Dynamic Chemistry

Title: Dynamic Chemistry
Rating: K+
Summary: Part Four (Believe it or not) of the Chemistry series. The Doctor decides to run a little experiment and gets Rose to play an integral part.
Disclaimer: Still don't own them, but keep checking, I might own them next time I post a fic.
A/N: Big thanks again to the beta without whom you guys would not have these fics . So thanks saganamidreams . Also, a huge, huge thank you to every single person who reviewed, you each have a comment below because I'm waiting impatiently for aforementioned beta to finish betaing and because I love you all so much.

This is the penultimate fic. The next one will be the last in this series, sadly. This one is very, very long. I could cut it down, but I can't be bothered. For all of you who called the last one smut without being smut, well, not sure what my response should be so I shan't respond.
The chemistry featured in this fic is not only real, it is also my experiment from last week. It followed the synthesis session in which I had to prepare phosphates in solution…you don't care do you? No…

Part 1: Subtle Chemistry
Part 2: Chemical Symmetry
Part 3: The Sweetness Triangle

Meet me in the lab after breakfast.

That was what the note on top of her bedside table said. Maybe there was something on the back; maybe it said 'just joking' or 'so we can go somewhere else'. But the only way she was going to find out if it said something else was if she picked it up and, since she'd managed to spend a good ten minutes just staring at it, unable to move from where she'd woken up, turning it over was looking rather improbable.

Was he serious? He'd been flashing smiles at her every time she'd looked at him the night before. Locking eyes with her any chance he got and he got plenty. After he'd scampered off following their little conversation, ostensibly to get food, she hadn't seen him for two hours. Not that she'd been looking: no, she'd spent those two hours face up on her bed staring at the ceiling. She probably would have continued to stare at that ceiling for eternity had he not knocked on her door and told her to come and help with fixing the TARDIS.

Funny, he'd seemed somewhat embarrassed with his lack of TARDIS fixing skills before, now he wanted her to come and bear witness to his failure. So she did. And that's what it had been. No plan behind it, no hidden agenda, he'd just fiddled and sworn and glared at the ship until it seemed to be working again and then, hands on hips, both jackets discarded and sleeves rolled up, he'd turned to Rose, flashed her a grin, thanked her for her help (what help?) and walked off.

What the hell was going on?

He'd found her again about a half hour later and dragged her into the kitchen, explaining that they would go on an adventure the next day and that until then they could amuse themselves with cards. Okay, now he was going to try to pull a trick. Who played cards when you could be off exploring the universe? Sure, it would be silly to rush off on an adventure since the day was almost over, but why did it seem like he was insisting on her being next to him and yet not actually trying to achieve anything?

Frustrating as hell was what it was.

So they'd played cards for well over two hours, had a light dinner and then he'd walked her to her room. The cards and dinner would have been boring had her brain not been going a thousand miles an hour trying to work out what all his little smiles meant. As for walking her to her room; he'd never done it before and she'd been convinced that he'd make his move, for better or worse, once there.

But he hadn't, he'd just continued to smile, opened her door for her, said good night and then disappeared. He was playing the game. She'd provoked him and he'd complied. It just hadn't occurred to her that he'd be this good at it.

Next thing she knew, she'd woken up with a piece of paper on her table, long, spindly writing across it. Meet him at the lab. Was it just going to be another little bit of nothing? Or was this his hidden agenda? Get her back into the lab. She remembered what had happened the last time. He'd tricked her. But it had been worth it.

The clock ticked past eight in the morning; early for her, and there was no way she was hungry enough to be bothered with breakfast. She'd go the lab now, surprise him, maybe catch him setting up his new trick.

As it was, when she walked through the door ten minutes later she found herself in an interesting position. The door had been open and, to avoid pacing outside for the next three hours, she'd forced herself through without hesitation. The Doctor was at the bench, as usual, perched on his chair, fiddling with something. Taking advantage of her position, she watched as he carefully measured out the tiniest amount of water and added it to a glass beaker.

Letting her eyes flicker off him for the first time since she'd entered, she noticed the other glassware. As far as chemistry went, this seemed pretty boring. So why was her heart racing as she watched him? She pushed the thought away and went back to observing as he added drops from several others flasks of water and then set the beaker down carefully.

It occurred to her that they might not all be water but she was distracted from trying to think of things that look like water but aren't when she heard him sigh and lean back in his seat. For the briefest amount of time, she saw the most adorable expression settle on his face. Eyes half closed, large contented smile and lips slightly parted as he breathed steadily. Fascinated, she watched the gentle rise and fall of his chest through the thin cotton shirt as his lab coat fell undone to his sides.

"Rose!"

Damn, she'd been checking him out and now she'd been caught checking him out. Looking back up at him, she noticed his shocked expression and tried a smile.

Sitting up straight, he wiped the startled, vulnerable look from his face. He took the safety glasses out of his hair and ran a hand through it. Recovering enough to sound relatively normal he asked, "Since when are you up this early?"

Moving further in, she leaned against the bench a few meters from him. "I got your note." Yeah, subtle Rose, he'll never guess why you rushed here.

"So you've had breakfast?" he countered quickly. This despite the fact he could already feel his brain slowing down and his thoughts clouding over.

"I wasn't hungry."

"Yeah, sure you weren't-" he muttered under his breath.

"What?" When had he become so cheeky? And when had she become so susceptible to it? Choosing to change the subject, she looked to the clear liquids. "So what are these then?"

Looking to the twenty six beakers of intricately matrixed chemicals, the Doctor couldn't help but be more interested in Rose. She'd come. That was good. But he hadn't expected her to arrive so soon; he was lucky he'd finished most of the preparation already. Lucky he hadn't been able to sleep and had been up at five with nothing to do. He angled a glance at her, she was to his right and looked completely out of place in his lab. He sighed and got up; he had to do it, even if it could very well destroy his ability to think forever.

She watched as he went to the back of the lab, to a tall cupboard nestled between the big analytical equipment she couldn't identify and opened the doors. He made sure to keep his back between what he was getting and Rose, half because a well-aimed comment could cause problems and half because it gave him time to try to lose the apprehensive, excited look from his face.

Turning around, he walked briskly back to her side, sat down, then handed over the lab coat, glasses and a hair tie. She held it all in her arms, taking note of each item before dumping them on the bench, yanking her jacket off and replacing it with the lab coat.

He was lucky he'd sat down. He hadn't accounted for the fact she'd have to take her jacket off to put the lab coat on and the enticing band of soft skin he glimpsed as her shirt rode up made his breath catch in his throat and his mouth go dry.

Stupid random variables.

He'd convinced himself to treat this as an experiment; that was the only way he could reconcile his terror at having to deal with Rose and his utter inability to not watch her, provoke her, challenge her. So, as a chemist, he needed an experiment to try to determine the cause, the precise effects and their usefulness in everyday life. So he was controlling the environment, setting the rules, and seeing where her characteristics took her. Yeah, that sounded nice and objective: this was a chemistry experiment. Not another excuse to touch her, breathe her in and generally just be around her an annoying voice inside his head insisted.

But there were always variables that you forgot to take into account. Her flashing too much skin at him was one of them.

Looking back to her, he found her with lab coat on, glasses perched on top of her head. "You should wear them properly," he said indicating the glasses.

She shrugged. "When you do, I will."

He would have insisted, but the glasses weren't really needed for the day's reactions. "You have to put your hair up."

She grinned, tilting her head and asking, "Why?"

"Because," he really had to get a better hold on his smart-assed, double meaning wit, "We're playing with fire."

Her expression fell away fast to reveal a slightly shocked look that he hadn't really expected his words to elicit. Maybe he was getting better at her version of chemistry. He grinned at that thought.

Not questioning again, she pulled her hair up into a high ponytail and sat down in her chair to await further instructions. Bringing him back to the matter at hand, he realized he had no idea where to start. He didn't want to bore her. But at the same time, he didn't want to go too fast. And he still needed to finish the prep.

Still pondering how to introduce her to chemical experimentation, he leant over to grab her seat and drag her so she was sitting right next to him. "Right," he looked up to find her staring at him, so he pointed to the equipment in front of them, throwing her a knowing smile and waiting until she'd managed to drag her gaze from his.

Explaining the purpose and name of each piece of glassware took him ten minutes, going over the different designs, advantages and disadvantages of burettes, beakers, flasks, conicals, test tubes and pipettes. He continuously glanced back over at her, making sure he hadn't lost her in the tedious details and to check she was still listening.

"Okay," the Doctor sighed once finished, "The point of this –"

"There's a point?"

Okay, so maybe glassware wasn't really that interesting, but the smile took the edge off, the cheekiness in her voice assuring him that she was still interested.

"Yes, there's a point." Picking up a beaker of clear water, he said,"This is a sample of the TARDIS water. The other day I got some unusual interference in some of my analytical calculations and I think it might be from phosphates. Phosphates that really shouldn't be present in such high concentrations."

"Phosphates are poisonous?" Rose asked, fingers tracing the bottom of the beaker.

The Doctor shivered and looked away, dragging his mind back to the subject at hand. "Nothing to worry about. Phosphates are good for body chemistry unless there are too many of them or they react with other metals. I'm more interested in where in the TARDIS's system they're coming from."

"Ah," she poked him in the arm, her tongue darting out in a way he identified as endearing. "You're just trying to understand the TARDIS through chemistry, instead of actually becoming competent at fixing her."

A lop-sided grin; he supposed that was what he was doing, but the phosphate levels were higher than he expected and he was interested. "It's a simple experiment, but it will serve as your introduction to chemical research."

He leaned forward and dragged several bottles of chemicals to the front of the bench. "First, we need to get five standards prepared. These are mixtures in which we know the concentration of phosphate, say one hundred micrograms per liter. We shine light on it and measure how much of the light it absorbs. The higher the concentration, the higher the absorbance. If we do it right, we get a linear graph of concentration verses absorbance, called a calibration plot. Next, we run the water sample, read the absorbance and find the corresponding concentration on the graph. And then we know how much phosphates are in our water."

She'd sort of followed that. It sounded interesting anyway. And the numerous chemicals he was rearranging in front of her were strangely enticing.

He stood up, edging in front of her and getting to work. Rose watched silently as he moved liquid around, measuring out tiny amounts precisely and putting them into flasks. His hands were steady as he worked, far more steady then any she'd ever seen, nimbly moving around half-filled beakers, his eyes always focused on one thing or another, not always coordinating with his hands which seemed to have eyes of their own. Precise and sure. It wasn't like anything she'd ever seen, but it was fascinating.

Twenty minutes passed filled only with the sound of glass tapping against glass and drops of liquid hitting calm aqueous surfaces. Occasionally, the Doctor would sigh, or his brow would crease and he'd bite his lip in a way Rose had never seen before. He didn't tell Rose to do anything, didn't even try to explain to her what he was doing, but twice he stopped and looked at her, reassuring himself she was still there, watching what he was doing. Both time, his eyes met hers and they both smiled.

At the end of it he seemed proud, and indeed the dilutions he'd just done would usually have taken him longer, but that wasn't why he was beaming. He was beaming because both times he'd checked, she'd been staring, both at him and what he was doing, a tiny crease between her eyes as she concentrated, her head resting on her hand as she leant against the table.

As he sat back, Rose turned in her seat to look at him, her knees just brushing against his thigh as she once again failed to resist temptation, letting her eyes wander as he stretched, his lab coat falling to his sides. His neck cracked as he twisted, his body releasing the tension for the first time in twenty minutes.

As his eyes slid to hers, she avoided his look, instead turning her gaze to the results of his tedious work. Eight pieces of glassware, almost round down the bottom with tall, narrow necks - conical flasks, if she remembered correctly - each about half filled with clear liquids. She wondered what was really hiding behind the guise of transparency and awaited the next revelation.

When Rose had first arrived, the Doctor had stretched, hands pushed up above his head, releasing the tension that gathered in every muscle from so long perched over the work bench. He'd been surprised when he'd glanced up and seen her watching him so intently. It was the first time anyone in the universe had seen him stretch, so he could be forgiven for not realizing exactly what sort of picture he must have presented, and exactly what sort of chemistry was going on inside Rose's head.

This time, having finished, he found himself giving in to temptation and deliberately repeating his earlier motion. He leaned back, pulling his muscles out until they stopped aching with the tension precision synthesis caused. All the while he followed Rose's gaze out of the corner of his eye.

Unable to stop herself, her eyes swept over him, observing, calculating and then she seemed to realize he was watching her and brought her eyes back up to meet his. The game was on.

"Okay, now that I've bored you to death with a synthesis you can't understand, you can actually do something useful." She glared at the instructive, almost cynical tone in his voice, but their up-turned lips assured each other it wasn't serious.

"To each of these," he explained, "a single drop of phenolphthalein needs to be added." He picked up an opaque bottle and left it in front of Rose.

"One drop?" she asked. "Is that a chemical term."

"No. Just add a drop." He passed her one of the conical flasks.

Skeptical, she unscrewed the lid and picked up an eye-dropper. "This one?"

Nodding, he took the eye dropper from her hands and picking up his ever-present whiteboard marker, scribbled something on the top of it. He passed it back, "Go ahead."

Sighing, she deposited a single drop into the flask. Nothing happened, but since when did a single drop do anything exciting? He passed her the next one and she realized that this was all there was for her to do.

Seven flasks later, she turned to him. It was strange: even though nothing had happened, she was still proud of herself and the mystery that surrounded the transparent liquids was still enthralling. "Now what?"

He screwed on the lid and pushed the bottle away, pulling another one forward. "Sodium hydroxide. Just a drop, but make sure you pick up the flask and swirl it around. And use another eyedropper. Chemistry might be subtle but if you mix the wrong things together, it all goes wrong."

Letting him label another eyedropper, she allowed her hand to brush his as he passed it back to her. He just grinned and seemed to have no reaction. "Stick to the chemistry at hand please." Cheeky bastard.

She added a drop of the sodium hydroxide and picked it up to swirl it around. Looking to the Doctor to make sure she wasn't doing it wrong, she wondered why he was smiling ever so slightly.

Raising an eyebrow won her an explanation. "You see, it only takes the subtlest of changes, a single reactive droplet and the dynamic changes." Even if she had no idea what he meant, the breathlessness of his voice was rather exciting. "Look."

She happened to glance down and gaped in surprise. The liquid had gone the most stunning fluoro pink she'd ever seen. She pulled it up to eye level, admiring the change. The Doctor, who admittedly knew this was fairly basic chemistry, just passed her the next flask and enjoyed her expressions as she stared at the reactions playing out in front of her.

"It's a dynamic mixture," he whispered in her ear once they'd finished; immediately she lost interest in the pink solutions and froze at how close he was. "The sodium hydroxide is dominant, so it controls the color. But you add something stronger, something better at controlling the dynamic and the color's gone."

"And what would it be that we need to add, Doctor?" His breath was hot and his voice was seductive. She knew perfectly well she sounded uncontrolled and submissive but there wasn't really much she could do about it, was there?

"An acid." Damn him, his voice was back to normal, he'd pulled back and was sitting in his chair, hands in pockets as he cheerily eyed everything but Rose. She almost huffed in annoyance. "Sulfuric to be exact."

She took the bottle marked sulfuric and snatched the marker from in front of him to label a new eyedropper. Once again, she added just a single drop of the chemical, swirled and watched with wonder as the pink color disappeared completely.

It took her less than a minute. The Doctor wished it had taken longer because his hands were clenched inside his pockets and his hearts were racing. Just because he'd found himself a better actor than expected didn't mean close proximity and Rose's breathless voice weren't effecting him.

"Right." He'd been silent too long and he could see Rose trying to work out why. "Now you just have to add eight mil of my own special mixture and then we can run the absorbances."

He leaned across in front of her, rather dismayed when his racing heart seemed to stutter to a stop as her scent and heat hit him like a wall. Hurriedly, he grabbed the reagent he'd organized earlier and placed it carefully in front of her. He retreated then, perching on the edge of his seat, trying to simultaneously commit to memory the subtle hints of vanilla, the air just after it had rained and something he could only label as Vienna in the spring - all of which he now recognized as Rose – and trying dismiss it from his mind because it was everything hazy.

"So," she interrupted his thoughts, "how do I use this?"

The Doctor realized she'd probably never seen an autodispenser and an idea crept into his mind, too insistent to ignore. He preferred it when his thoughts were hazy. "It's not difficult." He slipped out of his seat and walked around to stand behind her, "It's already set at eight mils, just lift it and push down."

She didn't move. His instructions vague when applied to a glass jar filled with more clear liquid and, instead of a lid, a long thin tap with a square box on top. There was a little adjustable arrow pointing to '8ml'; she would have been happy to get eight mils out of it and into the flasks, if she could figure out what to lift and what to push.

Despite the intelligent, scientific, unemotional voice in his head screaming 'No, no, no, no, no; stupid, stupid, stupid, stupid, and completely not objective,' the Doctor leaned forward, his arms gliding over her shoulders and down her arms until his hands hovered just over hers where they rested in her lap.

Rose froze, if froze was the right word when it felt like the temperature had just doubled. He was so close, she could feel his strange heartbeat; she was certain if she closed her eyes she could have tracked his movement based solely on the flow of air and heat. His breath, hot but subtle, was tickling her ear. "Here," he said and suddenly that single word became the most seductive she'd ever heard.

His hovering hands descended onto hers, his fingers entwining with hers as he guided them up to the autodispenser. Slowly, he settled her hands beneath his on the cool plastic, allowing them enough freedom to move, but not to get it wrong. He slid a finger between hers, resting on the top of the square box, and guided her to pull it up. Slipping his hands from hers, the rest of him didn't move an inch.

How was she supposed to concentrate with him that close? One hand snaked out; she watched it, trying to discern its target. He picked up the first conical flask and left it sitting under the tap. "Push it."

Watching from behind her as she shivered at his words, he found it ironic that he'd been talking to himself as much as her. Thankfully it had made sense she was slowly working through the conical flasks, adding the combined reagent and doing her best not to brush up against him. Difficult since he was hovering behind her. Both were immensely grateful for the chair back between them.

Yet some part of his brain was telling him to go for it. Everything he'd observed in the course of the experiment said that it was perfectly clear that she wanted him. Their chemistry was undeniable; it had been proven through his research.

Rose was finished, but she wasn't moving, not an inch; even her breathing was shallow as she tried to stop her chest from falling and rising. Leaning forward, unable and unwilling to resist, he let his lips just hover above the nape of her neck, his hands moving to grasp her elbows. He breathed deeply, her scent so distinct he thought it impossible to replicate, even with his chemical ability.

The same voice that had urged him to move behind her and guide her hands, the same one that told him the results of his experiment guaranteed him success should he push it further, tempted him with ideas of her taste. He knew taste and smell were closely tied, one was similar to the other, but there were certain tastes that couldn't be guessed from smell and that voice moved him that little bit closer.

Softly, so softly she would have thought she imagined it, his lips skimmed the back of her neck, then pressed insistently a moment later. Involuntarily, instinctively her head drifted sideways, a temptation he found impossible to resist.

Just the hint of taste was there, his lips danced on her skin, light and quick, teasing. Swallowing, he nestled his mouth in the crook of her neck, hair and nose brushing against her sensitized skin. Every atom of his existence was straining against the need to take her in, to give in and just let go. And finally, just one infinitesimal atom gave in and something in his brain snapped.

He paused as piece after piece of his resolve disappeared; he knew he couldn't stop it, couldn't resist her. And with that thought, he gave in, opening his mouth and scraping teeth and tongue over her flesh, indulging in the chemistry that almost blew his mind with action.

Under his lips, he felt her throat vibrate and heard a moan escape, he responded by instinct, whispering against her skin, "Rose."

Turning her head to him, she caught his gaze and leaned forward. How? Why? What? didn't matter. She watched him as he lifted his head, she knew what was coming next, was sure of it. "Wait," it was her own voice and she hated him for listening to it. "I don't –"

The surprise and hurt that appeared on his face, the speed with which he backed away, looking anywhere but at her, made her want to disappear. But she couldn't. All she could do was watch as he left, walking quickly out the door.

I don't what? Why had she even said anything? What on god's earth had possessed her to interrupt? She felt like screaming, but couldn't, like breaking something, but didn't dare. She just sat there, silent and unmoving.

Something in their chemistry had gone wrong.

What did you all think? I am very eager to know…