Textbook Solution: Chemical Changes and Reactions | IGCSE Cambridge Science for Year 6 - Class 6 PDF Download

Making Compounds

Ans: Atoms of different elements can be distinguished in a particle diagram by using different colors or shapes for each element. Additionally, labeling each type of atom with its chemical symbol can further clarify distinctions.

Q2. Explain how you can tell from the particle diagram that magnesium and magnesium oxide are in the solid state, and oxygen is in the gas state.

Ans: In the particle diagram, magnesium and magnesium oxide can be identified as solids by their tightly packed and orderly arrangement of particles, indicating strong intermolecular forces typical of solids. Oxygen, shown as dispersed individual particles or pairs, suggests it is in the gaseous state where particles are well-separated and free-moving.

Q3. Describe how the properties of magnesium oxide are different from the elements it is made from.

Ans: Magnesium oxide is an ionic compound formed from magnesium and oxygen. It differs from its constituent elements by being more stable, having a high melting point, and being an insulator of electricity. In contrast, magnesium is a reactive metal with relatively low melting point and is a good conductor of electricity, whereas oxygen is a gas that supports combustion.

Q4. a) Name two elements mentioned. b) Name the compound. c) This is a chemical reaction. Explain what this means.

Ans: a) The two elements mentioned are iron and sulfur. b) The compound formed is iron sulfide. c) A chemical reaction involves the rearrangement of atoms to form new substances. In this case, iron and sulfur atoms react to form iron sulfide, a new chemical compound, indicating that their original properties have changed.

Q5. Give four ways in which you can tell that a reaction has happened.

Ans: Four ways to tell that a reaction has occurred include: 1) Change in color, 2) Formation of a gas (bubbles), 3) Release or absorption of heat, and 4) Formation of a precipitate (a solid that forms from solution).

Q6. A stoppered test tube contains carbon dioxide from a reaction. A few drops of limewater are added, the stopper is replaced and the test tube is shaken. Describe what you would see.

Ans: When limewater is added to a test tube containing carbon dioxide and shaken, it turns cloudy. This cloudiness is due to the formation of calcium carbonate, indicating the presence of carbon dioxide.

Q7. Describe how you can use a wooden splint to show that a reaction produces hydrogen gas and not oxygen gas.

Ans: To test for hydrogen gas, you can use a glowing wooden splint; when it is inserted into a container with hydrogen gas, the gas will ignite and produce a distinctive 'pop' sound. This reaction does not occur with oxygen, which instead supports the glowing splint to relight if it is still glowing.

Forming Precipitates

Q1. Give the difference between a soluble substance and an insoluble substance.

Ans: A soluble substance is one that can dissolve in a solvent, such as water, forming a solution. An insoluble substance, on the other hand, does not dissolve in the solvent and remains separate, often settling at the bottom or floating on top.

Q2. Describe the difference between a mixture of water and a soluble substance, and a mixture of water and an insoluble substance.

Ans: A mixture of water and a soluble substance results in a homogeneous solution where the solute is completely dissolved and uniformly distributed throughout the solvent. In contrast, a mixture of water and an insoluble substance does not form a uniform solution; the insoluble substance does not dissolve and instead remains visibly separate within the mixture.

Q3. Suggest a reason why the particle model in the diagram does not show all the atoms involved.

Ans: The particle model typically does not show all the atoms involved in order to simplify the representation and focus on illustrating key concepts and interactions. Showing every atom could make the diagram overly complex and difficult to understand, especially for educational purposes where clarity and understanding of the concept are prioritized.

Neutralisation Reactions

Q1. What does the reading of 9.11 on the pH meter in figure 5.16 tell you about the blue solution?

Ans: The reading of 9.11 on the pH meter indicates that the blue solution is slightly alkaline, as pH values above 7 are considered alkaline.

Q2. What does the result in figure 5.17 tell you about the solution being tested?

Ans: The result in figure 5.17, which shows Universal Indicator paper and a pH color chart, allows you to determine the pH of a solution by matching the color of the indicator paper to the chart. This helps to visually estimate whether the solution is acidic, neutral, or alkaline based on the color match.

Q3. What pH does a neutral substance have?

Ans: A neutral substance has a pH of 7.

Q4. Sometimes, acidic or alkaline solutions need to be neutralised. Describe how you could neutralise an acidic solution and how you would know that neutralisation had happened.

Ans: To neutralise an acidic solution, you can gradually add an alkali to it until the pH reaches around 7, indicating a neutral solution. You would know that neutralisation has occurred when the pH indicator, such as Universal Indicator, changes color to indicate a neutral pH.

Q5. What colour would Universal Indicator change to if added to a neutral solution? Use figure 5.17 to help you.

Ans: If Universal Indicator is added to a neutral solution, it would change to green, as indicated by the pH color chart in figure 5.17 for a pH of 7.

Q6. The simple experiment shown in figure 5.18 could be changed by using a measuring cylinder to add the alkali. How could you show the way that the pH changes as you neutralise the acid with an alkali? How could you improve the experiment to measure the pH?

Ans: To show how the pH changes as you neutralise the acid with an alkali, you could use a pH meter to continuously monitor the pH. To improve the experiment, adding a burette instead of a measuring cylinder could allow for more precise measurement of the alkali added, providing more controlled and accurate pH adjustments.

End of the Chapter Review

Q1. List two ways in which you could tell that a chemical reaction had happened.

Ans: You could tell a chemical reaction had happened by observing a change in color or the formation of a gas.

Q2. Describe how you would test for hydrogen gas.

Ans: To test for hydrogen gas, you can hold a lit splint near the gas; if hydrogen is present, it will produce a distinct 'pop' sound.

Q3. Describe how you would test for oxygen gas.

Ans: To test for oxygen gas, you can introduce a glowing splint into a sample of the gas. If the splint reignites, oxygen is present.

Q4. What is a precipitate?

Ans: A precipitate is a solid that forms from a solution during a chemical reaction and settles out of the solution.

Q5. How can you tell that a precipitate has been made in a reaction?

Ans: You can tell a precipitate has been made in a reaction when you observe a solid forming in the solution, which often settles at the bottom of the container.

Q6. Copy and complete the following sentences about reacting acids and alkalis together.

Ans: When an acid and an alkali react together they produce a salt and water solution. This type of reaction is called neutralization.

Q7. Bees and wasps are insects which use a sting to defend themselves.

a) What does this information tell you about bee stings and wasp stings?

Ans: This information tells us that bee stings are more acidic (pH of 2) and wasp stings are more alkaline (pH of 10).

b) Suggest the pH of a solution that could be used to neutralise a bee sting.

Ans: A solution with a pH around 7 to 9 could be used to neutralize a bee sting.

c) Suggest the pH of a solution that could be used to neutralise a wasp sting.

Ans: A solution with a pH around 4 to 6 could be used to neutralize a wasp sting.

Q8. A student heated some green copper carbonate powder. The powder bubbled and turned black.

a) Give two pieces of evidence that a chemical reaction took place.

Ans: The evidence that a chemical reaction took place includes the bubbling which indicates gas release and the color change from green to black.

b) How can you tell that one of the products was a gas?

Ans: You can tell one of the products was a gas because the bubbling observed during the reaction usually signifies gas formation.

c) The student thinks that carbon dioxide was formed. Describe a test to show this.

Ans: To test for carbon dioxide, you can pass the gas through limewater (calcium hydroxide solution). If carbon dioxide is present, the limewater will turn cloudy.

Q9. Hydrochloric acid reacts with sodium hydroxide. Sodium chloride and water are formed in the reaction.

a) Will you see a precipitate form in this reaction? Give a reason for your answer.

Ans: No, a precipitate will not form in this reaction because both sodium chloride and water are soluble.

b) Name the type of reaction this is.

Ans: This is a neutralization reaction.

Q10. Iron reacts with copper oxide. Copper and iron oxide form in the reaction. Draw a particle diagram to model this reaction.

Ans: (Not displayed but described) The diagram should show iron atoms reacting with copper oxide molecules resulting in copper atoms and iron oxide molecules.

Q11. Some students did an experiment to measure the change in pH when an alkali was added to an acid. They added an alkali to an acid a little at a time using a measuring cylinder. They measured the pH every time they added the alkali.

a) Plot a line graph of the results on graph paper. Make sure that you plot the pH on the vertical (y) axis and volume of alkali added on the horizontal (x) axis.

Ans: The graph should show a gradual increase in pH from 5.0 to 9.0 as more alkali is added.

b) Describe what you can see happening from the results on the graph.

Ans: From the graph, you can see that as the volume of alkali added increases, the pH of the mixture gradually increases, indicating that the acid is being neutralized by the alkali.

Q12. Sodium thiosulfate solution and dilute hydrochloric acid are clear, colourless solutions. They react together to make four products:

Explain three ways in which you might be able to tell that a reaction is happening.

Ans: You could tell a reaction is happening by observing a temperature change, the formation of a precipitate, or the evolution of a gas.

b) Name the type of reaction this is. Give a reason for your answer.

Ans: This is a double displacement reaction because two compounds exchange ions to form two new compounds.

c) Describe how you could test sodium thiosulfate to see if it is an alkali.

Ans: To test if sodium thiosulfate is an alkali, you could use pH paper or a pH meter. If it shows a pH greater than 7, it is an alkali.

Q13. Use table 5.2 in Chapter 5 Topic 2 to help you answer this question.

A student mixes sodium sulfate solution with calcium hydroxide solution and with copper chloride solution.

a) For each mixture, predict whether a precipitate will form.

Ans: A precipitate will form in the mixture of calcium hydroxide and copper chloride because they react to form calcium chloride and copper hydroxide, where copper hydroxide is a precipitate.

b) Name any precipitate formed in your answer to part (a).

Ans: The precipitate formed is copper hydroxide.

c) Suggest the names of two solutions that will react together to make a precipitate of iron hydroxide.

Ans: Iron sulfate solution and sodium hydroxide solution will react to form a precipitate of iron hydroxide.

End of Stage Review

Q1. Why should you wear eye protection when using this substance?

Ans: Eye protection is necessary when using this substance because it is corrosive, as indicated by the symbol on the label, and can cause severe damage to the eyes.

Q2. Copy and complete the table, to show the missing indicator colours.

Ans:

Q3. Which solution in the table is the most strongly acidic?

Ans: Vinegar is the most strongly acidic solution in the table, as indicated by the red color in Methyl orange.

Q4. What further test would you need to do to identify which solution is the most strongly alkaline?

Ans: To identify the most strongly alkaline solution, you could use a pH meter to measure the exact pH of each solution, particularly those that are pink with Phenolphthalein, indicating they are alkaline.

Q5. Name a metal used to make cell phones.

Ans: Copper is a metal used to make cell phones.

Q6. Give one property that makes metals suitable for the electrical components of the phone.

Ans: Conductivity is a property that makes metals suitable for the electrical components of the phone, as it allows the metal to transmit electrical signals effectively.

Q7. Describe why alloys of gold are often used, rather than pure gold.

Ans: Alloys of gold are often used rather than pure gold because they are harder and more durable, making them more suitable for the wear and tear on the electronic components in which they are used.

Q8. Give two other ways to tell that a reaction has happened.

Ans: Two other ways to tell that a reaction has happened are the production of a gas (bubbling or fizzing) and the formation of a precipitate (a solid that forms and settles out of a liquid mixture).

Q9. When hydrochloric acid is mixed with silver nitrate solution, the mixture turns cloudy white. Name this type of reaction.

Ans: This type of reaction is called a precipitation reaction.