Definition: Given a question and a context passage, generate the answer having the word or phrase from the context passage. Here, the answer should be a shortest continous span from the passage.
Input: Context: A chemical equation is a symbolic representation of a chemical reaction. It is a shorthand way of showing how atoms are rearranged in the reaction. The general form of a chemical equation was introduced in this chapters lesson "Introduction to Chemical Reactions." It is: Reactants ! Products Consider the simple example in Figure 8.4. When carbon (C) reacts with oxygen (O2 ), it produces carbon dioxide (CO2 ). The chemical equation for this reaction is: C + O2 ! CO2 The reactants are one atom of carbon and one molecule of oxygen. When there is more than one reactant, they are separated by plus signs (+). The product is one molecule of carbon dioxide. If more than one product were produced, plus signs would be used between them as well. Some chemical equations are more challenging to write. Consider the reaction in which hydrogen (H2 ) and oxygen (O2 ) combine to form water (H2 O). Hydrogen and oxygen are the reactants, and water is the product. To write a chemical equation for this reaction, you would start by writing symbols for the reactants and products: Equation 1: H2 + O2 ! H2 O Like equations in math, equations in chemistry must balance. There must be the same number of each type of atom in the products as there is in the reactants. In equation 1, count the number of hydrogen and oxygen atoms on each side of the arrow. There are two hydrogen atoms in both reactants and products. There are two oxygen atoms in the reactants but only one in the product. Therefore, equation 1 is not balanced. Coefficients are used to balance chemical equations. A coefficient is a number placed in front of a chemical symbol or formula. It shows how many atoms or molecules of the substance are involved in the reaction. For example, two molecules of hydrogen would be written as 2H2 . A coefficient of 1 usually isnt written. Coefficients can be used to balance equation 1 (above) as follows: Equation 2: 2H2 + O2 ! 2H2 O Equation 2 shows that two molecules of hydrogen react with one molecule of oxygen to produce two molecules of water. The two molecules of hydrogen each contain two hydrogen atoms. There are now four hydrogen atoms in both reactants and products. Is equation 2 balanced? Count the oxygen atoms to find out. Balancing a chemical equation involves a certain amount of trial and error. In general, however, you should follow these steps: 1. Count the number of each type of atom in reactants and products. Does the same number of each atom appear on both sides of the arrow? If not, the equation is not balanced, and you need to go to step 2. 2. Add coefficients to increase the number of atoms or molecules of reactants or products. Use the smallest coefficients possible. 3. Repeat steps 1 and 2 until the equation is balanced. Helpful Hint When you balance chemical equations, never change the subscripts in chemical formulas. Changing subscripts changes the substances involved in the reaction. Change only the coefficients. Work through the Problem Solving examples below. Then do the You Try It! problems to check your understand- ing. If you need more help, go to this URL: (14:28). MEDIA Click image to the left or use the URL below. URL: Problem Solving Problem: Balance this chemical equation: N2 + H2 ! NH3 Hints for balancing 1. Two N are needed in the products to match the two N (N2 ) in the reactants. Add the coefficient 2 in front of NH3 . Now N is balanced. 2. Six H are now needed in the reactants to match the six H in the products. Add the coefficient 3 in front of H2 . Now H is balanced. Solution: N2 + 3H2 ! 2NH3 Problem: Balance this chemical equation: CH4 + O2 ! CO2 + H2 O Solution: CH4 + 2O2 ! CO2 + 2H2 O You Try It! Problem: Balance these chemical equations: Zn + HCl ! ZnCl2 + H2 Cu + O2 ! CuO Why must chemical equations be balanced? Its the law! 
 Question: symbolic representation of a chemical reaction
Output:
chemical equation