化学反应是指分子破裂成原子，原子重新排列组合生成新分子的过程。在反应中常伴有发光、发热、变色、生成沉淀物等，判断一个反应是否为化学反应的依据是反应是否生成新的分子或是分子内部的原子重组。
核反应不属于化学反应。离子反应是化学反应。

What are chemical reactions
什么是化学反应?
A chemical reaction is a process in which substances interact to form different substances by breaking forming or rearranging their chemical bonds.
化学反应是物质相互作用,通过断裂、形成或重新排列它们的化学键,形成不同的物质的过程·
Substances that take part in a chemical reaction may be ionic or covalent compounds
参与化学反应的物质可以是离子化合物或共价化合物,
as well as the atoms ions or molecules of some elements.
也可以是一些元素的原子、离子或分子·
Let's look at an example of a chemical reaction.
让我们来看一个化学反应的例子·
Photosynthesis is a chemical reaction in which plants make food by using the sun's energy to combine carbon dioxide from the air and water from the soil into a sugar called glucose and oxygen.
光合作用是一个化学反应,植物利用太阳的能量,将空气中的二氧化碳和土壤中的水结合,形成一种叫做葡萄糖的糖和氧气·
In this reaction carbon dioxide and water are called reactants because they're the substances changing by combining or reacting together.
在这个反应中,二氧化碳和水被称为反应物,因为它们通过结合或反应而发生变化·
Glucose and oxygen are called products because they're the new substances produced by the reaction.
葡萄糖和氧气被称为产物,因为它们是反应生成的新物质·
Scientists write a chemical reaction in the form of a chemical equation.
科学家用化学方程式来表示化学反应·
A chemical equation includes the reactants' chemical formulas on the left and the products' chemical formulas on the right.
化学方程式包括反应物的化学式在左边,产物的化学式在右边·
Notice that the reactants and the products are built from the same elements carbon oxygen and hydrogen.
注意,反应物和产物都由相同的元素组成,碳、氧和氢·
Products must contain the same amount and type of elements that were in the reactants
产物必须包含与反应物中相同数量和类型的元素,
and products never contain different elements than the reactants.
且产物中不会包含与反应物不同的元素·
In this reaction the chemical formulas for the reactants are CO2 for carbon dioxide and H2O for water.
在这个反应中,反应物的化学式是二氧化碳CO2和水H2O·
The chemical formulas for the products are C6H12O6 for glucose and O2 for oxygen.
产物的化学式是葡萄糖C6H12O6和氧气O2·
We are not done yet because the equation is not balanced.
我们还没完成,因为这个方程式没有平衡·
This is necessary due to the law of conservation of matter
这是必要的,因为质量守恒定律指出,
it states that matter can't be created or destroyed but can only change forms.
物质不能被创造或销毁,只能改变形式·
So to balance the equation
因此,为了平衡方程式,
we have to make sure that the number of atoms of each element in the reactants is equal to those in the products
我们必须确保反应物中每种元素的原子数量等于产物中该元素的数量,
this is because a chemical reaction must abide by the law of conservation of matter no atoms are created or destroyed.
因为化学反应必须遵守质量守恒定律,没有原子被创造或销毁·
Here's the balanced version of this equation.
这是该方程式的平衡版本·
It shows six molecules of carbon dioxide reacting with six molecules of water to produce one molecule of glucose and six molecules of oxygen.
它显示了六个二氧化碳分子与六个水分子反应,生成一个葡萄糖分子和六个氧气分子·
The numbers in front of the formulas are called coefficients.
公式前面的数字被称为系数·
They indicate the number of molecules of each reactant and product.
它们表示每种反应物和产物的分子数量·
If there's no coefficient it's understood to mean one molecule of that substance.
如果没有系数,则意味着该物质的一个分子·
To review
回顾一下,
in a chemical reaction chemical bonds in reacting substances are broken
在化学反应中,反应物中的化学键断裂
then reformed to make different substances.
然后后重新形成,生成不同的物质·
The reacting substances are called reactants.
反应物称为反应物·
The substances produced are called products.
产生的物质称为产物·
A chemical equation is a written expression of a chemical reaction
化学方程式是化学反应的书面表达,
it includes the chemical formulas of both the reactants and the products.
包含了反应物和产物的化学式·
In a balanced chemical equation
在平衡的化学方程式中,
the number of atoms of each element in the reactants equals those in the products.
反应物中每种元素的原子数等于产物中该元素的原子数·
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