The principle of matter conservation may be considered as an approximate physical law that is true only in the classical sense, without consideration of special relativity andquantum mechanics. It is approximately true except in certain high energy applications.
A particular difficulty with the idea of conservation of "matter" is that "matter" is not a well-defined word scientifically, and when particles that are considered to be "matter" (such as electrons and positrons) are annihilated to make photons (which are often not considered matter) then conservation of matter does not take place over time, even within isolated systems. However, matter is conserved to such an extent that matter conservation may be safely assumed in chemical reactions and all situations in which radioactivityand nuclear reactions are not involved.
Even when matter is not conserved, the collection of mass and energy within the system are conserved.
Open systems and thermodynamically closed systems
Mass is also not generally conserved in open systems. Such is the case when various forms of energy are allowed into, or out of, the system (see for example, binding energy). However, again unless radioactivity or nuclear reactions are involved, the amount of energy escaping such systems as heat, work, or electromagnetic radiation is usually too small to be measured as a decrease in system mass.
The law of mass conservation for isolated systems (totally closed to all mass and energy), as viewed over time from any single inertial frame, continues to be true in modern physics. The reason for this is that relativistic equations show that even "massless" particles such as photons still add mass and energy to isolated systems, allowing mass (though not matter) to be strictly conserved in all processes where energy does not escape the system. In relativity, different observers may disagree as to the particular value of the conserved mass of a given system, but each observer will agree that this value does not change over time as long as the system is isolated (totally closed to everything).
In general relativity, the total invariant mass of photons in an expanding volume of space will decrease, due to the red shift of such an expansion (see Mass in general relativity). The conservation of both mass and energy therefore depends on various corrections made to energy in the theory, due to the changing gravitational potential energy of such systems. 
All metals show certain physical & chemical properties like malleability, ductility and a lustrous surface. Almost all metals release hydrogen gas with dilute acids. But the reactivity of metals towards various reactants is not the same.
Some metals like alkali & alkaline earth metals (group-1 & 2) are very reactive & react vigorously with a reactant. But some metals like gold & platinum are least reactive and passive for almost all reactants. Some metals like copper release hydrogen gas with dilute acid. Hence, there must be some criteria for understanding the reactivity of different metals and predicting the products of different reactions.
Reactivity Series of Metals
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Elements are mainly classified under metals & non-metals. There are some elements, which have intermediate features. They are known as metalloids.
Almost all metals are reactive and react vigorously with various compounds. In the whole periodic table, more than 75% elements are metallic in nature.
The reactivity series or activity series is an empirical arrangement of metals, in order of "reactivity" from highest to lowest. In other words, the most reactive metal is presented at the top and the least reactive metal at the bottom.
Hence potassium is the most reactive metal and platinum the least reactive one. In the whole series, only two non-metals are included, which are carbon & hydrogen. Carbon helps in predicting the products formed during the extraction of iron in blast furnace and hydrogen is included because non-metals below it will not react with dilute acids.
In the reactivity series, as we move from bottom to top, the reactivity of metals increases. Metals present at the top of the series can lose electrons more readily to form positive ions and corrode or tarnish more readily. They require more energy to be separated from their ores, and become stronger reducing agents, while metals present at the bottom of the series are good oxidizing agent.
By using the reactivity series, one can predict the products of displacement reaction. Each element in the reactivity series can be replaced from a compound by any of the elements above it. For example, magnesium metal can displace zinc ions in a solution.
Mg(s) + Zn2+ →→Zn(s) + Mg2+
The interval between metals in the reactivity series represents the reactivity of those metals towards each other.
If the interval between elements is larger, they will react more vigorously. The topmost five elements, form lithium to sodium are known as very active metals; hence they react with cold water to produce the hydroxide and hydrogen gas. For example, sodium forms sodium hydroxide and hydrogen gas with cold water.
2Na + 2H2O→→2NaOH + H2
From magnesium to chromium, elements are considered as active metals and they will react with very hot water or steam and form the oxide and hydrogen gas. For example, aluminum reacts with steam to form aluminum oxide and hydrogen gas.
2Al + 3H2O→→Al2O3+ 3H2
From iron to lead, metals can replace hydrogen from various acids like Hydrochloric acid, dilute sulfuric and nitric acids. Oxides of these metals undergo reduction when heated with hydrogen gas, carbon, or carbon monoxide. Till copper, metals can combine directly with oxygen and form metal oxide. Elements present at the bottom from mercury to gold are often found in the native form in nature and their oxides show thermal decomposition under mild conditions.