O AR ATMOSFÉRICO.
Autor: PROFESSOR CARLOS DOMINGUES | Publicado: 2014-02-23T19:07:00-03:00


O AR ATMOSFÉRICO.



Muitas são as
situações do nosso dia-a-dia em que percebemos a presença de ar. Quando
sentimos a brisa suave no nosso rosto, quando o vento sopra forte balançando os
galhos das árvores, quando respiramos e sentimos o ar entrando e saindo dos
nosso pulmões, estamos percebendo a presença do ar.



Não podemos
ver o ar nem tocá-lo. Ele é&nbsp;invisível,&nbsp;incolor&nbsp;(não
tem cor) e&nbsp;inodoro&nbsp;(não tem cheiro). Mas existe, tem peso e
ocupa espaço.







De que é
feito o ar?







A matéria
pode se apresentar na natureza no estado sólido, líquido e gasoso. O ar se
apresenta no estado gasoso, é uma mistura de gases.&nbsp; O gás de maior
quantidade é o gás&nbsp;nitrogênio&nbsp;ou azoto, que forma cerca
de&nbsp;78%&nbsp;do ar. Isso quer dizer que, em 100 litros de ar, há 78
litros de nitrogênio. Depois vem o oxigênio&nbsp;com cerca de&nbsp;21%.
O 1% restante inclui&nbsp;argônio, o gás carbônico e outros gases. Esta
é a proporção de gases no ar seco. Mas normalmente, há também&nbsp;vapor de
água&nbsp;(em quantidade variável) e poeira. Certos gases vindos das
indústrias ou de outras fontes podem também estar presentes.







 
 
  
  
  
  
  
  
  
  
  
  
  
  
 
 
 

 



Gás
Oxigênio e a Combustão







O gás
oxigênio é um gás de importância fundamental para os processos vitais do nosso
planeta, utilizado na&nbsp;respiração&nbsp;da maioria dos seres vivos.
As algas e as plantas também absorvem oxigênio na respiração, mas, pela
fotossíntese, liberam esse gás, possibilitando a sua renovação contínua no
ambiente.



A maior
parte do oxigênio inspirado é utilizado pelos seres vivos na produção de
energia que mantém seus sistemas vitais.






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&nbsp;Quando
o motor do carro funciona, por exemplo, a gasolina combina-se com o oxigênio do
ar. A gasolina ou a outra substância que está sendo queimada é chamada de&nbsp;combustível,
e o oxigênio é chamado de comburente. Comburente é, portanto, a substância
que provoca a combustão.



No caso da
vela acesa, o comburente é o oxigênio do ar. O combustível é a parafina da
vela. Mas, para começar a combustão, é preciso aquecer o combustível. No caso
da vela, acendemos o pavio com um fósforo. O calor da chama do pavio aquece a
parafina que se combina com o oxigênio e é queimado.







A
combustão libera energia química que está armazenada no combustível. Essa energia aparece sob a forma de
calor e luz.&nbsp; Com a energia da combustão o ser humano movimenta veículos a
gasolina, a gás, a óleo diesel ou a álcool, e cozinha alimentos no fogão. Essa
energia pode ser liberada também em usinas&nbsp;termelétricas, que transformam energia de
combustíveis, como o carvão e o petróleo em energia elétrica.







Depois que
a vela se queima, sobra um pouco de parafina. Mas a quantidade que sobra é bem
menor. Para aonde foi então a parafina que falta?







 



A combustão
transforma o combustível, que no caso é a parafina, em&nbsp;vapor de água e gás
carbônico. Ocorre aqui o que se chama de transformação química ou&nbsp;reação química. As substâncias presentes na parafina
transformam-se em outras substâncias:&nbsp;o gás carbônico e a água.







O
oxigênio e a respiração celular







Um animal
mantido em um recipiente fechado morre logo - mesmo que haja comida suficiente.
Por quê?



Quase todos
os seres vivos empregam o oxigênio num processo que libera energia para as suas
atividades.&nbsp;Sem oxigênio, a maiorias dos seres vivos não consegue
energia suficiente para se manter vivo.&nbsp;Esse processo é chamado
de&nbsp;respiração celular.







O processo que envolve a entrada de oxigênio em nossos
pulmões e a saída de gás carbônico é chamado de respiração pulmonar. Dos
pulmões o ar entra e, pela corrente sanguínea, é levado para dentro de
estruturas microscópicas que formam o nosso corpo, as células. Nas células
ocorre a respiração celular, onde o oxigênio combina-se com substâncias
químicas do alimento (principalmente com o açúcar, a glicose) e libera energia.
Além disso, produz-se também gás carbônico e água.Veja um resumo da respiração
celular:







Glicose + oxigênio = gás
carbônico + água







A
diferença entre respiração celular e combustão







Tanto na
respiração celular quanto na combustão da maioria das substâncias, ocorre a
produção de gás carbônico e vapor de água. Mas a respiração é um processo mais
complicado e demorado do que a combustão: a respiração ocorre em etapas. A
glicose, por exemplo, é transformada em uma série de substâncias até virar gás
carbônico e água.



Se a
respiração ocorresse da mesma forma que a combustão, a energia seria liberada
muito rapidamente, e o calor faria a temperatura do organismo aumentar tanto
que provocaria a morte. Em vez disso, na respiração a energia é liberada aos
poucos, sem a temperatura da célula aumentar muito.