Purity & Separating Mixtures (OCR GCSE Chemistry A (Gateway)): Flashcards
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What is a pure substance in chemistry?
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What is a pure substance in chemistry?
A pure substance contains only one element or compound with no other substances mixed in.
For example, pure water contains only H2O molecules.
True or False?
A mixture of salt and water is a pure substance.
False.
Salt and water are two different compounds physically combined, making it a mixture, not a pure substance.
What is a mixture?
A mixture contains two or more elements or compounds that are physically combined but not chemically joined.
The chemical properties of each substance in the mixture remain unchanged.
Why can substances in a mixture be separated by physical means?
The substances in a mixture are not chemically bonded, so their chemical properties stay unchanged and physical methods such as filtration or distillation can separate them.
Pure substances melt at a .......... and .......... temperature, while mixtures melt over a .......... of temperatures.
Pure substances melt at a specific and sharp temperature, while mixtures melt over a range of temperatures.
How can melting point data be used to identify whether a sample is pure or impure?
A pure substance melts at a single sharp temperature that matches its data table value.
An impure sample melts over a broader range at a lower temperature than expected.
What shape does the cooling curve of a pure substance show during freezing, and why?
The cooling curve shows a flat horizontal plateau during freezing.
This is because a pure substance changes state at a single fixed temperature, so the temperature does not change while it freezes.
True or False?
Air is a pure substance because it has a fixed composition.
False.
Air is a mixture of gases including nitrogen, oxygen and carbon dioxide. Its components can be separated by physical means.
What does it mean if a measured melting point is close to the data table value?
It means the sample is close to pure.
The smaller the difference between the measured and actual melting point, the purer the sample.
What is relative atomic mass (Ar)?
Relative atomic mass (Ar) is the average mass of the atoms of an element measured relative to 1/12 of the mass of one atom of carbon-12.
It is calculated from the mass number and relative abundances of all isotopes of that element.
How do you calculate the Mr of a compound?
Add up the relative atomic masses (Ar) of every atom in the formula, taking into account how many of each atom are present.
For example, Mr of H2O = (2 × 1) + 16 = 18.
What is relative formula mass (Mr)?
Relative formula mass (Mr) is the total mass of a molecule or formula unit, found by adding the relative atomic masses of all atoms in the formula.
Why does an element's relative atomic mass account for all its isotopes?
Different isotopes of the same element have different masses, so the relative atomic mass is a weighted average that reflects the relative abundance of each isotope.
The relative atomic mass of an element is the .......... of the two numbers shown on the .........., and has the symbol ............
The relative atomic mass of an element is the larger of the two numbers shown on the periodic table, and has the symbol Ar.
True or False?
The relative atomic mass of an element is the smaller of the two numbers shown on the periodic table.
False.
The relative atomic mass is the larger of the two numbers shown on the periodic table.
What does the Law of Conservation of Mass tell us about Mr values in a balanced equation?
The sum of the relative formula masses of the reactants equals the sum for the products.
Mass is neither created nor destroyed in a chemical reaction.
What is the Mr of CO2? (Ar: C = 12, O = 16)
Mr of CO2 = 12 + (2 × 16) = 44.
What is relative molecular mass?
Relative molecular mass is the average mass of one molecule of an element or compound measured relative to 1/12 of the mass of one atom of carbon-12.
It applies to molecular substances.
What is an empirical formula?
An empirical formula is the simplest whole number ratio of atoms of each element in a compound.
What is a molecular formula?
A molecular formula shows the actual number of atoms of each element present in one molecule of a compound.
An empirical formula shows the .......... whole number .......... of atoms of each element in a compound.
An empirical formula shows the simplest whole number ratio of atoms of each element in a compound.
How do you find the empirical formula from a molecular formula?
Divide the number of atoms of each element by their highest common factor to give the simplest whole number ratio.
True or False?
The empirical formula of a compound always differs from its molecular formula.
False.
Sometimes they are the same. CO2 is an example where the simplest ratio and the actual formula are identical.
Why can the empirical formula and molecular formula sometimes be the same?
If the ratio of atoms in the molecule is already in its simplest whole number form, the empirical and molecular formulas are identical.
CO2 is an example.
How do you find the empirical formula of an ionic compound from a 3D lattice diagram?
Identify the ions in the lattice and balance their charges so the overall charge is zero.
The resulting formula is the empirical formula.
What is the equation for calculating the percentage by mass of an element in a compound?
% mass = (Ar × number of atoms of the element) ÷ Mr of the compound × 100.
Multiply by 100 to convert the answer to a percentage.
A compound has the formula C2H5. What type of formula is this, and what does it tell you?
C2H5 is an empirical formula. It tells you the simplest whole number ratio of carbon to hydrogen atoms is 2:5, but not the actual number of atoms in one molecule.
What is an alloy?
An alloy is a mixture of metals, or a metal mixed with one or more other elements.
Why are alloys usually harder than the pure metals they are made from?
Alloys contain atoms of different sizes, which distort the regular arrangement of atoms.
This makes it harder for layers to slide over each other, so the alloy is harder and less malleable than the pure metal.
Alloys contain atoms of .......... sizes, which .......... the regular arrangement, making it harder for layers to .......... over each other.
Alloys contain atoms of different sizes, which distort the regular arrangement, making it harder for layers to slide over each other.
True or False?
Adding a different-sized atom to a pure metal makes it easier for layers of atoms to slide over each other.
False.
Atoms of different sizes distort the regular arrangement, making it harder for layers to slide. The alloy becomes harder and less malleable than the pure metal.
What is a formulation?
A formulation is a mixture that has been designed as a useful product, made by following an exact recipe with carefully measured quantities of each component.
What is the composition of brass, and what type of material is it?
Brass contains 70% copper and 30% zinc. It is an alloy and also an example of a formulation.
Give an example of an alloy and two other types of formulation.
Brass (copper and zinc) is an example of an alloy.
Paint
Medicines
True or False?
Every component in a formulation is present in a carefully measured quantity.
True.
Formulations are made to an exact recipe, with each component in precise quantities so that it provides the required properties.
What is filtration used to separate?
Filtration is used to separate an undissolved solid from a mixture of that solid and a liquid or solution.
What is crystallisation used to separate?
Crystallisation is used to separate a dissolved solid from a solution when the solid is much more soluble in hot solvent than in cold.
During filtration, what happens to the solid and what happens to the liquid?
The solid particles are too large to pass through the filter paper and remain as a residue.
The liquid passes through as the filtrate.
Why does crystallisation produce crystals as the saturated solution cools?
As the solution cools, the solubility of the solid decreases, so the excess dissolved solid comes out of solution and forms crystals.
What is simple distillation used to separate?
Simple distillation is used to separate a liquid from a dissolved solid, or a pure liquid from a mixture of liquids with very different boiling points.
In simple distillation, the solution is heated so the liquid .........., travels through the condenser, .........., and is collected as a pure liquid.
In simple distillation, the solution is heated so the liquid evaporates, travels through the condenser, condenses, and is collected as a pure liquid.
What is fractional distillation used to separate?
Fractional distillation is used to separate two or more miscible liquids that have different boiling points, such as ethanol and water.
True or False?
Fractional distillation and simple distillation can both be used to separate two miscible liquids with similar boiling points.
False.
For two miscible liquids with similar boiling points, fractional distillation is needed. Simple distillation is less effective when boiling points are close together.
A student has a mixture of rock salt (sand, salt and water). Which two separation techniques are needed?
Filtration separates the sand (undissolved solid) from the water.
Crystallisation separates the dissolved salt from the water.
Why is the fractionating column in fractional distillation filled with glass beads?
The glass beads increase the surface area inside the column.
Substances with higher boiling points cool and condense on the beads, while the substance with the lower boiling point continues rising to the condenser.
What is chromatography used for?
Chromatography is used to separate the components of a mixture and provide information to help identify them.
In paper chromatography, what are the mobile phase and stationary phase?
The mobile phase is the liquid solvent that moves through the paper.
The stationary phase is the chromatography paper itself.
Why do different substances travel different distances up the chromatography paper?
Separation depends on how each substance is distributed between the two phases. Substances with higher solubility in the mobile phase are carried further up the paper, while substances with stronger adsorption to the stationary phase travel shorter distances.
In paper chromatography, the solvent travels up the paper by .......... action, carrying substances based on their .......... in the solvent.
In paper chromatography, the solvent travels up the paper by capillary action, carrying substances based on their solubility in the solvent.
What is the Rf value?
The Rf value (retention factor) is the ratio of the distance moved by a compound to the distance moved by the solvent.
It is used to identify unknown substances by comparing with reference values under the same conditions.
What is the formula for calculating an Rf value?
Rf = distance moved by the compound ÷ distance moved by the solvent front.
Both distances are measured from the baseline, and Rf values have no units.
In gas chromatography, what are the mobile phase and stationary phase?
The mobile phase is an unreactive carrier gas such as nitrogen.
The stationary phase is a liquid or gel coated on a solid support inside the column.
True or False?
A pure substance will produce only one spot on a chromatogram.
True.
A pure substance contains only one component, so it produces a single spot. A mixture produces multiple spots as its components separate.
What does the number of peaks on a gas chromatogram tell you?
The number of peaks tells you the number of compounds present in the mixture.
Each peak represents one component of the mixture.
Why is a pencil line used to mark the baseline in paper chromatography, rather than pen?
Ink from a pen would dissolve in the solvent and run up the paper along with the samples, contaminating the chromatogram.
Pencil marks are insoluble in most solvents.
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