1.Pure substance X is solid at room temperature. If the substance is heated to 230’C is melted gradually. If     cooled to room temperature, the liquid can not be frozen.

a.       Is it posibble X of an element or a compound. Explain it !

b.      Does it a chemical change occured? If so can it be said to undergo an endotherm changing,based on the information provided?

c.       Can it be said that liquid is an element, based on the information provided?
 

a. I think X is a compound because the compound is a pure chemical substance consisting of two or more elements that can be further broken down into its constituent elements by chemical reactions. 

b. Chemical changes that occur in the automatic reaction endothermic.because the reactions are accompanied by heat transfer from the environment to the system (heat absorbed by the system from its environment), characterized by a decrease in the ambient temperature around the system as well as the reactions that require energy or absorb energy from the environment when the reaction occurs, the reaction generally produces cold temperatures. or endothermic reaction is a chemical reaction that absorbs heat. Heat is taken from the environment so that the enthalpy increases.In endothermic reactions absorb energy systems. Therefore, the enthalpy of the system will increase, meaning that the enthalpy of the product (HP) is greater than the enthalpy of the reactants (HR). Consequently, the change in enthalpy (ΔH), namely the difference between the enthalpy of the products with the enthalpy of the reactants is positive.  

c.yes can,as simple substance (singular) who is considered as the composition of the universe,because the fluid that includes elements At room temperature (25 C) elements can be solid, liquid or gas and will continue to change over energized.

2.When a candle that weight 10gr is burned in oxygen,carbon dioxide and water vapor formed by combustion the weight more than 10gr.was this case match with the law of conversation mass. Explain it !
 

      Law of conservation of mass expressed by Antoine Laurent Lavoisier, which reads: "In a reaction, the mass of substances before and after the reaction is the same", in other words, the mass can not be created and can not be destroyed. This means that as long as there is no reaction occurs atoms reactants and reaction products were missing. 

      Since mass and energy related, in a system that gets / expend energy, the mass in very small amounts will be created / missing from the system. However, in almost all events that involve changes in energy, law of conservation of mass can be used as mass changed very little.

That is, the energy that arises in an incident resulting in the loss of mass. Instead, the energy absorbed by the event will be accompanied by the creation of a number of materials. However, calculations show that the change in the reaction mass is so small that can be ignored. Thus, the law of conservation of mass is still in effect, and the modern version reads:
"In a chemical reaction can not be detected mass change"

      3.When carbon burns in oxygen under limited number,it will form two gaseous compounds. Suggest the way to differentiate the two compounds with one another.
      
      Carbon compounds or commonly known as organic compound is a compound constituent elements consist of carbon atoms and hydrogen atoms, oxygen, nitrogen, sulfur, halogen, or fosfor.Sifat physical and chemical properties of organic compounds can distinguish each other . When some chemical and physical properties of organic compounds and inorganic compounds simple that tells whether included in the organic or inorganic compounds such as the current state of the heating, conductivity, and ionization (dissociation) and solubility.

      4.After mendeev compiled the periodic table,he concluded that the atomic weights of certain elements was wrong ruling,and this conclusion was apparently correct. How mendeev  was able to predict that several  atomic weights were wrong? Why his predictions are not always right. Explain !

      Mendeleev periodic believes that the law can be used to predict the existence and properties of some new element compounds, as well as to correct the atomic weights of some elements are already known. However, historian and author of chemistry are emphasized in the legal aspects of prediction. Mendeleev ability to accommodate elements that are known to have contributed positively in the periodic system. For example, the quote that accompanies her awarded the Davy Medal by the Royal Society of London does not mention any of his predictions       

    
      5.When an aqueous solution of mercury chloride is added to an aqueous solution of silver nitrate,a white solid forms. Identify the white solid and write the balanced equation for the reaction that occurs! 
        
      HgCl2 + 2AgNO3 ---> Hg (NO3) 2 + 2AgCl

The resulting white powder is silver nitrate (AgNO3), because pure silver has a bright white color so silver can form monovalent ions in solution is colorless