Skip to content Skip to navigation Skip to collection information

OpenStax_CNX

You are here: Home » Content » Siyavula textbooks: Grade 10 Physical Science » Precipitation reactions

Navigation

Table of Contents

Lenses

What is a lens?

Definition of a lens

Lenses

A lens is a custom view of the content in the repository. You can think of it as a fancy kind of list that will let you see content through the eyes of organizations and people you trust.

What is in a lens?

Lens makers point to materials (modules and collections), creating a guide that includes their own comments and descriptive tags about the content.

Who can create a lens?

Any individual member, a community, or a respected organization.

What are tags? tag icon

Tags are descriptors added by lens makers to help label content, attaching a vocabulary that is meaningful in the context of the lens.

This content is ...

Affiliated with (What does "Affiliated with" mean?)

This content is either by members of the organizations listed or about topics related to the organizations listed. Click each link to see a list of all content affiliated with the organization.
  • Bookshare

    This collection is included inLens: Bookshare's Lens
    By: Bookshare - A Benetech Initiative

    Comments:

    "Accessible versions of this collection are available at Bookshare. DAISY and BRF provided."

    Click the "Bookshare" link to see all content affiliated with them.

  • Siyavula: Physical Science display tagshide tags

    This collection is included inLens: Siyavula textbooks: Physical Science
    By: Free High School Science Texts Project

    Click the "Siyavula: Physical Science" link to see all content affiliated with them.

    Click the tag icon tag icon to display tags associated with this content.

Recently Viewed

This feature requires Javascript to be enabled.

Tags

(What is a tag?)

These tags come from the endorsement, affiliation, and other lenses that include this content.
 

Precipitation reactions

Module by: Free High School Science Texts Project. E-mail the author

Precipitation reactions

Sometimes, ions in solution may react with each other to form a new substance that is insoluble. This is called a precipitate.

Definition 1: Precipitate

A precipitate is the solid that forms in a solution during a chemical reaction.

Demonstration : The reaction of ions in solution

Apparatus and materials:

4 test tubes; copper(II) chloride solution; sodium carbonate solution; sodium sulphate solution

Figure 1
Figure 1 (CG10C8_004.png)

Method:

  1. Prepare 2 test tubes with approximately 5 ml of dilute Cu(II) chloride solution in each
  2. Prepare 1 test tube with 5 ml sodium carbonate solution
  3. Prepare 1 test tube with 5 ml sodium sulphate solution
  4. Carefully pour the sodium carbonate solution into one of the test tubes containing copper(II) chloride and observe what happens
  5. Carefully pour the sodium sulphate solution into the second test tube containing copper(II) chloride and observe what happens

Results:

  1. A light blue precipitate forms when sodium carbonate reacts with copper(II) chloride
  2. No precipitate forms when sodium sulphate reacts with copper(II) chloride

It is important to understand what happened in the previous demonstration. We will look at what happens in each reaction, step by step.

  1. Reaction 1: Sodium carbonate reacts with copper(II) chloride.
    When these compounds react, a number of ions are present in solution: Cu2+, Cl--, Na+ and CO32--. Because there are lots of ions in solution, they will collide with each other and may recombine in different ways. The product that forms may be insoluble, in which case a precipitate will form, or the product will be soluble, in which case the ions will go back into solution. Let's see how the ions in this example could have combined with each other:
    Cu2+ + CO32-- CuCO3
    Cu2+ + 2Cl-- CuCl2
    Na+ + Cl-- NaCl
    Na+ + CO32-- Na2 CO3
    You can automatically exclude the reactions where sodium carbonate and copper(II) chloride are the products because these were the initial reactants. You also know that sodium chloride (NaCl) is soluble in water, so the remaining product (copper carbonate) must be the one that is insoluble. It is also possible to look up which salts are soluble and which are insoluble. If you do this, you will find that most carbonates are insoluble, therefore the precipitate that forms in this reaction must be CuCO33. The reaction that has taken place between the ions in solution is as follows:
    2Na+ + CO32-- + Cu2+ + 2Cl-- CuCO3 + 2Na+ + 2Cl--
  2. Reaction 2: Sodium sulphate reacts with copper(II) chloride.
    The ions that are present in solution are Cu2+, Cl--, Na+ and SO42-2-. The ions collide with each other and may recombine in different ways. The possible combinations of the ions are as follows:
    Cu2+ + SO42-- CuSO4
    Cu2+ + 2Cl-- CuCl2
    Na+ + Cl-- NaCl
    Na+ + SO42-- Na2SO4
    If we look up which of these salts are soluble and which are insoluble, we see that most chlorides and most sulphates are soluble. This is why no precipitate forms in this second reaction. Even when the ions recombine, they immediately separate and go back into solution. The reaction that has taken place between the ions in solution is as follows:
    2Na+ + SO42-- + Cu2+ + 2Cl-- 2Na+ + SO42-- + Cu2+ + 2Cl--

Table 1 shows some of the general rules about the solubility of different salts based on a number of investigations:

Table 1: General rules for the solubility of salts
Salt Solubility
Nitrates All are soluble
Potassium, sodium and ammonium salts All are soluble
Chlorides All are soluble except silver chloride, lead(II) chloride and mercury(II) chloride
Sulphates All are soluble except lead(II) sulphate, barium sulphate and calcium sulphate
Carbonates All are insoluble except those of potassium, sodium and ammonium

Testing for common anions in solution

It is also possible to carry out tests to determine which ions are present in a solution.

Test for a chloride

Prepare a solution of the unknown salt using distilled water and add a small amount of silver nitrate solution. If a white precipitate forms, the salt is either a chloride or a carbonate.

Cl-- + Ag+ + NO3-- AgCl + NO3-- (AgCl is white precipitate)

CO32-- + 2Ag+ + 2NO3-- Ag2CO3 + 2NO3-- (Ag2CO3 is white precipitate)

The next step is to treat the precipitate with a small amount of concentrated nitric acid. If the precipitate remains unchanged, then the salt is a chloride. If carbon dioxide is formed, and the precipitate disappears, the salt is a carbonate.

AgCl + HNO3 (no reaction; precipitate is unchanged)

Ag 2 CO 3+2 HNO 32 AgNO 3+H2O+ CO 2 Ag 2 CO 3+2 HNO 32 AgNO 3+H2O+ CO 2 (precipitate disappears)

Test for a sulphate

Add a small amount of barium chloride solution to a solution of the test salt. If a white precipitate forms, the salt is either a sulphate or a carbonate.

SO 42-+ Ba 2++ Cl - BaSO 4+ Cl - SO 42-+ Ba 2++ Cl - BaSO 4+ Cl - (BaSO44 is a white precipitate)

CO 32-+ Ba 2++ Cl - BaCO 3+ Cl - CO 32-+ Ba 2++ Cl - BaCO 3+ Cl - (BaCO33 is a white precipitate)

If the precipitate is treated with nitric acid, it is possible to distinguish whether the salt is a sulphate or a carbonate (as in the test for a chloride).

BaSO 4+ HNO 3 BaSO 4+ HNO 3 (no reaction; precipitate is unchanged)

BaCO 3+2 HNO 3 Ba ( NO 3)2+H2O+ CO 2 BaCO 3+2 HNO 3 Ba ( NO 3)2+H2O+ CO 2 (precipitate disappears)

Test for a carbonate

If a sample of the dry salt is treated with a small amount of acid, the production of carbon dioxide is a positive test for a carbonate.

Acid + CO32-- CO2

If the gas is passed through limewater and the solution becomes milky, the gas is carbon dioxide.

Ca(OH)2 + CO2 CaCO3 + H2O (It is the insoluble CaCO3 precipitate that makes the limewater go milky)

Test for bromides and iodides

As was the case with the chlorides, the bromides and iodides also form precipitates when they are reacted with silver nitrate. Silver chloride is a white precipitate, but the silver bromide and silver iodide precipitates are both pale yellow. To determine whether the precipitate is a bromide or an iodide, we use chlorine water and carbon tetrachloride (CCl44).

Chlorine water frees bromine gas from the bromide and colours the carbon tetrachloride a reddish brown.

Chlorine water frees iodine gas from an iodide and colours the carbon tetrachloride purple.

Precipitation reactions and ions in solution

  1. Silver nitrate (AgNO33) reacts with potassium chloride (KCl) and a white precipitate is formed.
    1. Write a balanced equation for the reaction that takes place.
    2. What is the name of the insoluble salt that forms?
    3. Which of the salts in this reaction are soluble?
    Click here for the solution
  2. Barium chloride reacts with sulphuric acid to produce barium sulphate and hydrochloric acid.
    1. Write a balanced equation for the reaction that takes place.
    2. Does a precipitate form during the reaction?
    3. Describe a test that could be used to test for the presence of barium sulphate in the products.
    Click here for the solution
  3. A test tube contains a clear, colourless salt solution. A few drops of silver nitrate solution are added to the solution and a pale yellow precipitate forms. Which one of the following salts was dissolved in the original solution?
    1. NaI
    2. KCl
    3. K22CO33
    4. Na22SO44
    (IEB Paper 2, 2005) Click here for the solution

Threats to the Hydrosphere

It should be clear by now that the hydrosphere plays an extremely important role in the survival of life on Earth and that the unique properties of water allow various important chemical processes to take place which would otherwise not be possible. Unfortunately for us however, there are a number of factors that threaten our hydrosphere and most of these threats are because of human activities. We are going to focus on two of these issues: overuse and pollution and look at ways in which these problems can possibly be overcome.

  1. Pollution
    Pollution of the hydrosphere is also a major problem. When we think of pollution, we sometimes only think of things like plastic, bottles, oil and so on. But any chemical that is present in the hydrosphere in an amount that is not what it should be is a pollutant. Animals and plants that live in the hydrosphere are specially adapted to surviving within a certain range of conditions. If these conditions are changed (e.g. through pollution), these organisms may not be able to survive. Pollution then, can affect entire aquatic ecosystems. The most common forms of pollution in the hydrosphere are waste products from humans and from industries, nutrient pollution e.g. fertiliser runoff which causes eutrophication (this was discussed in chapter ref 7) and toxic trace elements such as aluminium, mercury and copper to name a few. Most of these elements come from mines or from industries.
  2. Overuse of water
    We mentioned earlier that only a very small percentage of the hydrosphere's water is available as freshwater. However, despite this, humans continue to use more and more water to the point where water consumption is fast approaching the amount of water that is available. The situation is a serious one, particularly in countries such as South Africa which are naturally dry and where water resources are limited. It is estimated that between 2020 and 2040, water supplies in South Africa will no longer be able to meet the growing demand for water in this country. This is partly due to population growth, but also because of the increasing needs of industries as they expand and develop. For each of us, this should be a very scary thought. Try to imagine a day without water...difficult isn't it? Water is so much a part of our lives, that we are hardly aware of the huge part that it plays in our daily lives.

Discussion : Creative water conservation

As populations grow, so do the demands that are placed on dwindling water resources. While many people argue that building dams helps to solve this water-shortage problem, the reality is that dams are only a temporary solution and that they often end up doing far more ecological damage than good. The only sustainable solution is to reduce the demand for water, so that water supplies are sufficient to meet this. The more important question then is how to do this.

Discussion:
Divide the class into groups, so that there are about five people in each. Each group is going to represent a different sector within society. Your teacher will tell you which sector you belong to from the following: Farming, industry, city management or civil society (i.e. you will represent the ordinary 'man on the street'). In your groups, discuss the following questions as they relate to the group of people you represent: (Remember to take notes during your discussions, and nominate a spokesperson to give feedback to the rest of the class on behalf of your group)

  • What steps could be taken by your group to conserve water?
  • Why do you think these steps are not being taken?
  • What incentives do you think could be introduced to encourage this group to conserve water more efficiently?

It is important to realise that our hydrosphere exists in a delicate balance with other systems and that disturbing this balance can have serious consequences for life on this planet.

Group Project : School Action Project

There is a lot that can be done within a school to save water. As a class, discuss what actions could be taken by your class to make people more aware of how important it is to conserve water.

Summary

  • The hydrosphere includes all the water that is on Earth. Sources of water include freshwater (e.g. rivers, lakes), saltwater (e.g. oceans), groundwater (e.g. boreholes) and water vapour. Ice (e.g. glaciers) is also part of the hydrosphere.
  • The hydrosphere interacts with other global systems, including the atmosphere, lithosphere and biosphere.
  • The hydrosphere has a number of important functions. Water is a part of all living cells, it provides a habitat for many living organisms, it helps to regulate climate and it is used by humans for domestic, industrial and other use.
  • The polar nature of water means that ionic compounds dissociate easily in aqueous solution into their component ions.
  • Ions in solution play a number of roles. In the human body for example, ions help to regulate the internal environment (e.g. controlling muscle function, regulating blood pH). Ions in solution also determine water hardness and pH.
  • Water hardness is a measure of the mineral content of water. Hard water has a high mineral concentration and generally also a high concentration of metal ions e.g. calcium and magnesium. The opposite is true for soft water.
  • pH is a measure of the concentration of hydrogen ions in solution. The formula used to calculate pH is as follows: pH = -log[H33O++] or pH = -log[H++]. A solution with a pH less than 7 is considered acidic and more than 7 is considered basic (or alkaline). A neutral solution has a pH of 7.
  • Gases such as CO22, NO22 and SO42-2- dissolve in water to form weak acid solutions. Rain is naturally acidic because of the high concentrations of carbon dioxide in the atmosphere. Human activities such as burning fossil fuels, increase the concentration of these gases in the atmosphere, resulting in acid rain.
  • Conductivity is a measure of a solution's ability to conduct an electric current.
  • An electrolyte is a substance that contains free ions and is therefore able to conduct an electric current. Electrolytes can be divided into strong and weak electrolytes, based on the extent to which the substance ionises in solution.
  • A non-electrolyte cannot conduct an electric current because it dooes not contain free ions.
  • The type of substance, the concentration of ions and the temperature of the solution affect its conductivity.
  • A precipitate is formed when ions in solution react with each other to form an insoluble product. Solubility 'rules' help to identify the precipitate that has been formed.
  • A number of tests can be used to identify whether certain anions are present in a solution.
  • Despite the importance of the hydrosphere, a number of factors threaten it. These include overuse of water, and pollution.

Summary Exercise

  1. Give one word for each of the following descriptions:
    1. the change in phase of water from a gas to a liquid
    2. a charged atom
    3. a term used to describe the mineral content of water
    4. a gas that forms sulphuric acid when it reacts with water
    Click here for the solution
  2. Match the information in column A with the information in column B by writing only the letter (A to I) next to the question number (1 to 7)
    Table 2
    Column AColumn B
    1. A polar moleculeA. H22SO44
    2. molecular solutionB. CaCO33
    3. Mineral that increases water hardnessC. NaOH
    4. Substance that increases the hydrogen ion concentrationD. salt water
    5. A strong electrolyteE. calcium
    6. A white precipitateF. carbon dioxide
    7. A non-conductor of electricityG. potassium nitrate
     H. sugar water
     I. O22
    Click here for the solution
  3. For each of the following questions, choose the one correct answer from the list provided.
    1. Which one of the following substances does not conduct electricity in the solid phase but is an electrical conductor when molten?
      1. Cu
      2. PbBr22
      3. H22O
      4. I22
      (IEB Paper 2, 2003) Click here for the solution
    2. The following substances are dissolved in water. Which one of the solutions is basic?
      1. sodium nitrate
      2. calcium sulphate
      3. ammonium chloride
      4. potassium carbonate
      (IEB Paper 2, 2005) Click here for the solution
  4. The concentration of hydronium and hydroxyl ions in a typical sample of seawater are 10-8-8 and 10-6-6 respectively.
    1. Is the seawater acidic or basic?
    2. Calculate the pH of this seawater.
  5. Three test tubes (X, Y and Z) each contain a solution of an unknown potassium salt. The following observations were made during a practical investigation to identify the solutions in the test tubes: A: A white precipitate formed when silver nitrate (AgNO33) was added to test tube Z. B: A white precipitate formed in test tubes X and Y when barium chloride (BaCl22) was added. C: The precipitate in test tube X dissolved in hydrochloric acid (HCl) and a gas was released. D: The precipitate in test tube Y was insoluble in hydrochloric acid.
    1. Use the above information to identify the solutions in each of the test tubes X, Y and Z.
    2. Write a chemical equation for the reaction that took place in test tube X before hydrochloric acid was added.
    (DoE Exemplar Paper 2 2007) Click here for the solution

Collection Navigation

Content actions

Download:

Collection as:

PDF | EPUB (?)

What is an EPUB file?

EPUB is an electronic book format that can be read on a variety of mobile devices.

Downloading to a reading device

For detailed instructions on how to download this content's EPUB to your specific device, click the "(?)" link.

| More downloads ...

Module as:

PDF | EPUB (?)

What is an EPUB file?

EPUB is an electronic book format that can be read on a variety of mobile devices.

Downloading to a reading device

For detailed instructions on how to download this content's EPUB to your specific device, click the "(?)" link.

| More downloads ...

Add:

Collection to:

My Favorites (?)

'My Favorites' is a special kind of lens which you can use to bookmark modules and collections. 'My Favorites' can only be seen by you, and collections saved in 'My Favorites' can remember the last module you were on. You need an account to use 'My Favorites'.

| A lens I own (?)

Definition of a lens

Lenses

A lens is a custom view of the content in the repository. You can think of it as a fancy kind of list that will let you see content through the eyes of organizations and people you trust.

What is in a lens?

Lens makers point to materials (modules and collections), creating a guide that includes their own comments and descriptive tags about the content.

Who can create a lens?

Any individual member, a community, or a respected organization.

What are tags? tag icon

Tags are descriptors added by lens makers to help label content, attaching a vocabulary that is meaningful in the context of the lens.

| External bookmarks

Module to:

My Favorites (?)

'My Favorites' is a special kind of lens which you can use to bookmark modules and collections. 'My Favorites' can only be seen by you, and collections saved in 'My Favorites' can remember the last module you were on. You need an account to use 'My Favorites'.

| A lens I own (?)

Definition of a lens

Lenses

A lens is a custom view of the content in the repository. You can think of it as a fancy kind of list that will let you see content through the eyes of organizations and people you trust.

What is in a lens?

Lens makers point to materials (modules and collections), creating a guide that includes their own comments and descriptive tags about the content.

Who can create a lens?

Any individual member, a community, or a respected organization.

What are tags? tag icon

Tags are descriptors added by lens makers to help label content, attaching a vocabulary that is meaningful in the context of the lens.

| External bookmarks