratio instead. (Measure to the centre of the spot)
Define the term mixture
Mixture - in a mixture, the various substances are mixed together and no chemical reactions occur. Mixtures can be made form elements and/or compounds. The various components can be in any proportion, for example you can put any amount of sugar into your cup of tea and coffee (until it becomes saturated)
define the term molecule
Molecules - a group of atoms bonded together, smallest unit of a chemical compound that can take part in a chemical reaction. Bonded with a covalent bond
in simple distillation, why is the water put in from the lower end?
Notice that the water is always fed into the way of the condenser on the lower end. That way it fills the condenser jacket better and If the flow of water stops for any reason the condenser jacket remains full of water
What is paper chromatography?
Paper chromatography can be used to separate a variety of mixtures. However, we mainly use it to separate inks and food colourings. Most inks and food colourings are not just made up of one colour but contain a mixture of dyes. Paper chromatography can also be used to separate a mixture of colourless substances such as sugars, but then some method must be used to make the spots visible on the paper. Overall, paper chromatography is used to separate soluble coloured chemicals or substances.
What are the differences in boiling point between pure and impure substances?
Pure substances, such as elements are pure compounds, melt and boil at fixed temperatures. For example, the melting point of water is 0°C and the boiling point 100°C. However, mixtures usually melt or boil over a range of temperatures. impure substances impurities lowers the melting point of a substance and raises the boiling point (A mixture is not a pure substance. If a sample contains only a small amount of an unwanted substance, the unwanted substance might be called an impurity.)
What is the equation for Rf value
Rf = distance moved by solute (spots) from the pencil line / distance moved by the solvent front (from the pencil line)
What is room temperature
Room temperature is different in different places but in science It is usually taken to a mean a temperature between 20°C and 25°C. because there is not just one fixed value, for changes of state that occur near room temperature we must be careful when making comparisons and make clear what value is being used as room temperature?
What is simple distillation
Simple distillation can be used to separate components of a solution, with different boiling points. Although we can use crystallization to separate sodium chloride from a sodium chloride solution, we can also collect the water if we use simple distillation. The water boils and is condensed back to a liquid by a condenser. The salt remains in the flask. You could, collect the salt from the solution as well as collecting pure water. The sodium chloride solution eventually becomes so concentrated that the salt will crystallize out.
What are the three states of matter?
Solids, liquids and gases are known as the three states of matter.
Solubility equation
Solubility (g/100g) = mass of solute / mass of solvent x 100
how are different dyes different?
The dyes that make up the mixture will be different in two different ways: - The affinity they have for the paper ( how attracted and how well they 'stick' to the paper) - How soluble they are in the solvent which moves up the paper However if the dye doesn't move from the pencil line during the experiment, then the dye is not soluble in the solvent you are using. You need a different solvent. If the dye moves up the paper with the solvent front, the dye to soluble in that solvent and, again you have to try a different solvent
What is in the fractional column to make separation easier
The fractionating column is often packed with glass beads or something similar, although the separation of ethanol and water in the lab works perfectly well with just an empty column. Beads help due to the larger surface area in the column helps separation of the two vapours.
Example of how to see if a solution of pure or not
The melting point can be very useful in determining whether or not a substance is pure. In order to determine whether your substance is pure or not you can measure the melting point. You would record the temperature at which your sample starts to melt, and then you would record the temperature at which it has fully melted to completely form a liquid. Eg: Asprin is a white powder that melts at 138°C. if the melting point of the sample you made was 128-134°C you can see that it is quite impure because it melts over a wide range of temperatures (below the melting point of pure aspirin)
Arrangement of particles in gas
The particles in a gas are moving around randomly at high speed in all directions. In a gas, the particles are much further apart and there are (almost) no forces of attraction between them.
What is solubility
The solubility of a solid in a solvent at a particular temperature is usually defined as 'the mass of solute which must dissolve in 100g of solvent at that temperature to from a saturated solution'. In other words, it is the maximum mass of solute that dissolves in 100g of solvent at a particular temperature. For example, the solubility of sodium chloride (common salt) in water at 25°C is about 35g per 100g of water. A saturated solution is a solution which contains as much dissolved solid as possible at a particular temperature. There must be some undissolved solute present.
What are solubility curves?
The solubility of solids changes with temperature and you can plot this on a solubility curve. Most solids have solubility curves like those for the salts shown in this solubility curve. Their solubility increases with temperature - either dramatically or just a little
Interconnections between the three states of matter: changing between liquid and gas:
There are two different ways you can change from a liquid to a gas called boiling and evaporation Boiling: Boiling occurs when a liquid is heated so strongly that the particles are moving fast enough to overcome all the forces of attraction between them. The stronger the forces of attraction between particles, the higher the boiling point of the liquid. This is because more energy is needed to overcome these forces of attraction. If a gas cooled, the particles eventually move slowly enough that forces of attraction between them start to form and hold them together as a liquid. The gas condenses. Evaporation: Evaporation is different. In any liquid or gas, the average speed of the particles varies with the temperature. But at each temperature, some particles will be moving faster than the average and others more slowly. Some very fast particles at the surface of the liquid will have enough energy to overcome the forces of attraction between the particles - they will break away to form a gas. This is evaporation. You don't see any bubbling; the liquid just slowly disappears if it was open to the air. If the liquid is in a closed container, particles in the gas will also be colliding with particles at the surface of the liquid. If they are moving slowly enough they will be held by the attractive forces and become part of the liquid. In closed container evaporation and condensation will both be occurring at the same time.
Ammonia and hydrochloric acid practical
This experiment relies on the reaction between ammonia (NH3) and hydrogen chloride (HCl) gases to give white solid ammonium chloride (NH4Cl): Requires eye protection and avoidance of skin contact and inhalation of any fumes. NH3 (g) + HCl (g) -> NH4Cl (s) Ammonia + hydrogen chloride -> sodium chloride Pieces of cotton are soaked in concentrated ammonia solution ( as a source of ammonia gas) and concentrated hydrochloric acid ( as a source of hydrogen chloride gas). These are placed in the ends of a long glass tube with rubber bungs to stop the poisonous gasses escaping Ammonia particles and hydrogen chloride particles diffuse along the tube. A white ring of solid ammonium chloride forms when they meet. The white ring of ammonium chloride takes time to form (as it takes some time for the particles of ammonia and hydrogen chloride to diffuse along the tube) and appears closer to the hydrochloric acid end. Ammonia particles are lighter than hydrogen chloride particles and therefore faster. The ammonia particles travel further away from the ammonia end.
practical - Investigating the composition of dye with paper chromatography
We can investigare the composition of a mixture of coloured dyes using paper chromatography. To do this we will carry out the following steps: 1) Draw a pencil line across the piece of the chromatography paper; this line should be around 1cm from the bottom of the paper. Do not use a pen as the colours in the ink may move up the chromatography paper with the solvent. 2) Put a spot ( use a treat pipette or a capillary tube) of the mixture of dyes on the pencil line and allow it to dry 3) Suspend the chromotgraphy paper in a beaker that contains a small amount of solvent so that the bottom of the paper goes into the solvent. It is important that the solvent is below the pencil line so that the inks/colourings don't just dissolve in the solvent. 4) Put a lid (such as a watch glass) on the beaker so that the atmosphere becomes saturated with the solvent. This is to stop evaporation of the solvent from the surface of the paper. 5) When the solvent has moved up the paper to about 1cm from the top, remove the paper from the beaker and draw a pencil line to show where the solvent got to. The highest level of solvent on the paper at any time is called the solvent front. 6) Leave the paper to dry so that all the solvent evaporates For the solvent you can use water or a non-aqueous solvent ( a solvent other than water). Which solvent you use depends on the substances that are present in the mixture. A suitable solvent is usually found by experimenting different ones.
How to make a pure salt from rock salt?
We can use filtration and crystallization to obtain pure salt from rock salt. Rock salt consists of salt contaminated by various other earthy or rocky impurities. These impurities are insoluble in water. If you crush the rock salt and mix it with hot water, the salt dissolves, but the impurities don't. the impurities can be filtered off, and remain on the filter paper. The filtrate is then a salt solution. The solid salt can be obtained from the solution by crystallization. This is typical way you can separate any mixture of two solids, one of which is soluble in water and one of which isn't.
Examples of solubility experiment
We need to calculate the mass of the solid and also the mass of water evaporated from the solution: Mass of crystals = 38.00 - 25.72 = 12.28g Mass of water = 58.00 - 38.00 = 20.00g 12.28g of solid is the maximum mass that dissolves in 20.00g of water. Therefore 5 times as much would dissolve in 100g in water. That works out at 61.4g. the solubility of potassium nitrate at 40°C is therefore 61.4 per 100g of water More generally, we can calculate the solubility of a substance in 100g of solvent using the equation: Solubility (g/100g) = mass of solute / mass of solvent x 100
When you are making a solution what happens to the forces?
When you make a solution, the attractive forces between the particles In the solute (the solid) are being broken. At the same time, new attractive forces are being formed between the solvent particles and the solute particles. Whether a particular solid is soluble in any solvent depends on whether the new attractive forces are strong enough to over come the old ones.
Diffusion in gases
You can show diffusion in gases very easily by using apparatus in this photo. The lower gas jar contains bromine gas; the top one contains air. If the lids are removed, the brown colour of the bromine diffuses upwards until both gas jars are uniformly brown ( the particles also diffuse downwards). The bromine particles and air particles move around at random to give an extra mixture - both gas jars contain air and bromine particles. Doing this in a fume cupboard wearing eye protection and chemical resistant gloves. Irritation of bromine by anyone with breathing difficulties may produce a reaction, possibly delayed. Medical attention may be needed
How can you use chromatography with a mixture?
You can use paper chromatography to identify the particular dyes in a mixture. If you think that your mixture (m) could contain dyes d1, d2, d3, and d4, you can carry out an experiment to determine this. Paper chromatography can be used to analyze a mixture. Lines will not be present on you paper, but they have been added here to help you measure the distances. A pencil line is drawn on a larger sheet of paper and pencil marks are drawn along the line to show the original positions of the various dyes placed on the line. One spot is your unknown mixture; the others are single known dyes. The chromatogram is then allowed to develop as before. The mixture (m) has spots corresponding to dyes d1, d3 and d4. They have the same colour as spots in the mixture, and have travelled the same distance on the paper. Although dye d2 is the same colour as one of the spots in the mixture, it has travelled a different distance and so must be a different compound.
Example of solubility curve
You can use solubility curves to work out what mass of crystals you would get if you cooled a saturated solution. Consider the solubility curves for potassium nitrate (KNO3) in the solubilty curve above. At 90°C 200g of potassium nitrate dissolves in 100g of water. At 30°C only 50g will dissolve. Therefore, is we have a solution containing 200g of potassium nitrate dissolved in 100g of water and let it cool down from 90°C to 30°C, 150g of potassium nitrate must be released from the solution, which it does as crystals. We say that nitrate must be released from the solution, which it does as crystals. We say that potassium nitrate crystallizes out of the solution or precipitates out of the solution.
what is the difference between pure salt and rock salt
rock salt contains impurities that are not soluble in water. however, pure salt is soluble in water