Tests for gases

A student investigates the reaction that occurs when zinc carbonate is heated.

Figure 1 shows the apparatus the student uses.

What is the name of the piece of apparatus labelled G in Figure 1?

State what apparatus should be used to heat the zinc carbonate in Figure 1.

The word equation for the reaction that occurs when zinc carbonate is heated is

zinc carbonate zinc oxide + carbon dioxide

The carbon dioxide bubbles through the limewater.

State what is seen when carbon dioxide bubbles through the limewater.

Another student investigates the change in mass that takes place when zinc carbonate is heated.

Figure 2 shows the results the student records at the start of the investigation.

Using the results in Figure 2, calculate the mass of zinc carbonate in the test tube.

mass of zinc carbonate =_........ g

The student then

• heats the test tube containing the zinc carbonate for five minutes

• places the hot test tube on a bench mat to cool

• finds the mass of the test tube and contents.

Describe what the student should do next to make sure that all the zinc carbonate has reacted completely.

This question is about metals and their reactions with acids.

Place ticks () in boxes to show which elements in the table are metals.

Use the periodic table to help you answer this question.

Three equal sized pieces of the metals E, F, G and H are put into separate test tubes containing equal volumes of dilute sulfuric acid.

The results in Figure 7 show the test tubes after half a minute.

Arrange the metals E, F, G and H in order of reactivity.

The bubbles that form are hydrogen.

Which row of the table shows the test and result that confirm the gas is hydrogen?

When zinc reacts with very dilute nitric acid, zinc nitrate is formed.

The formula of zinc nitrate is Zn(NO₃)₂

Calculate the relative formula mass of zinc nitrate, Zn(NO₃)₂

(relative atomic masses: Zn = 65, N = 14, O = 16)

relative formula mass =_........

A student investigates the reaction between a metal and a dilute acid using the apparatus shown in Figure 8.

Cotton wool is placed in the top of the apparatus as shown, to stop any acid from spraying out.

The total mass of the apparatus is shown at the start of the reaction and after the reaction has ended.

start of reaction end of reaction

Figure 8

State two ways the student would know that the reaction has ended.

Malachite, Cu₂CO₃(OH)₂, is a mineral containing copper.

When heated, malachite decomposes to form copper oxide, carbon dioxide and water.

Which apparatus should be used to heat the malachite and test for carbon dioxide?

When carbon dioxide is bubbled through limewater, a precipitate forms. This precipitate solid makes the limewater cloudy.

Name the precipitate that makes the limewater cloudy.

The equation for the decomposition of malachite when it is heated is

Calculate the maximum mass of copper oxide that can be formed when 22.1 g of malachite is decomposed completely.

(relative atomic masses: Cu = 63.5, O = 16.0, C = 12.0, H = 1.0)

maximum mass of CuO =_........

Copper can be extracted from copper oxide by heating the copper oxide with carbon.

A student investigated two methods to extract the maximum mass of copper from copper oxide.

Figure 3 shows the two methods.

Comment on the strengths and weaknesses of these two methods.

The elements in group 7 of the periodic table are called the halogens.

Give the symbol of the element astatine.

Use the periodic table to help you.

What is the colour of chlorine gas at room temperature?

Damp blue litmus paper can be used to test for chlorine gas.

Use words from the box to complete the sentences.

When damp blue litmus paper is added to chlorine gas there are two colour changes.

The damp blue litmus paper changes from blue to_........ .

The colour of the litmus paper then changes to_........ .

Figure 1 shows the atomic number and boiling point of some of the halogens.

Describe the trend in the boiling points of the first three halogens in Figure 1.

Using the information in Figure 1, predict the boiling point of iodine.

Write your prediction on the answer line below.

Which gas made up the greatest proportion of the Earth’s early atmosphere?

Figure 6 shows the composition by volume of the Earth’s atmosphere today.

Give the name of the gas in Earth’s atmosphere today that makes up the largest proportion.

Using the information in Figure 6, calculate the percentage of argon and other gases in the Earth’s atmosphere today.

The percentage of oxygen in air can be measured using the apparatus shown in Figure 7.

The oxygen in the air reacts with the heated zinc.

Explain one change that would be seen during the reaction.

Name the product formed when the zinc reacts with oxygen in this experiment.

Figure 8 shows the electron configuration of an argon atom.

State why argon is unreactive.

Argon makes up 0.93% of the Earth’s atmosphere today.

Calculate the volume of argon, in cm³, in 4000 cm³ of air.

Sodium carbonate has the formula Na₂CO₃.

Sodium carbonate contains Na⁺ ions and CO₃^2– ions.

The atomic number of sodium is 11.

What is the electronic configuration of the Na⁺ ion?

Explain why solid sodium carbonate cannot conduct electricity but a solution of sodium carbonate can conduct electricity.

Calculate the percentage by mass of sodium in sodium carbonate, Na₂CO₃.

(relative atomic masses: C = 12, O = 16, Na = 23)

percentage by mass of sodium =_........

*(c) A student has three solids, A, B and C.

The solids are sodium carbonate, powdered zinc and copper oxide, but the student does not know which solid is which.

The student reacted each solid with dilute sulfuric acid.

Figure 15 shows the student’s observations and the results of tests on any gases produced.

Use the observations and results in Figure 15 to identify which solid is which.

Your answer should include

• how each test result helps you to identify the solid

• word equations to support your answer.

This question is about the atmosphere.

Describe the test to show that a gas is oxygen.

Copper reacts with oxygen to form copper oxide.

2.100 g of copper will react completely with 0.529 g of oxygen.

In an experiment, 4.200 g of copper is heated with 50.000 g of oxygen until the reaction is complete.

Calculate the mass of oxygen remaining at the end of the experiment.

Helium, neon and argon are all inert.

Explain, in terms of electrons, why these gases are inert.

Two pieces of steel can be joined by heating the metal pieces with a very hot flame.

This process is often carried out in an argon atmosphere rather than in air.

Which property makes argon gas suitable for this use?

*(d) Figure 16 shows how plant life and the atmosphere of Earth have changed over time.

Explain the effect that plant life has had on the Earth’s atmosphere and the temperature of the Earth.

You should refer to the information in Figure 16 including

• the plant life

• the amounts of carbon dioxide

• the amounts of oxygen

This question is about the atmosphere.

Describe the test to show that a gas is oxygen.

Copper reacts with oxygen to form copper oxide.

2.100 g of copper will react completely with 0.529 g of oxygen.

In an experiment, 4.200 g of copper is heated with 50.000 g of oxygen until the reaction is complete.

Calculate the mass of oxygen remaining at the end of the experiment.

mass of oxygen =_........

Helium, neon and argon are all inert.

Explain, in terms of electrons, why these gases are inert.

Two pieces of steel can be joined by heating the metal pieces with a very hot flame.

This process is often carried out in an argon atmosphere rather than in air.

Which property makes argon gas suitable for this use?

Carbon dioxide is removed from the atmosphere by plants and stored in plants and soil as carbon compounds.

Figure 5 shows the relative amounts of carbon stored in plants and soils in different environments.

relative amount of carbon stored per unit area

Figure 5

It has been suggested that preserving coastal ecosystems is more effective than reforestation in the mitigation of climate change.

Describe how the data in Figure 5 supports this suggestion.

Damp iron wool reacts with oxygen in the air. A student uses the apparatus in Figure 7 to investigate the percentage of oxygen in the atmosphere.

The initial volume of air in the measuring cylinder was 18.0 cm³.

The student left the apparatus overnight.

The volume of gas in the measuring cylinder the next day was 14.5 cm³.

To the nearest whole number, what percentage of the air has reacted with the iron wool?

Describe one improvement the student could make to this method to ensure that all of the oxygen in the measuring cylinder has reacted.

When hydrocarbon fuels are burned, the products are water and carbon dioxide.

Describe what needs to be done to the apparatus in Figure 8 to collect the water and show that carbon dioxide has been produced.

A hydrocarbon, C_xH_y, is burned in excess oxygen, forming 26.4 g of carbon dioxide and 5.4 g of water.

The relative formula mass of C_xH_y is 78.

Calculate the molecular formula of the hydrocarbon C_xH_y.

(relative atomic masses: H = 1.0, C = 12; relative formula masses: H₂O = 18, CO₂ = 44)

molecular formula =_........

Figure 13 shows part of the reactivity series of metals.

Which metal reacts when added to cold water?

A student investigates the reactivity of four different metals.

The student adds an equal‑sized piece of each metal to separate test tubes containing dilute hydrochloric acid.

The student’s observations for zinc and copper are recorded in Figure 14.

Use the information in Figure 13 and in Figure 14 to predict the observations for the reactions of magnesium and of iron with dilute hydrochloric acid.

magnesium

iron

When metals react with acids, hydrogen gas is produced.

Describe the test to show that the gas is hydrogen.

When magnesium reacts with hydrochloric acid, magnesium chloride and hydrogen are formed.

Complete the balanced equation for the reaction.

*(c) There are three common methods of obtaining metals from the Earth’s crust:

• mine the pure metal

• mine the metal ore and heat it with carbon

• mine the metal ore and electrolyse the molten compound.

The method used to obtain a metal is linked to its position in the reactivity series of metals.

Aluminium, gold, iron, and silver are some commonly used metals.

Use the reactivity series in Figure 13 to state and explain the method chosen to obtain each of these four metals.

Chlorine gas can be prepared by reacting concentrated hydrochloric acid with solid potassium manganate(VII).

Figure 10 shows the apparatus used.

Figure 11 shows the hazard symbols for concentrated hydrochloric acid, potassium manganate(VII) and chlorine gas.

Use the information in Figure 11 to help you answer (a)(i) and (a)(ii).

What are the hazards associated with potassium manganate(VII)?

Explain one precaution that should be taken when preparing the sample of chlorine gas.

precaution

reason

State the purpose of the delivery tube.

Suggest why damp blue litmus is placed at the top of the gas jar.

In the reaction, potassium manganate(VII), KMnO₄, reacts with hydrochloric acid to form manganese chloride, MnCl₂, potassium chloride, chlorine and water.

Write the balanced equation for the reaction.

Magnesium carbonate has the formula MgCO₃.

Magnesium carbonate contains Mg²⁺ and CO₃²⁻ ions.

The atomic number of magnesium is 12.

What is the electronic configuration of the Mg²⁺ ion?

Explain why solid magnesium carbonate cannot conduct electricity but solid magnesium can.

Calculate the percentage by mass of magnesium in magnesium carbonate, MgCO₃.

(relative atomic masses: C = 12.0, O = 16.0, Mg = 24.0)

*(c) A student has two separate test tubes containing sulfuric acid.

The student adds a spatula measure of magnesium carbonate, MgCO₃, to the first test tube and a piece of magnesium to the second test tube.

Explain what the student would see in each test tube and the tests that they should carry out to identify the gases produced.

Your answer should include word equations for the reactions that would take place.

This question is about gases.

When sodium is added to water, hydrogen gas is produced.

Which observation shows that a gas has been produced?

Some damp litmus paper is placed in a gas. The litmus paper is bleached.

Which gas bleaches damp litmus paper?

When calcium carbonate is heated it decomposes.

When 5.000 g of calcium carbonate is heated, the mass of solid remaining is 2.800 g.

Calculate the mass of carbon dioxide that has been released.

Give your answer to three significant figures.

A diagram of an atom of helium is shown in Figure 15.

Explain, using Figure 15, why helium is inert.

Helium is used to fill balloons.

State one property of helium, apart from it being inert, that makes it suitable for filling balloons.

*(e) Figure 16 shows the relative amounts of three gases in the early atmosphere compared to the composition of today’s atmosphere.

Natural processes and human activities have altered the relative amounts of these gases in the atmosphere.

Explain how the relative amount of each of the gases in Figure 16 has increased or decreased over time.

When chromium reacts with oxygen, chromium oxide is formed.

Write the word equation for this reaction.

What type of reaction occurs when chromium reacts with oxygen?

Calculate the relative formula mass of chromium oxide, Cr₂O₃.

(relative atomic masses: O = 16, Cr = 52)

relative formula mass =_........

Three different metals are added to separate test tubes of acid.

The observations are shown in Figure 4.

Place the metals in order of reactivity from most to least reactive.

Hydrogen is given off when magnesium reacts with acid. The hydrogen is tested by collecting the gas in a test tube and igniting it.

What is the safest way to ignite the gas?

State the observation made in this test that shows that the gas is hydrogen.

Iron is extracted by heating iron oxide with carbon. Electrolysis of molten iron oxide is not used to extract iron.

State why iron can be extracted by heating iron oxide with carbon.

State why electrolysis is not used to extract iron.

Figure 9 shows a sample of hydrogen peroxide solution decomposing to form water and oxygen gas.

Write the word equation for hydrogen peroxide solution decomposing.

In this reaction hydrogen peroxide is a solution, water is a liquid and oxygen is a gas.

Draw one straight line from each substance to its correct state symbol.

Describe the test to show the gas produced is oxygen.

Figure 10 shows the electron arrangement for an atom of hydrogen and an atom of oxygen.

Complete the dot and cross diagram in Figure 11 for a molecule of water, H₂O.

Draw outer shell electrons only.

Liver contains the enzyme catalase.

A piece of liver was added to another sample of hydrogen peroxide solution.

Figure 12 shows the results.

Figure 13 shows a graph of the volume of oxygen produced from the hydrogen peroxide with and without liver.

Figure 13

Complete the missing label on the axis of the graph.

Describe what the graph shows about the difference in decomposition of hydrogen peroxide with and without liver.

Describe how the apparatus in Figure 12 could be modified to find the volume of gas produced when the liver is added to the hydrogen peroxide.

This question is about potassium and zinc.

Which of the following temperatures is most likely to be the melting point of potassium?

Explain how the electronic configuration of an atom of potassium is related to its position in the periodic table.

Potassium reacts with oxygen to form potassium oxide.

Describe the test to show that a gas is oxygen.

Potassium oxide is ionic.

Write the electronic configurations for the ions in potassium oxide, K₂O.

Figure 6 shows two gas syringes connected by a glass tube.

Inside the glass tube there are some pieces of zinc. Zinc reacts with oxygen at a temperature of over 225 °C. Not all the oxygen reacts at once, the oxygen reacts only when in contact with the zinc.

Devise a plan to find the volume of oxygen contained in a known volume of air, using the apparatus shown in Figure 6.

Figure 11 shows the apparatus that can be used to electrolyse sodium sulfate solution using inert electrodes.

Hydrogen is produced at the negative electrode during electrolysis.

Describe the test to show the gas is hydrogen.

What is the name of gas X that forms at the positive electrode?

State what is meant by the term electrolysis.

The sodium sulfate solution was made by dissolving 28.4 g of sodium sulfate in water to make 250 cm³ of solution.

Calculate the concentration of this solution in g dm⁻³.

Give your answer to three significant figures.

concentration =_........ g dm

The ions present in sodium sulfate are

Write the formula of sodium sulfate using this information.

In Figure 11, the gases given off at the electrodes are collected in test-tubes. However, the actual volume of gases cannot be measured using these test-tubes.

Suggest what apparatus could be used in place of the test-tubes in Figure 11 to measure the volume of gases given off.

State what could be added into the circuit to show a current is flowing during electrolysis.

The word equation for the reaction between copper carbonate and dilute sulfuric acid is

copper sulfuric copper carbon

Complete the balanced equation for this reaction.

Calculate the relative formula mass of copper carbonate, CuCO₃. (relative atomic masses: C = 12.0, O = 16.0, Cu = 63.5)

relative formula mass of CuCO₃ =_........

What is the chemical test to show that a gas is carbon dioxide?

Figure 12 shows a conical flask containing dilute sulfuric acid. Copper carbonate is added to the acid in the flask. The copper carbonate is added one spatula measure at a time until the reaction has finished.

State two observations that would show the reaction has finished.

*(ii) Describe how you would obtain a solution of copper sulfate from the mixture and how you would obtain pure, dry copper sulfate crystals from this solution.

Your description should include the apparatus you would use.

You may wish to use diagrams in your answer.

Figure 3 shows the apparatus that can be used to electrolyse sodium sulfate solution using inert electrodes.

Hydrogen is produced at the negative electrode during electrolysis.

Describe the test to show the gas is hydrogen.

What is the name of gas X that forms at the positive electrode?

State what is meant by the term electrolysis.

The sodium sulfate solution was made by dissolving 28.4 g of sodium sulfate in water to make 250 cm³ of solution.

Calculate the concentration of this solution in g dm⁻³.

Give your answer to three significant figures.

concentration =_........ g dm

The ions present in sodium sulfate are

Write the formula of sodium sulfate using this information.

The word equation for the reaction between copper carbonate and dilute sulfuric acid is

copper sulfuric copper carbon

Complete the balanced equation for this reaction.

Calculate the relative formula mass of copper carbonate, CuCO₃. (relative atomic masses: C = 12.0, O = 16.0, Cu = 63.5)

relative formula mass of CuCO₃ =_........

What is the chemical test to show that a gas is carbon dioxide?

Figure 4 shows a conical flask containing dilute sulfuric acid. Copper carbonate is added to the acid in the flask. The copper carbonate is added one spatula measure at a time until the reaction has finished.

State two observations that would show the reaction has finished.

The electronic configuration of carbon is 2.4 The electronic configuration of oxygen is 2.6

Draw a dot and cross diagram for a molecule of carbon dioxide.

Show outer electrons only.

Figure 3 shows the structure of two monomers.

Monomer B contains a carboxylic acid group.

Describe what you would see when a small amount of solid sodium carbonate is added to a solution of monomer B.

When monomer A and monomer B react together they polymerise to form a polymer and one other product.

Name the type of polymerisation that takes place and name the other product.

type of polymerisation_........

name of other product_........

A naturally occurring polymer is made by combining monomers called nucleotides.

Give the name of this natural polymer.

Some polymerisation reactions produce ammonia as a waste product.

A student is given a sample of pure, dry ammonia gas.

The student suggests the following method to test for ammonia gas.

step 1 take some dry, blue litmus paper step 2 place the dry litmus paper into the dry gas step 3 observe any change in colour of the litmus paper

This test for ammonia will not work.

Give two changes that should be made to this test for it to work.

Alcohols can be dehydrated.

Complete the balanced equation for the dehydration of butan‑1‑ol by drawing the structures of the two products in the boxes. Name the two products.

(a) Water, acidified with sulfuric acid, is decomposed by electrolysis. The water is decomposed to produce hydrogen and oxygen.

A sample of hydrogen is mixed with air and ignited.

State what would happen.

Throughout the experiment the volume of hydrogen and the volume of oxygen are measured at two-minute intervals.

The results are shown in Figure 2.

Describe, using the data in Figure 2, what the results show about the volumes of hydrogen and of oxygen produced in this experiment.

Molten lead bromide is electrolysed.

The products of this electrolysis are

Calcium nitrate and calcium carbonate are both ionic compounds.

Calcium nitrate mixed with water behaves as an electrolyte. Calcium carbonate mixed with water does not behave as an electrolyte.

Explain, in terms of solubility and movement of ions, this difference in behaviour.

When molten zinc chloride is electrolysed, zinc ions, Zn²⁺, form zinc atoms.

Write the half equation for this reaction.

Plants release oxygen into the atmosphere.

What is the name of the process that releases oxygen into the atmosphere?

The atmosphere contains 21% of oxygen.

Figure 1 shows an incomplete bar chart of the main gases in the atmosphere.

Complete the bar chart by showing the percentage of oxygen in the atmosphere.

Calculate the volume of oxygen present in 300 cm³ of air.

(volumes are measured under the same conditions of temperature and pressure)

An atom of an element has an atomic number and a mass number.

Draw one straight line from each of these to the numbers of subatomic particles it shows to be present in an atom.

number of subatomic particles in an atom

Which test shows a gas is oxygen?

Fluorine, chlorine, bromine, iodine and astatine are elements in group 7.

Describe the test to show that a gas is chlorine.

Bromine reacts with hydrogen to form hydrogen bromide. Hydrogen bromide dissolves in water to form a solution.

State the name of the solution formed.

There is a trend in the colour and the state of the halogens at room temperature.

Predict the colour and state of astatine at room temperature.

Bromine, chlorine and iodine are dissolved in water to make aqueous solutions. Potassium iodide solution is added to each of these solutions.

Figure 6 shows the observations.

Explain the observations shown in the table.

Fluorine reacts vigorously with iron to produce iron(III) fluoride, FeF₃.

Write the balanced equation for this reaction.

The molecular formula of butene is C₄H₈.

Which of the following is the empirical formula of butene?

Calculate the relative formula mass of butene, C₄H₈.

(relative atomic masses: H = 1, C = 12)

relative formula mass_........

When burnt completely in air, butene forms carbon dioxide and water.

Balance the equation for this reaction by putting numbers in the spaces provided.

Describe the test to show that a gas is carbon dioxide.

Substance X is a gas at room temperature. It is a simple molecular, covalent substance.

Which row of the table shows the properties that substance X is most likely to have?

Diamond has a giant covalent structure.

State one property of diamond that is the result of its giant covalent structure.

State two characteristic properties of metals.

Acids are used to make salts.

Give the name of the acid used to make chlorides.

Salts of metals can be prepared by reacting the metal with an acid to produce the salt and hydrogen.

Describe the test to show that the gas is hydrogen.

Nickel is a metal.

Explain how the structure of a nickel atom, Ni, changes when it forms a nickel ion, Ni²⁺

A nickel sulfate solution is made by dissolving 23.5 g of nickel sulfate to make 250 cm³ of solution.

Calculate the concentration of the solution in g dm^–3.

concentration =_........ g dm

Excess solid nickel carbonate is added to dilute sulfuric acid in a beaker.

nickel sulfuric nickel carbon

Nickel sulfate is formed in solution.

Describe how a sample of pure, dry nickel sulfate crystals can be obtained from the mixture of nickel sulfate solution and excess solid nickel carbonate in the beaker.

(a) Salts of metals can be prepared by reacting the metal with an acid to produce the salt and hydrogen.

Describe the test to show the gas is hydrogen.

Nickel is a metal.

Explain how the structure of a nickel atom, Ni, changes when it forms a nickel ion, Ni²⁺.

A nickel sulfate solution is made by dissolving 23.5 g of nickel sulfate to make 250 cm³ of solution.

Calculate the concentration of the solution in g dm⁻³.

concentration =_........ g dm

Excess solid nickel carbonate is added to dilute sulfuric acid in a beaker.

nickel sulfuric nickel carbon

Nickel sulfate is formed in solution.

Describe how a sample of pure, dry nickel sulfate crystals can be obtained from the mixture of nickel sulfate solution and excess solid nickel carbonate in the beaker.

Hydrogen peroxide decomposes to form water and oxygen.

The rate of this reaction can be found by measuring the volume of oxygen formed after different time intervals.

Hydrogen peroxide solution is placed in a conical flask. The apparatus is set up as shown in Figure 5.

State the name of the piece of apparatus labelled Z in Figure 5.

At the end of the reaction the bung is removed from the conical flask. A glowing splint is put into the gas in the flask.

State what you would see.

A solid catalyst can be used for this reaction.

The experiment is repeated under identical conditions but with the catalyst added.

In the experiment with the catalyst added

decomposed is larger than when no catalyst is present

At the end of the experiment with the catalyst added, the mass of the catalyst remaining is found.

The method used to find the mass of the catalyst remaining is filter the mixture of products and catalyst determine the mass of the filter paper and solid catalyst subtract the mass of a filter paper from the mass of filter paper and solid catalyst.

This method would not give the accurate mass of catalyst remaining.

Which of the following needs to be done to give a more accurate mass?

A given mass of catalyst is more effective if it has a large surface area.

State how you could increase the surface area of some lumps of solid catalyst.

The experiment is repeated three times once using a more dilute solution of hydrogen peroxide once using a lower temperature once using a larger flask

In each case, all other conditions are kept the same.

Circle the word that shows the change in the rate of decomposition in each case.

Complete the balanced equation for the reaction and fill in the two missing state symbols.

(a) A chlorine atom contains 17 electrons, 18 neutrons and 17 protons.

State the mass number of this chlorine atom.

Give the electronic configuration of this chlorine atom.

Describe what you would see if damp, blue litmus paper is placed into chlorine gas.

Chlorine exists as diatomic molecules.

In a molecule, two chlorine atoms are joined by a covalent bond.

Describe what is meant by a covalent bond.

Explain why chlorine is a gas, rather than a liquid, at room temperature.

When the gas hydrogen chloride, HC l, is dissolved in water, a solution forms. Blue litmus paper dipped in this solution turns red.

State why the litmus paper turns red.

Figure 13 lists the halogens in the order in which they appear in group 7 of the periodic table.

The melting points of four of the halogens are given.

Estimate the melting point of iodine.

Bromine reacts with heated iron.

Give the name of one halogen that would react with iron more vigorously than bromine.

(a) A chlorine atom contains 17 electrons, 18 neutrons and 17 protons.

State the mass number of this chlorine atom.

Give the electronic configuration of this chlorine atom.

Describe what you would see if damp, blue litmus paper is placed into chlorine gas.

Chlorine exists as diatomic molecules.

In a molecule, two chlorine atoms are joined by a covalent bond.

Describe what is meant by a covalent bond.

Explain why chlorine is a gas, rather than a liquid, at room temperature.

When the gas hydrogen chloride, HC l, is dissolved in water, a solution forms. Blue litmus paper dipped in this solution turns red.

State why the litmus paper turns red.