The Himalayan mountain chain is an example of a continental collision. 40 million years ago, the Indian plate collided with the Eurasian plate. The enormous pressure forces resulting from this shock caused a gigantic mountain uplift. There are 3000 km of the Himalayas with Mount Everest being the highest peak (8848m). Further north, behind the front line, the Eurasian plate is raised over 2.5 million km² (6 times the size of California - 10 times that of UK) forming the Tibetan Plateau with an average altitude of over 4000m.

This phenomena is one of a number of examples of the theory of the continental drift and plate tectonics model.

An electric current is moving electric charges. In a metal, the electrons are moving while in a solution, only the dissolved ions are likely to be able to move.

Sodium chloride is the chemical name for salt. It is an ionic solid made up of sodium (Na⁺) and chloride (Cl^-) ions. These charged ions are liberated in solution and can move if a voltage is applied across two electrodes. The circulating current can light a lamp.

Sugar, also called sucrose, is a molecular solid. In solution, the molecules are dispersed in the water but they are not attracted to the electrodes because they are electically neutral. The current does not circulate and the lamp is not lighted.

The pH of an aqueous solution (water based) is a number that indicates the nature of the solution as acidic, neutral or basic.

• pH < 7: The solution is acidic.
• pH = 7: The solution is neutral.
• pH > 7: The solution is basic.

pH measurement provides direct information on the presence of certain ions: H⁺ hydrogen ion and the hydroxide ion OH^- in the solution. The H⁺ ion is directly related to the acidity and OH^- ion is linked to the basicity of the solution.

• pH < 7: An acidic solution that contains more H⁺ than OH^- ions.
• pH = 7: A neutral solution that contains an equal number of H⁺ and OH^- ions.
• pH > 7: A basic solution containing more OH^- ions than H⁺ ions.

The pH (potential hydrogen) is a measure of the concentration of H⁺ ions. These ions, react with many elements including water. Strong acids or strong bases are corrosive. They must be handled with great care.

An atom is the smallest particle of matter that can combine chemically with another atom or molecule.  It has a nucleus, composed of protons and neutrons, around which electrons orbit.
These particles have mass and another property called electric charge:

• The proton carries a positive one charge noted by +e
• The electron carries a charge equal to the proton but opposite in charge (-e). The electron is sometime noted by e^-.
• The neutron carries no charge and is therefore neutral

The number of protons in an atom signifies its name:

• 1 proton:  Hydrogen
• 2 protons:  Helium
• 17 protons:  Chlorine
• …

An atom has as many protons as electrons. The total charge is zero which is why matter is usually neutral.

In some cases, with the help of an external energy input, an atom can lose or gain one or more electrons. This is the phenomenon of ionization.

Example:  An atom of sodium (Na) has 11 protons and 11 electrons. If it loses an electron, it  then has 11 protons but only 10 electrons. A positive charge +e is no longer balanced and the resulting sodium ion is positively charged. The sodium ion, Na⁺, indicates that it carries a charge +e.

A molecule is an assembly of atoms that share electrons to achieve greater stability.

These exercises allow you to verify that you have mastered the graphic method of determining the time constant of a system.

In an electrical circuit composed of a constant voltage source, E, in series with a resistance, R, and a condenser (capacitor), C, theory indicates that the response of the circuit to a voltage step, E, is a curve of exponential nature.

The exponential form is directly related to the values ​​of R and C. Specifically the RC product that is homogeneous to a time (in seconds). τ = RC is called the time constant of the circuit.  This is an indicator of the rate of change in the circuit experiencing a disturbance (by a voltage step).  The smaller the value, the more rapidly steady state will be reached. The observation of the curve of voltage, or even the current, can approach the value of the time constant as illustrated in the animation.

Man has known, for a very long time that matter can be electrified. However, it is very complex to maintain a separation of charges for the circulation of electric current. It was not until the late eighteenth century and the voltaic cell that we had our first current generator.  

In 1836, John Daniell, a British physician (1790-1845), invented a new one cell battery made of two compartments containing respectively ionic aqueous solutions of copper sulphate in which a strip of copper metal has been inserted and zinc sulfate in which a zinc metal strip has been inserted.  

The two compartments are connected by a salt bridge. This device can generate a sustainable voltage of 1.1 V. 

Test and evaluate your knowledge of combustion reactions by answering questions in minimum time and errors. The final assessment takes into account both time and number of attempts needed to complete the test.

Test and evaluate your knowledge by responding to the questions in a minimum amount of time, with a minimum number of errors.  The evaluation obtained at the end takes into account the time needed and the number of tries needed to complete the test.

Remark: There are 8 pages of questions.