Reactions of metals with oxygen, water, and acids
In this topic you will:
- describe the reactions of some metals with oxygen
- compare how reactive some metals are with oxygen
- describe the reactions of some metals with water
- compare how reactive some metals are with water
- describe the reactions of some metals with dilute acid
- compare how reactive some metals are with dilute acid
Key words
- collapses
- inert
- prevent
- reactive
- rusts
- reactivity
- sandpaper
- reagents
- salt
Metals and oxygen
In previous topics you learned about the properties of metals. Now you are going to investigate how different metals react with oxygen.
Think Like a Scientist
You will investigate how different metals react when heated in air. Air contains oxygen, and some metals will react with it to form oxides. Follow the steps below and record your observations carefully.
Equipment: safety glasses, Bunsen burner, heatproof mat, tongs, small pieces of metal such as magnesium, zinc, iron, and copper.
Step 1: Take a small piece of one of the metals.
Step 2: Hold it in tongs and heat it in a Bunsen flame.
Step 3: Record what you observe in a table. Describe what happened to the metal.
Step 4: Repeat Steps 1–3 for each of the other metals.
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Magnesium is usually the most reactive—it burns with a bright white flame and forms magnesium oxide quickly when heated in air.
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I wore safety glasses, used tongs to handle hot metals, kept flammable materials away from the flame, and conducted the experiment on a heatproof mat.
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Sodium and potassium are highly reactive and dangerous when heated in air—they can catch fire or explode, so they are unsafe for school use.
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Gold and silver are expensive and unreactive. They do not easily react with air when heated, so they are not useful for this type of investigation.
Looking at the reactions of metals with oxygen
Many metals react with oxygen if they get hot enough. When you look carefully at the reactions of metals with oxygen, it is possible to identify which metals are more reactive. For example, magnesium is more reactive than iron because magnesium reacts more quickly than iron. This reaction between metals and oxygen is an oxidation reaction.
Some metals react very quickly with oxygen without even being heated. When pieces of sodium, potassium or calcium are taken from their containers, they appear dull. When the pieces are cut, the surface is shiny. The shiny surface soon becomes dull because the metal reacts with the oxygen in the air. The surface becomes covered with a new substance – the oxide of the metal. These metals are so reactive that they have to be stored under oil to prevent them reacting with the water vapour in the air. The layer of metal oxide on the surface prevents any more of the metal from reacting with the air or water vapour.
The general word equation for this reaction is:
metal + oxygen ⟶ metal oxide
Some metals, such as gold, do not react with oxygen. They are generally unreactive. They are described as inert.
Silver reacts slowly with the air. If a silver object is not cleaned it goes black over time, as silver oxide is formed.
Questions
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They are very soft and can be cut easily, which is not typical of most metals that are usually hard.
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Sodium is highly reactive and can be dangerous to touch with bare hands. Gloves protect the scientist from burns or chemical reactions.
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sodium + oxygen ⟶ sodium oxide
The reaction between iron and oxygen
When iron is left in damp air it rusts. The iron reacts with oxygen to form an orange-brown solid, called iron oxide. This is known as rust.
iron + oxygen ⟶ iron oxide
This is not a very useful reaction because it means that the iron changes and no longer has the same properties. A strong iron girder can become rusted and fall apart. This could mean that a building collapses.
The reaction between iron and oxygen only takes place when both water and oxygen are present. The water is not part of the equation, but it is needed for the reaction to happen. The reaction takes a long time to happen – iron is not very reactive with oxygen.
What causes iron to rust?
A new iron nail is placed in each of four test tubes, as in the diagram.
Test tube 1 contains nothing, apart from the nail, and is open to the air.
Test tube 2 contains water and the nail is half in the water. The tube is open to the air. So, this tube has air and water.
Test tube 3 has calcium chloride in the bottom. The calcium chloride absorbs water so the air inside the tube is dry. The tube is stoppered.
Test tube 4 has water that has been boiled to remove as much dissolved gas as possible. On top of the boiled water is a layer of oil. This stops any air entering the water. The tube is stoppered.
| Tube number | Contains | Result |
|---|---|---|
| 1 | moist air | nail is rusted |
| 2 | water and air | nail is very rusty |
| 3 | dry air | no rust |
| 4 | boiled water covered with oil, no air | small amount of rust |
Questions
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To prevent rusting, iron must be kept away from both water and oxygen.
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Test tube 2 caused the fastest rusting because it contained both air and water.
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To make the experiment fair by ensuring only the conditions change, not the material being tested.
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By adding calcium chloride, which absorbs moisture from the air.
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A layer of oil is added on top of boiled water to stop air from reaching the nail.
How can iron be protected?
There are ways that iron can be protected so that it does not rust.
Metals and water
In last topic you learned about the reactions of metals with oxygen. Now you are going to investigate how different metals react with water.
Think Like a Scientist
You will investigate how different metals react with water. Some metals may have an oxide layer that prevents visible reactions, so sandpaper is used to clean the surface.
Equipment: test tubes, test tube rack, sandpaper, forceps, small pieces of metal such as magnesium, zinc, iron, and copper.
Step 1: Use sandpaper to clean the surface of one metal sample.
Step 2: Place the cleaned metal into a test tube of cold water.
Step 3: Record your observations. If no reaction occurs, try again using hot water.
Step 4: Repeat Steps 1–3 for the remaining metal samples.
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Magnesium is usually the most reactive—it may fizz and produce bubbles in cold or warm water, indicating hydrogen gas is being released.
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Example order (may vary): Magnesium > Zinc > Iron > Copper. Magnesium reacts most vigorously, while copper often shows no visible reaction.
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Hot water has more energy, which can speed up the reaction and help overcome activation energy barriers, allowing slower-reacting metals to react.
Reactions of sodium and potassium with water
Some metals are too reactive for you to test in water. Sodium and potassium react very vigorously. They have to be stored under oil to prevent them from reacting with the water vapour in the air.
In these two reactions, the metal reacted with water to produce hydrogen and the metal hydroxide.
metal + water ⟶ metal hydroxide + hydrogen
Questions
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sodium + water ⟶ sodium hydroxide + hydrogen
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Wear safety goggles and gloves, use small amounts of metal, and conduct the reaction behind a safety screen or shield.
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They are stored under oil to stop them reacting with moisture and oxygen in the air, which could cause a dangerous reaction.
Reactions of other metals with water
Some other metals react less vigorously with water – for example, calcium and magnesium. In the experiment shown in the diagram, a piece of calcium has been placed at the bottom of a beaker and covered with water. A filter funnel has been placed upside down over the metal. The gas given off is collected in a test tube by the displacement of water.
Questions
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The gas is hydrogen. You can test for hydrogen by bringing a lit splint near it—if it makes a squeaky pop, hydrogen is present.
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The more reactive metal will produce bubbles of gas more quickly and possibly cause a temperature rise in the water. You compare the rate of reaction.
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Use equal masses or surface areas of metal, the same volume and temperature of water, and carry out the test for the same time.
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calcium + water ⟶ calcium hydroxide + hydrogen
Some of the metals that do not react with water may react with steam. Even magnesium will react more rapidly with steam than with water.
In the reaction shown here, magnesium is heated. From time to time, the heat is directed at the ceramic wool. The ceramic wool has been soaked in water, which when heated produces steam. In this reaction the magnesium reacts with water, which is in the form of a gas. Magnesium oxide and hydrogen are formed. The hydrogen gas that is given off can be burnt.
The word equation for this reaction is:
magnesium + water (g) ⟶ magnesium oxide + hydrogen
The (g) after water indicates it is water in the form of a gas, in this case steam. Steam is formed by boiling water and is very hot, whereas water vapour is made up of water particles in the air at lower temperatures. Some metals, such as gold, do not react with water at all.
Questions
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Steam particles move faster and have more energy than water particles, increasing the frequency and energy of collisions with magnesium, so the reaction happens faster.
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Gold, silver, and platinum are examples of metals that do not react with water.
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It means the element is unreactive and does not easily take part in chemical reactions under normal conditions.
A familiar reaction
You will probably remember the reaction of magnesium with dilute hydrochloric acid. This is the word equation for this reaction:
magnesium + hydrochloric acid ⟶ magnesium chloride + hydrogen
Magnesium chloride is an example of a salt. When a metal reacts with an acid, the products are a salt and hydrogen.
The general equation for this reaction is:
metal + acid ⟶ salt + hydrogen
Questions
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magnesium + sulfuric acid ⟶ magnesium sulfate + hydrogen
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Magnesium sulfate
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You would see bubbles of gas (hydrogen) forming and the magnesium gradually dissolving.
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zinc + nitric acid ⟶ zinc nitrate + hydrogen
Think Like a Scientist
Sofia and Marcus are investigating how different metals react with acid. You will help plan and carry out the investigation to compare reactivity using safe and accurate methods.
Part 1: Planning the Investigation
Review the equipment shown and decide which items are safe and suitable. Consider the following questions:
- What will they change?
- What will they keep the same?
- How will they measure reactivity?
- How will they stay safe?
- What equipment will they use?
Create a results table and write a short plan to share with your teacher before starting.
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Sodium and potassium are too reactive and dangerous to use in acid—they can cause explosions or fires, so they must not be used.
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They should use a smaller measuring cylinder (e.g. 50 mL) for greater accuracy when measuring small amounts of dilute acid.
Part 2: Carrying Out the Investigation
Once your plan has been checked, test a selection of metals in dilute acid and record the results carefully. Observe the amount of fizzing or bubbling to determine reactivity.
Step 1: Follow your plan after it has been approved.
Step 2: Use the correct safety equipment.
Step 3: Test each metal and observe carefully.
Step 4: Record your results and rank the metals by reactivity.
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Magnesium typically reacts the most vigorously with dilute acid, producing many bubbles quickly as hydrogen gas is released.
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Copper is usually the least reactive metal tested; it does not visibly react with dilute acid under normal conditions.
Summary: Reactions of Metals with Oxygen, Water, Steam, and Acids
- With Oxygen: Most reactive metals burn or tarnish to form metal oxides (e.g., magnesium + oxygen ⟶ magnesium oxide). This is an oxidation reaction.
- With Water: Some metals (e.g., potassium, sodium, calcium) react vigorously, forming metal hydroxide and hydrogen (e.g., sodium + water ⟶ sodium hydroxide + hydrogen).
- With Acids: Many metals react with dilute acids to form a salt and hydrogen gas (e.g., zinc + sulfuric acid ⟶ zinc sulfate + hydrogen).
The speed and products of these reactions depend on the metal’s position in the reactivity series and the nature of the reacting substance (oxygen, water, acid, or steam).