How can we observe a chemical change? (1 Viewer)

[minijumbuk]

I know this is a pretty stupid question to ask, but I'm dying without the answer

I know about observing the following factors:
- Colour change
- Heat change
- New substances formed

But, say if we add cold milk into hot tea. Initially brown, the milk slowly turns it into white. Now there is a heat change, and a colour change.

So how do we know if this physical or a chemical change? I know it's obviously a chemical change, but what conclusive evidence is there, that could be made simply by observation?

 

[JasonNg1025]

Mmm, I'm not very sure myself, but at this stage I think we tend to predict any reaction that will occur instead of telling whether one did or not. Your three signs are correct and usually signify a chemical reaction:

- Colour - does not always mean a chemical reaction is taking place, but sometimes there is a colour change when a chemical reaction does take place. So if we mix blue paint with yellow paint, we get green paint but no chemical reaction has necessarily occurred.

- Heat - also a result of a chemical reaction, but again, does not necessarily mean one occurs. Good example is the one you gave.

- New substance - by far the best way - it actually does mean a chemical reaction has occurred. Also, no chemical reaction occurs without producing a new substance.

Milk + Tea --> Milk Tea... Milk Tea is a mixture of milk and tea (and maybe sugar), so that means no new substances are formed. Although there is a colour and heat change, no chemical reaction occurs. Bubbles in solution are also a good way to test for chemical reaction, means a gas is <i>produced</i>.

At this level, as I mentioned earlier, I guess we just predict what will cause a reaction and what won't. We know all the general reactions such as acid - base, acid - carbonate etc. Just apply what you know to the situation and see if you can make a reaction that makes sense. For example, with displacement reactions, to tell, you either look at it to see if new metal deposits form (should be quite obvious), or you just look at your standard potential table and try and predict if one metal will displace another.

 

[tommykins]

回复: Re: How can we observe a chemical change?

Odour is also produced.

 

[minijumbuk]

Re: 回复: Re: How can we observe a chemical change?

Odour is also produced.

Does that actually work?

I mean, if you smell milk, tea, or milk tea, they'll still have different odours.

And Jason: I know that the things like displacement reactions etc... are obviously chemical reactions. That's only because we studied it before... But if we have no prior knowledge of a certain field in chemistry, how is one to know a reaction was physical or chemical?

And what does it actually mean by "new substance", rather than precipitates?

 

[tommykins]

回复: Re: 回复: Re: How can we observe a chemical change?

Does that actually work?

I mean, if you smell milk, tea, or milk tea, they'll still have different odours.

And Jason: I know that the things like displacement reactions etc... are obviously chemical reactions. That's only because we studied it before... But if we have no prior knowledge of a certain field in chemistry, how is one to know a reaction was physical or chemical?

And what does it actually mean by "new substance", rather than precipitates?

I think the odour is related to the productino of a gas.

 

[minijumbuk]

Re: 回复: Re: How can we observe a chemical change?

H₂O + CO₂ --> H₂CO₃, a new substance.
Ask yourself;
[1] is there a change in state?
*if {{answer}}, {{answer}} to physical.

[2] is there a change in the way the chemicals' composition?
*if {{answer}}, {{answer}} to chemical.

[3] is there a change in the way the chemicals' nuclear arrangement?
*if {{answer}}, {{answer}} to nuclear.
Yeah?

What if there was no gas produced? You wouldn't smell the odour, would you?
And how can you observe the change in chemical composition and nuclear arrangement?

 

[tommykins]

Re: 回复: Re: How can we observe a chemical change?

@tommykins: Why is it that when you reply "回复" comes up?

I don't know, the forum is in chinese on my laptop for some god-awful reason. haven't been able to fix.

 

[JasonNg1025]

Re: 回复: Re: How can we observe a chemical change?

And Jason: I know that the things like displacement reactions etc... are obviously chemical reactions. That's only because we studied it before... But if we have no prior knowledge of a certain field in chemistry, how is one to know a reaction was physical or chemical?

Hmm, I'm not sure, maybe the thing is that if we haven't studied that field of chemistry they will give us an equation, confirming that a chemical reaction has occurred. It's not that common that they'd give us a chemical reaction outside of the course, without us knowing that it is a chemical reaction.

Sorry, I'm not much of a help here, but stick to the ways of detection above. For nuclear, I guess you'd be feeling giddy if a nuclear reaction happened in your face

 

[cinDii]

cold milk in hot tea..colour change? yer i get tht the colour changes from brown to white as u sed..but the white is actually the milk fat particles that is suspended in the tea..by the naked eye it seems there is a colour change but its jus suspended fat particles..
sorry random comment..ill shuddup now...>_<
i hate chem T_T