Chemical Analysis (SQA National 5 Chemistry): Flashcards

Filtration is a technique used to separate an insoluble solid from a liquid, trapping the solid as a residue in the filter paper while the liquid passes through as the filtrate.

The liquid that passes through the filter paper is called the filtrate. It is a solution consisting of the solvent and any dissolved substances.

False.

The tare button must be pressed before adding the substance. Pressing it after the substance is added would not zero out the mass of the container.

Collection over water is used for gases that are insoluble in water. The gas displaces the water in an inverted measuring cylinder placed in a water-filled trough.

1. Collection over water — used for insoluble gases such as hydrogen.

2. Upward displacement of air — used for soluble gases denser than air such as carbon dioxide.

3. Downward displacement of air — used for soluble gases less dense than air such as ammonia.

A Bunsen burner is a common piece of heating apparatus used for general-purpose direct heating of substances that are not flammable.

False.

Ammonia is less dense than air so downward displacement of air must be used. The gas enters an inverted jar and pushes air out from the bottom.

An electric hotplate has no open flame, which eliminates the risk of a flammable liquid catching fire. It also provides gentler and more controllable heating than a Bunsen burner.

When using a Bunsen burner for strong, direct heating, you should use the blue flame, as it is hotter and cleaner than the yellow safety flame.

Place an empty container on the balance and press the tare button to zero the display. Carefully add the substance until the desired mass is reached and wait for the reading to settle before recording it.

A salt is a compound formed when the hydrogen atom in an acid is replaced by a metal ion.

1. React: add the insoluble base in excess to gently warmed dilute acid until no more reacts.

2. Filter: remove the excess unreacted solid base to leave a pure salt solution as the filtrate.

3. Evaporate and crystallise: heat the filtrate to evaporate some water then allow it to cool and crystallise.

4. Dry: remove the crystals and pat them dry.

True.

Adding the base in excess guarantees that all the acid is neutralised. The excess unreacted base is then removed by filtration.

When preparing copper(II) sulfate from copper(II) oxide and sulfuric acid, the mixture is filtered to remove excess oxide, giving a pure salt solution in the filtrate.

In the second run the indicator is omitted so that it does not contaminate the salt solution. The exact volumes found in the first titration are repeated to neutralise the acid and alkali without any added dye.

A precipitation reaction is a reaction in which two soluble solutions are mixed and an insoluble salt forms as a solid product called the precipitate.

True.

When two soluble salt solutions are mixed, the ions can combine to form an insoluble salt that precipitates out of solution as a solid.

1. Dissolve each soluble reactant in water and mix the two solutions together in a beaker.

2. Filter the mixture to collect the insoluble precipitate as the residue.

3. Wash the residue with distilled water to remove traces of the other soluble ions.

4. Dry the solid in an oven.

To check that a salt solution is saturated, dip a cold glass rod into the solution and observe whether crystals form on it.

Slow evaporation over about a week produces larger crystals because ions have more time to arrange into an ordered structure. Heating the solution causes rapid evaporation and produces smaller crystals.

An electrolyte is an ionic solution or molten ionic compound that can conduct electricity because it contains ions that are free to move.

A solid ionic compound does not conduct because its ions are held in a fixed lattice and cannot move. When dissolved in water the ions are free to move, so the solution does conduct electricity.

False.

In the solid state, ions are locked in a fixed lattice and cannot move. Ionic compounds only conduct electricity when molten or dissolved in water.

In an electrochemical cell, the two half-cells are connected by an ion bridge (a strip of filter paper soaked in an ionic solution) to allow ions to flow and complete the circuit.

1. Two beakers containing solutions of the respective metal ions.

2. Two different metal electrodes.

3. A voltmeter connected by wires and crocodile clips.

4. An ion bridge to connect the two beakers.

5. Wires and crocodile clips to link the electrodes to the voltmeter.

True.

A DC (direct current) supply keeps the polarity of the electrodes constant, which is essential for consistent electrolysis. An AC supply would reverse the electrodes repeatedly.

Electrolysis is the process of using electricity to break down an ionic compound, causing ions to discharge at the electrodes.

Graphite is used because it conducts electricity but does not react with the electrolyte or the products formed at the electrodes. This means the electrode does not interfere with the results.

When setting up an electrochemical cell, a zinc rod is placed in a solution of zinc sulfate to form one half-cell, and the voltage is measured using a voltmeter.

A spirit burner is a piece of apparatus that contains the fuel and is lit to release heat energy, which is then transferred to the water in the calorimeter above it.

1. A spirit burner containing the fuel.

2. A copper can or beaker containing a known mass of water.

3. A thermometer to measure the temperature change of the water.

4. A tripod and gauze to support the can.

5. A digital balance to measure the mass of the spirit burner before and after.

6. Draught shields to minimise heat loss to the surroundings.

False.

The experimental value is always lower than the true value because a significant amount of heat is lost to the surroundings rather than being transferred to the water.

In a calorimetry experiment, the mass of fuel burned is calculated by subtracting the final mass of the spirit burner from its initial mass.

Copper is a much better conductor of heat than glass, so a copper can transfers heat energy from the flame to the water more efficiently and reduces the amount of heat absorbed by the container itself.

True.

Draught shields protect the flame from air currents, which reduces heat loss and means more of the energy released by the fuel is transferred to the water.

1. Heat loss to the surroundings from the flame and the beaker.

2. Incomplete combustion of the fuel, which releases less energy than complete combustion.

3. Heat energy being absorbed by the beaker rather than the water.

In a calorimetry experiment, the temperature change (ΔT) is calculated by subtracting the initial temperature from the final temperature of the water.

A titre is the volume of solution added from the burette to reach the end-point of a titration.

True.

For SQA National 5, concordant results must be within 0.2 cm³ of each other. Only concordant results are used to calculate the average titre.

The purpose of a titration is to accurately determine the exact volume of one solution needed to completely react with another solution. This data can be used to find an unknown concentration.

The end-point is the point at which the reaction in a titration is just complete, identified by a sharp, permanent colour change in the indicator.

The rough titre is not included because it is performed rapidly to estimate the end-point and is less accurate than subsequent careful titrations. Using it would make the average less reliable.

True.

Burette readings must be recorded to the nearest 0.05 cm³ to ensure accuracy. The eye should be kept level with the meniscus when taking a reading.

A standard solution is a solution whose concentration is precisely known. In a titration, it is used to find the concentration of the unknown solution.

The average titre to use is 14.95 cm³.

Titre 1 (14.90 cm³) and Titre 2 (15.00 cm³) are concordant because they differ by only 0.10 cm³, which is within 0.2 cm³. The average is (14.90 + 15.00) / 2 = 14.95 cm³.

The test for oxygen is to place a glowing splint into the test tube containing the gas. If oxygen is present, the glowing splint relights.

False.

A squeaky pop with a lit splint is the test for hydrogen, not oxygen. The test for oxygen uses a glowing splint, which relights in the presence of oxygen.

To test for carbon dioxide, the gas is bubbled through limewater. If CO₂ is present, the solution turns cloudy (or milky).

The test for hydrogen uses a lit (burning) splint held at the mouth of the test tube — a squeaky pop confirms hydrogen. The test for oxygen uses a glowing splint placed inside the tube — the splint relights if oxygen is present.

True.

Limewater is a unique test for carbon dioxide — it turns cloudy or milky only in the presence of CO₂. A lit splint being extinguished is not a unique test and should not be given as an answer.

Limewater is an aqueous solution of calcium hydroxide used as a chemical test for carbon dioxide gas. It turns cloudy or milky when CO₂ is bubbled through it.

A glowing splint is used because the test for oxygen checks whether the gas supports combustion. A glowing splint relights in pure oxygen, confirming its presence. A lit splint is used to test whether the gas itself burns, which is the test for hydrogen.

Hydrogen gas is often produced when an acid reacts with a reactive metal. It is identified by holding a lit (burning) splint at the mouth of the test tube.

A flame test is a laboratory technique used to identify a metal ion based on the characteristic colour it produces when a sample is heated in a Bunsen flame.

False.

Flame test colours are provided on page 6 of the SQA Data Booklet. The required skill is locating and reading the correct information from the booklet, not memorising it.

The wire loop must be cleaned to remove any contaminating ions from previous tests. If it is not cleaned, flame colours from different ions could mix, making it impossible to identify the correct result. The strong yellow of sodium is especially likely to mask other colours.

In a flame test, the wire loop is first cleaned by dipping it in dilute hydrochloric acid and heating it in a blue flame until no colour is produced.

A precipitate is an insoluble solid that forms when two solutions are mixed and react together in a precipitation reaction.

The reagent used to test for chloride ions is silver nitrate solution (AgNO₃ (aq)). A positive result is the formation of a white precipitate of silver chloride.

True.

Adding barium chloride solution (BaCl₂ (aq)) to a sample containing sulfate ions (SO₄^2-) produces a white precipitate of barium sulfate, confirming their presence.

To test for carbonate ions, a small amount of dilute acid is added to the sample. A positive result is fizzing (effervescence), as carbon dioxide gas is produced.

The three standard chemical tests are:

1. Silver nitrate solution — identifies chloride ions (Cl^-)

2. Barium chloride solution — identifies sulfate ions (SO₄^2-)

3. Dilute acid — identifies carbonate ions (CO₃^2-)

A labelled diagram is a simple 2D drawing of an experimental setup, drawn with clear lines and labels identifying all important parts of the apparatus.

True.

Every column heading in a results table must state the quantity and include its units, usually written after a forward slash, for example: Time / s.

A bar chart should be used when comparing data across different non-numerical categories, such as comparing the melting points of different named plastics. A line or scatter graph is used when plotting the relationship between two numerical sets of data.

When drawing a graph, the independent variable is plotted on the x-axis and the dependent variable is plotted on the y-axis.

Repeating an experiment and calculating an average improves the reliability of the data. It helps to smooth out any small random errors made during individual measurements.

False.

An anomalous result (outlier) is a reading that does not fit the pattern of the others and must be identified and discarded before calculating the average.

A full answer should follow the identify-improve-justify structure:

1. Identify — state a specific weakness or source of error in the method

2. Improve — suggest a specific practical change to the apparatus or procedure

3. Justify — explain why the change is an improvement

When drawing a line of best fit, you should draw a single, smooth line that shows the overall trend of the data. It does NOT have to pass through every point.

True.

Good scientific diagrams are simple 2D cross-section drawings with clear lines, labels on all important parts and no shading.

The student could place a white tile under the conical flask. This makes the colour change at the end-point much clearer and easier to see accurately.