Many, but not all, metals react with acids. Hydrogen gas forms as the metals react with the acid to form salts. This class experiment is often used in the introductory study of acids to establish that this behaviour is a characteristic property Show The experiment is done first on a smaller scale using test tubes (lesson 1 below), with no attempt to recover the salts formed. This establishes that hydrogen production is a characteristic property of the reaction of metals and acids. It can then be done on a larger scale (lesson 2 below), and the salts formed can be recovered by crystallisation. Lesson 1 is a series of test tube experiments in which each working group establishes as a common feature that hydrogen is given off as metals react with an acid – if the metal reacts at all. This should take around 40 minutes, and most classes should be able to do this version. Each working group needs a small selection of metals and acids to test. The range of metals and acids tested can be extended to a teacher demonstration in the concluding part of this lesson. Lesson 2, in which the salt formed is recovered by crystallisation, takes longer, and the class needs to be reliable enough in behaviour and manipulative skills to cope with the hazards involved in heating acidic solutions in beakers on tripods. The time taken for the reaction depends on the particle size of the metal used. Using small granules helps to reduce the time taken. EquipmentApparatusLesson 1
Lesson 2
Apparatus notes
ChemicalsLesson 1
Lesson 2
Health, safety and technical notes
ProcedureLesson 1
Lesson 2Apparatus for recovering salt formed by the reaction of zinc and sulfuric acid.
Teaching notesUse the student handout available with this resource and get the students to answer each question after observing the reaction. Safety is particularly relevant to younger students. Be aware of the problems associated with heating beakers or evaporating dishes on tripods, and with lifting such hot containers off a tripod after heating. Students should not be seated on laboratory stools while carrying out these operations. Using tongs of suitable size is a good way of lifting hot containers but some schools may not have these. If there is any doubt about the safety of this step, the teacher should first lift each beaker down onto the heatproof mat, using a thick cloth or wearing suitable thermal protection gloves, before the students add the zinc pieces. The same applies to moving the evaporating basin before pouring its contents into the crystallising dish. The procedure for safely testing the evolved hydrogen gas in the test tube reactions needs to be demonstrated at a suitable point in lesson 1. A loosely inserted cork allows sufficient build-up of gas in a slow reaction to enable a successful test. Nevertheless many students find it difficult to achieve a successful ‘pop’ test for hydrogen, so you may need to do follow-up demonstrations as well. This pair of experiments forms an important stage for younger students in developing an understanding of what an acid is. They need to understand how to generalise from sufficient examples, and to see the limits to that generalisation in metals that do not react. It may help to develop this discussion in the concluding stages of lesson 1 by additional demonstrations of other metals and acids. In particular dilute nitric acid (< 0.5 M) does produce hydrogen with moderately reactive metals such as magnesium and zinc, even though reactions are different at higher concentrations, and with other metals. By the end of the lesson, students should be able readily to draw the conclusion: Metal + acid → salt + hydrogen This experiment is also a good opportunity for students to learn how to draw up suitable tables for recording experimental observations. In lesson 2, selecting zinc and sulfuric acid as the example to follow through to producing crystals of the salt is governed by the need to have a salt that crystallises easily. Unfortunately the chlorides of magnesium and zinc are not easy to crystallise, while magnesium sulfate is so soluble that it takes longer to evaporate sufficiently. Iron(II) compounds may suffer from oxidation problems when the solution is evaporated, giving a visibly impure product. There is potential for producing hazardous fumes if classes are allowed to over-evaporate salt solutions, either from evaporation of any excess sulfuric acid or from decomposition of the salt. There is also a danger of hot material spitting out of the container. If crystals begin to appear, eg at the top edge of the solution, the Bunsen burner should be turned off immediately and the solution left to cool. Refer to CLEAPSS Laboratory Handbook Section 13.2.6 for a discussion. If older students perform these experiments, they can be asked to write symbol equations: Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g) and Mg(s) + H2SO4(aq) → MgSO4(aq) + H2(g) For reactions of these acids with iron or zinc, the students simply substitute Fe or Zn for Mg in these equations. Questions for studentsDownload these questions as a worksheet from the bottom of this article.
What reactions form a salt?salt, in chemistry, substance produced by the reaction of an acid with a base. A salt consists of the positive ion (cation) of a base and the negative ion (anion) of an acid. The reaction between an acid and a base is called a neutralization reaction.
How do metals form salts?As the reaction between metals and acids produces flammable hydrogen, chemists usually make salts by reacting a metal compound such as a metal carbonate with an acid. Acids take part in reactions in which salts are produced.
Does all metals react with acid to form salt?Many, but not all, metals react with acids. Hydrogen gas isformed as the metal reacts with the acid to form a salt. Many, but not all, metals react with acids. Hydrogen gas is formed as the metal reacts with the acid to form a salt.
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